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Top-Antitop Production and Decay at Threshold at the LHC in QCD Perturbation Theory
Authors:
Paolo Nason,
Giovanni Pelliccioli,
Emanuele Re,
Luca Rottoli
Abstract:
In this work we consider the production of a top-antitop pair at the LHC when the mass of the pair is relatively near to the nominal threshold, that is to say to twice the top pole mass. In this regime, enhanced perturbative corrections arise that can be computed to all orders in perturbation theory. We present three generators of the NLO+PS kind (Next-to-Leading-Order that can be interfaced to pa…
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In this work we consider the production of a top-antitop pair at the LHC when the mass of the pair is relatively near to the nominal threshold, that is to say to twice the top pole mass. In this regime, enhanced perturbative corrections arise that can be computed to all orders in perturbation theory. We present three generators of the NLO+PS kind (Next-to-Leading-Order that can be interfaced to parton showers) that include these threshold enhanced effects. Using these generators we address the following questions: what is the size of enhanced non-relativistic effects that are not already present in the well known NLO and NNLO perturbative results; what is the size of the contribution from these effects that can be loosely attributed to toponium production; and to what extent the finite width of the top quark affects threshold enhanced corrections. Our generators are relevant for the recent observation of enhanced $t{\bar t}$ production near threshold in the pseudoscalar channel by the ATLAS and CMS collaborations.
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Submitted 27 July, 2026; v1 submitted 10 July, 2026;
originally announced July 2026.
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On Thrust Resummation Ambiguities in $e^+e^-$ Annihilation into Hadrons
Authors:
Luca Buonocore,
Paolo Nason,
Luca Rottoli,
Paolo Torrielli
Abstract:
In $e^+e^-$ shape-variable studies, and in particular for the case of thrust, fixed-order QCD predictions are typically supplemented with the resummation of contributions enhanced near the two-jet limit.
In this work we examine whether different, yet legitimate, resummation prescriptions can induce significant differences in the resulting predictions. This can occur because formally equivalent p…
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In $e^+e^-$ shape-variable studies, and in particular for the case of thrust, fixed-order QCD predictions are typically supplemented with the resummation of contributions enhanced near the two-jet limit.
In this work we examine whether different, yet legitimate, resummation prescriptions can induce significant differences in the resulting predictions. This can occur because formally equivalent prescriptions may differ by terms that, although subleading, are characterised by asymptotic expansions and may therefore lead to slow convergence.
We first compare two alternative formulations of resummation: the conjugate-space (or Laplace-space) approach, in which resummation is performed in a variable conjugate to thrust, such that the observable factorises exactly in the soft-collinear limit; and the direct-space formulation, where resummation is instead carried out directly in the thrust variable. We show that, at double-logarithmic level, the inverse Laplace transform generates a convergent tower of subleading terms. Starting from leading-logarithmic accuracy, the expansion becomes asymptotic due to the presence of the Landau pole, leading to a mild log-factorial growth of the coefficients. When including the highest available logarithmic order in the resummation, matched to fixed-order results, we still find non-negligible differences between predictions obtained in the two spaces.
We then consider a formulation of the resummation that avoids certain approximations commonly used in the derivation of conjugate-space resummation. We observe that this also has a non-negligible numerical impact.
In general, we find that the systematics stemming from the adoption of different formalisms typically exceeds the quoted theoretical uncertainties, suggesting the need for more conservative theory-error estimates when using the thrust distribution in determinations of the strong coupling.
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Submitted 6 March, 2026;
originally announced March 2026.
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HOPPET v2 release note
Authors:
Alexander Karlberg,
Paolo Nason,
Gavin Salam,
Giulia Zanderighi,
Frédéric Dreyer
Abstract:
We document the three main new features in the v2 release series of the HOPPET parton distribution function evolution code, specifically support for N$^3$LO QCD evolution in the variable flavour number scheme, for the determination of hadronic structure functions for massless quarks up to N$^3$LO, and for QED evolution to an accuracy phenomenologically equivalent to NNLO QCD. Additionally we descr…
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We document the three main new features in the v2 release series of the HOPPET parton distribution function evolution code, specifically support for N$^3$LO QCD evolution in the variable flavour number scheme, for the determination of hadronic structure functions for massless quarks up to N$^3$LO, and for QED evolution to an accuracy phenomenologically equivalent to NNLO QCD. Additionally we describe a new Python interface, CMake build option, functionality to save a hoppet table as an LHAPDF grid and update our performance benchmarks, including optimisations in interpolating PDF tables.
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Submitted 9 December, 2025; v1 submitted 10 October, 2025;
originally announced October 2025.
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Forecasting UK Consumer Price Inflation with RaGNAR: Random Generalised Network Autoregressive Processes
Authors:
Guy P. Nason,
Henry Antonio Palasciano
Abstract:
This article forecasts CPI inflation in the United Kingdom using Random Generalised Network Autoregressive (RaGNAR) Processes. More specifically, we fit Generalised Network Autoregressive (GNAR) Processes to a large set of random networks generated according to the Erdős-Rényi-Gilbert model and select the best-performing networks each month to compute out-of-sample forecasts. RaGNAR significantly…
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This article forecasts CPI inflation in the United Kingdom using Random Generalised Network Autoregressive (RaGNAR) Processes. More specifically, we fit Generalised Network Autoregressive (GNAR) Processes to a large set of random networks generated according to the Erdős-Rényi-Gilbert model and select the best-performing networks each month to compute out-of-sample forecasts. RaGNAR significantly outperforms traditional benchmark models across all horizons. Remarkably, RaGNAR also delivers materially more accurate predictions than the Bank of Englan's four to six month inflation rate forecasts published in their quarterly Monetary Policy Reports. Our results are remarkable not only for their accuracy, but also because of their speed, efficiency and simplicity compared to the Bank's current forecasting processes. RaGNAR's performance improvements manifest both in terms of their root mean squared error and mean absolute percentage error, which measure different, but crucial, aspects of the methods' performance. GNAR processes demonstrably predict future changes to CPI inflation more accurately and quickly than the benchmark models, especially at medium- to long-term forecast horizons, which is of great importance to policymakers charged with setting interest rates. We find that the most robust forecasts are those which combine the predictions from multiple GNAR processes via the use of various model averaging techniques. By analysing the structure of the best-performing graphs, we are also able to identify the key components that influence inflation rates during different periods.
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Submitted 7 May, 2025;
originally announced May 2025.
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Spin Correlations in $t{\bar t}$ Production and Decay at the LHC in QCD Perturbation Theory
Authors:
Paolo Nason,
Emanuele Re,
Luca Rottoli
Abstract:
In this work we consider the QCD predictions for spin correlations in $t\bar{t}$ production in hadronic collisions. In view of recent tensions between experimental data and theoretical calculations, it has been argued that one should include in the predictions also the effects of the production of the $η_t$, i.e. the pseudoscalar $t\bar{t}$ bound state, or alternatively the full effects of the non…
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In this work we consider the QCD predictions for spin correlations in $t\bar{t}$ production in hadronic collisions. In view of recent tensions between experimental data and theoretical calculations, it has been argued that one should include in the predictions also the effects of the production of the $η_t$, i.e. the pseudoscalar $t\bar{t}$ bound state, or alternatively the full effects of the non-relativistic dynamics of the $t\bar{t}$ pair near threshold. This implies the resummation of all corrections that scale like powers of $α_s/v$ (where $v$ is the velocity of the top quark in the $t\bar{t}$ rest frame) which are dominated by values of $v$ of order $α_s$. In this work, we show that, since the observables that are usually considered for these studies are integrated cross sections up to a $t\bar{t}$ mass cut that is not small, it is possible to perform the calculation using perturbation theory, considering only the contributions that scale as the first few powers of $α_s/v$. We examine the implications of our approach by computing corrections to nominal Monte Carlo results for correlation-sensitive observables, and compare them with available data, showing that the tension with data is no longer present.
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Submitted 25 March, 2026; v1 submitted 30 April, 2025;
originally announced May 2025.
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Fits of $α_s$ from event-shapes in the three-jet region: extension to all energies
Authors:
Paolo Nason,
Giulia Zanderighi
Abstract:
This work is an extension of a previous publication [1] where we fitted the strong coupling $α_s$ together with the non-perturbative parameter $α_0$ from event-shape and jet-shape distributions using power corrections computed in the three-jet region. In ref. [1] only ALEPH data at the $Z$-pole were used in the fit. Here, instead, we include a large data sample from various $e^+e^-$ experiments at…
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This work is an extension of a previous publication [1] where we fitted the strong coupling $α_s$ together with the non-perturbative parameter $α_0$ from event-shape and jet-shape distributions using power corrections computed in the three-jet region. In ref. [1] only ALEPH data at the $Z$-pole were used in the fit. Here, instead, we include a large data sample from various $e^+e^-$ experiments at energies ranging from 22 to 207 GeV and revisited the treatment of theoretical uncertainties. We find that the inclusion of different energies, while not changing the central fit result considerably, helps to disentangle the dependence of perturbative and non-perturbative corrections. Our best fit result is $α_s(M_Z) = 0.1181 (+0.0002 -0.0005) (+0.0018 -0.0021)$, where the first error includes experimental uncertianties and the second one includes uncertainties associated with scale variation, mass effects, fit limits, non-perturbative schemes and non-perturbative uncertainties.
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Submitted 30 January, 2025;
originally announced January 2025.
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A multiscale method for data collected from network edges via the line graph
Authors:
Dingjia Cao,
Marina I. Knight,
Guy P. Nason
Abstract:
Data collected over networks can be modelled as noisy observations of an unknown function over the nodes of a graph or network structure, fully described by its nodes and their connections, the edges. In this context, function estimation has been proposed in the literature and typically makes use of the network topology such as relative node arrangement, often using given or artificially construct…
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Data collected over networks can be modelled as noisy observations of an unknown function over the nodes of a graph or network structure, fully described by its nodes and their connections, the edges. In this context, function estimation has been proposed in the literature and typically makes use of the network topology such as relative node arrangement, often using given or artificially constructed node Euclidean coordinates. However, networks that arise in fields such as hydrology (for example, river networks) present features that challenge these established modelling setups since the target function may naturally live on edges (e.g., river flow) and/or the node-oriented modelling uses noisy edge data as weights. This work tackles these challenges and develops a novel lifting scheme along with its associated (second) generation wavelets that permit data decomposition across the network edges. The transform, which we refer to under the acronym LG-LOCAAT, makes use of a line graph construction that first maps the data in the line graph domain. We thoroughly investigate the proposed algorithm's properties and illustrate its performance versus existing methodologies. We conclude with an application pertaining to hydrology that involves the denoising of a water quality index over the England river network, backed up by a simulation study for a river flow dataset.
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Submitted 17 October, 2024;
originally announced October 2024.
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The photon parton distribution function: updates and applications
Authors:
Aneesh Manohar,
Paolo Nason,
Gavin Salam,
Giulia Zanderighi
Abstract:
The photon parton distribution function (PDF) of the proton is crucial for precise comparisons of LHC cross sections with theoretical predictions. However, it was previously affected by very large uncertainties of around ${\cal O}(100\%)$ or dependent upon phenomenologically inspired models. In the paper~\cite{Manohar:2016nzj}, we demonstrated how the photon PDF could be determined using the proto…
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The photon parton distribution function (PDF) of the proton is crucial for precise comparisons of LHC cross sections with theoretical predictions. However, it was previously affected by very large uncertainties of around ${\cal O}(100\%)$ or dependent upon phenomenologically inspired models. In the paper~\cite{Manohar:2016nzj}, we demonstrated how the photon PDF could be determined using the proton structure functions $F_2$ and $F_L$ measured in electron--proton scattering experiments. We provided an explicit formula for the PDF, which can be systematically improved order by order in perturbation theory. We obtained a photon PDF with errors $\lesssim 2$\% for $10^{-4} < x < 0.1$. Here, we recall the underlying idea and method used to obtain this result, as well as the progress made since then.
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Submitted 22 August, 2024;
originally announced August 2024.
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Linear power corrections to single top production and decay at the LHC in the narrow width approximation
Authors:
Sergei Makarov,
Kirill Melnikov,
Paolo Nason,
Melih A. Ozcelik
Abstract:
We consider top quark production and decay in the narrow width approximation and study if the polarisation effects, that manifest themselves in correlations of angular distributions of particles from top quark decays and final state jets in the production sub-process, are affected by linear power corrections. We find that, in general, the answer to this question is affirmative. We also discuss how…
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We consider top quark production and decay in the narrow width approximation and study if the polarisation effects, that manifest themselves in correlations of angular distributions of particles from top quark decays and final state jets in the production sub-process, are affected by linear power corrections. We find that, in general, the answer to this question is affirmative. We also discuss how these non-perturbative corrections affect polarisation observables used to study single top production at the LHC. Finally, we point out that generic kinematic distributions of leptons from top quark decays are affected by linear power corrections, which may have implications for proposals to extract the top quark mass from such leptonic observables. On the other hand, we demonstrate that the distribution of the ``out-of-collision-plane'' component of the positron momentum is free from linear power corrections, making it an interesting candidate for the top quark mass measurement.
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Submitted 22 November, 2024; v1 submitted 1 August, 2024;
originally announced August 2024.
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An event generator for Lepton-Hadron Deep Inelastic Scattering at NLO+PS with POWHEG including mass effects
Authors:
Luca Buonocore,
Giovanni Limatola,
Paolo Nason,
Francesco Tramontano
Abstract:
We present a generator for lepton nucleon collisions in the DIS regime, focusing in particular on processes with a massive lepton and/or a massive quark in the final state. We have built a full code matching NLO QCD corrections to parton shower Monte Carlo programs in the POWHEG-BOX framework. Our code can be used to compute NLO+PS accurate fully differential predictions for neutral current and ch…
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We present a generator for lepton nucleon collisions in the DIS regime, focusing in particular on processes with a massive lepton and/or a massive quark in the final state. We have built a full code matching NLO QCD corrections to parton shower Monte Carlo programs in the POWHEG-BOX framework. Our code can be used to compute NLO+PS accurate fully differential predictions for neutral current and charged current processes, including processes with an incoming tau neutrino, and/or including charm quarks in the final state. We also made comparisons with available data and predictions for the new neutrino experiments at CERN.
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Submitted 7 June, 2024;
originally announced June 2024.
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Leveraging Non-Decimated Wavelet Packet Features and Transformer Models for Time Series Forecasting
Authors:
Guy P Nason,
James L. Wei
Abstract:
This article combines wavelet analysis techniques with machine learning methods for univariate time series forecasting, focusing on three main contributions. Firstly, we consider the use of Daubechies wavelets with different numbers of vanishing moments as input features to both non-temporal and temporal forecasting methods, by selecting these numbers during the cross-validation phase. Secondly, w…
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This article combines wavelet analysis techniques with machine learning methods for univariate time series forecasting, focusing on three main contributions. Firstly, we consider the use of Daubechies wavelets with different numbers of vanishing moments as input features to both non-temporal and temporal forecasting methods, by selecting these numbers during the cross-validation phase. Secondly, we compare the use of both the non-decimated wavelet transform and the non-decimated wavelet packet transform for computing these features, the latter providing a much larger set of potentially useful coefficient vectors. The wavelet coefficients are computed using a shifted version of the typical pyramidal algorithm to ensure no leakage of future information into these inputs. Thirdly, we evaluate the use of these wavelet features on a significantly wider set of forecasting methods than previous studies, including both temporal and non-temporal models, and both statistical and deep learning-based methods. The latter include state-of-the-art transformer-based neural network architectures. Our experiments suggest significant benefit in replacing higher-order lagged features with wavelet features across all examined non-temporal methods for one-step-forward forecasting, and modest benefit when used as inputs for temporal deep learning-based models for long-horizon forecasting.
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Submitted 13 March, 2024;
originally announced March 2024.
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Continuous Time Locally Stationary Wavelet Processes
Authors:
Henry Antonio Palasciano,
Marina I. Knight,
Guy P. Nason
Abstract:
This article introduces the class of continuous time locally stationary wavelet processes. Continuous time models enable us to properly provide scale-based time series models for irregularly-spaced observations for the first time, while also permitting a spectral representation of the process over a continuous range of scales. We derive results for both the theoretical setting, where we assume acc…
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This article introduces the class of continuous time locally stationary wavelet processes. Continuous time models enable us to properly provide scale-based time series models for irregularly-spaced observations for the first time, while also permitting a spectral representation of the process over a continuous range of scales. We derive results for both the theoretical setting, where we assume access to the entire process sample path, and a more practical one, which develops methods for estimating the quantities of interest from sampled time series. The latter estimates are accurately computable in reasonable time by solving the relevant linear integral equation using the iterative soft-thresholding algorithm due to Daubechies, Defrise and De~Mol. Appropriate smoothing techniques are also developed and applied in this new setting. Comparisons to previous methods are conducted on the heart rate time series of a sleeping infant. Additionally, we exemplify our new methods by computing spectral and autocovariance estimates on irregularly-spaced heart rate data obtained from a recent sleep-state study.
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Submitted 17 March, 2025; v1 submitted 19 October, 2023;
originally announced October 2023.
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Linear power corrections to top quark pair production in hadron collisions
Authors:
Sergei Makarov,
Kirill Melnikov,
Paolo Nason,
Melih A. Ozcelik
Abstract:
We compute, in the framework of renormalon calculus, the ${\cal O}(Λ_{\rm QCD})$ corrections to the production of $t\bar{t}$ pairs in hadron collisions under the assumption that $q \bar q \to t \bar t$ is the dominant partonic channel. This assumption is not applicable to top quark pair production at the LHC but it is valid for the Tevatron where collisions of protons and anti-protons were studied…
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We compute, in the framework of renormalon calculus, the ${\cal O}(Λ_{\rm QCD})$ corrections to the production of $t\bar{t}$ pairs in hadron collisions under the assumption that $q \bar q \to t \bar t$ is the dominant partonic channel. This assumption is not applicable to top quark pair production at the LHC but it is valid for the Tevatron where collisions of protons and anti-protons were studied. We show that the linear power correction to the total $t \bar t$ production cross section vanishes provided one uses a short-distance scheme for the top quark mass. We also derive relatively simple formulas for the power corrections to top quark kinematic distributions. Although small numerically, these power corrections exhibit interesting dependencies on top quark kinematics.
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Submitted 12 January, 2024; v1 submitted 10 August, 2023;
originally announced August 2023.
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Automatic Locally Stationary Time Series Forecasting with application to predicting U.K. Gross Value Added Time Series under sudden shocks caused by the COVID pandemic
Authors:
Rebecca Killick,
Marina I. Knight,
Guy P. Nason,
Matthew A. Nunes,
Idris A. Eckley
Abstract:
Accurate forecasting of the U.K. gross value added (GVA) is fundamental for measuring the growth of the U.K. economy. A common nonstationarity in GVA data, such as the ABML series, is its increase in variance over time due to inflation. Transformed or inflation-adjusted series can still be challenging for classical stationarity-assuming forecasters. We adopt a different approach that works directl…
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Accurate forecasting of the U.K. gross value added (GVA) is fundamental for measuring the growth of the U.K. economy. A common nonstationarity in GVA data, such as the ABML series, is its increase in variance over time due to inflation. Transformed or inflation-adjusted series can still be challenging for classical stationarity-assuming forecasters. We adopt a different approach that works directly with the GVA series by advancing recent forecasting methods for locally stationary time series. Our approach results in more accurate and reliable forecasts, and continues to work well even when the ABML series becomes highly variable during the COVID pandemic.
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Submitted 14 March, 2023;
originally announced March 2023.
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Linear power corrections to single top production processes at the LHC
Authors:
Sergei Makarov,
Kirill Melnikov,
Paolo Nason,
Melih A. Ozcelik
Abstract:
We discuss the linear power corrections to the electroweak production of top quarks at the LHC using renormalon calculus. We show how such non-perturbative corrections can be obtained using the Low-Burnett-Kroll theorem, which provides the first subleading term to the expansion of the real-emission amplitudes around the soft limit. We demonstrate that there are no linear power corrections to the t…
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We discuss the linear power corrections to the electroweak production of top quarks at the LHC using renormalon calculus. We show how such non-perturbative corrections can be obtained using the Low-Burnett-Kroll theorem, which provides the first subleading term to the expansion of the real-emission amplitudes around the soft limit. We demonstrate that there are no linear power corrections to the total cross sections of arbitrary processes of a single top production type provided that these cross sections are expressed in terms of a short-distance top quark mass. We also derive a universal formula for the linear power corrections to generic observables that involve the top-quark momentum.
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Submitted 19 May, 2023; v1 submitted 6 February, 2023;
originally announced February 2023.
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Fits of $α_s$ using power corrections in the three-jet region
Authors:
Paolo Nason,
Giulia Zanderighi
Abstract:
In this work we study the impact of recent findings regarding non-perturbative corrections in the three-jet region to $e^+e^-$ hadronic observables, by performing a simultaneous fit of the strong coupling constant $α_s$ and the non-perturbative parameter $α_0$. We extend the calculation of these power corrections, already known for thrust and C-parameter, to other $e^+e^-$ hadronic observables. We…
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In this work we study the impact of recent findings regarding non-perturbative corrections in the three-jet region to $e^+e^-$ hadronic observables, by performing a simultaneous fit of the strong coupling constant $α_s$ and the non-perturbative parameter $α_0$. We extend the calculation of these power corrections, already known for thrust and C-parameter, to other $e^+e^-$ hadronic observables. We find that for some observables the non-perturbative corrections are reasonably well behaved in the two-jet limit, while for others they have a more problematic behaviour. If one limits the fit to the three-jet region and to the well-behaved observables, one finds in general very good results, with the extracted value of $α_s$ agreeing well with the world average. This is the case in particular for the thrust and $C$-parameter for which notably small values of $α_s$ have been reported when non-perturbative corrections have been computed using analytic methods. Furthermore, the more problematic variables are also well described provided one stays far enough from the two-jet limit, while in this same region they cannot be described using the traditional implementation of power-corrections based on two-jet kinematics.
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Submitted 9 January, 2023;
originally announced January 2023.
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Resonant leptoquark at NLO with POWHEG
Authors:
Luca Buonocore,
Admir Greljo,
Peter Krack,
Paolo Nason,
Nudzeim Selimovic,
Francesco Tramontano,
Giulia Zanderighi
Abstract:
Recent progress in calculating lepton density functions inside the proton and simulating lepton showers laid the foundations for precision studies of resonant leptoquark production at hadron colliders. Direct quark-lepton fusion into a leptoquark is a novel production channel at the LHC that has the potential to probe a unique parameter space for large masses and couplings. In this work, we build…
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Recent progress in calculating lepton density functions inside the proton and simulating lepton showers laid the foundations for precision studies of resonant leptoquark production at hadron colliders. Direct quark-lepton fusion into a leptoquark is a novel production channel at the LHC that has the potential to probe a unique parameter space for large masses and couplings. In this work, we build the first Monte Carlo event generator for a full-fledged simulation of this process at NLO for production, followed by a subsequent decay using the POWHEG method and matching to the parton showers utilizing HERWIG. The code can handle all scalar leptoquark models with renormalisable quark-lepton interactions. We then comprehensively study the differential distributions, including higher-order effects, and asses the corresponding theoretical uncertainties. We also quantify the impact of the improved predictions on the projected (HL-)LHC sensitivities and initiate the first exploration of the potential at the FCC-hh. Our work paves the way toward performing LHC searches using this channel.
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Submitted 6 September, 2022;
originally announced September 2022.
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Linear power corrections to $e^+e^-$ shape variables in the three-jet region
Authors:
Fabrizio Caola,
Silvia Ferrario Ravasio,
Giovanni Limatola,
Kirill Melnikov,
Paolo Nason,
Melih Arslan Ozcelik
Abstract:
We use an abelian model to study linear power corrections which arise from infrared renormalons and affect event shapes in $e^+e^-$ annihilation into hadrons. While previous studies explored power corrections in the two-jet region, in this paper we focus on the three-jet region, which is the most relevant one for the determination of the strong coupling constant. We show that for a broad class of…
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We use an abelian model to study linear power corrections which arise from infrared renormalons and affect event shapes in $e^+e^-$ annihilation into hadrons. While previous studies explored power corrections in the two-jet region, in this paper we focus on the three-jet region, which is the most relevant one for the determination of the strong coupling constant. We show that for a broad class of shape variables, linear power corrections can be written in a factorised form, that involves an analytically-calculable function, that characterises changes in the shape variable when a soft parton is emitted, and a constant universal factor. This universal factor is proportional to the so-called Milan factor, introduced in earlier literature to describe linear power corrections in the two-jet region. We find that the power corrections in the two-jet and in the three-jet regions are different, a result which is bound to have important consequences for the determination of the strong coupling constant from event shapes. As a further illustration of the power of the approach developed in this paper, we provide explicit analytic expressions for the leading power corrections to the $C$-parameter and the thrust distributions in the $N$-jet region for arbitrary $N$, albeit in the abelian model.
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Submitted 5 December, 2022; v1 submitted 5 April, 2022;
originally announced April 2022.
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Event Generators for High-Energy Physics Experiments
Authors:
J. M. Campbell,
M. Diefenthaler,
T. J. Hobbs,
S. Höche,
J. Isaacson,
F. Kling,
S. Mrenna,
J. Reuter,
S. Alioli,
J. R. Andersen,
C. Andreopoulos,
A. M. Ankowski,
E. C. Aschenauer,
A. Ashkenazi,
M. D. Baker,
J. L. Barrow,
M. van Beekveld,
G. Bewick,
S. Bhattacharya,
N. Bhuiyan,
C. Bierlich,
E. Bothmann,
P. Bredt,
A. Broggio,
A. Buckley
, et al. (187 additional authors not shown)
Abstract:
We provide an overview of the status of Monte-Carlo event generators for high-energy particle physics. Guided by the experimental needs and requirements, we highlight areas of active development, and opportunities for future improvements. Particular emphasis is given to physics models and algorithms that are employed across a variety of experiments. These common themes in event generator developme…
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We provide an overview of the status of Monte-Carlo event generators for high-energy particle physics. Guided by the experimental needs and requirements, we highlight areas of active development, and opportunities for future improvements. Particular emphasis is given to physics models and algorithms that are employed across a variety of experiments. These common themes in event generator development lead to a more comprehensive understanding of physics at the highest energies and intensities, and allow models to be tested against a wealth of data that have been accumulated over the past decades. A cohesive approach to event generator development will allow these models to be further improved and systematic uncertainties to be reduced, directly contributing to future experimental success. Event generators are part of a much larger ecosystem of computational tools. They typically involve a number of unknown model parameters that must be tuned to experimental data, while maintaining the integrity of the underlying physics models. Making both these data, and the analyses with which they have been obtained accessible to future users is an essential aspect of open science and data preservation. It ensures the consistency of physics models across a variety of experiments.
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Submitted 26 February, 2025; v1 submitted 21 March, 2022;
originally announced March 2022.
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The strong coupling constant: State of the art and the decade ahead
Authors:
D. d'Enterria,
S. Kluth,
G. Zanderighi,
C. Ayala,
M. A. Benitez-Rathgeb,
J. Bluemlein,
D. Boito,
N. Brambilla,
D. Britzger,
S. Camarda,
A. M. Cooper-Sarkar,
T. Cridge,
G. Cvetic,
M. Dalla Brida,
A. Deur,
F. Giuli,
M. Golterman,
A. H. Hoang,
J. Huston,
M. Jamin,
A. V. Kotikov,
V. G. Krivokhizhin,
A. S. Kronfeld,
V. Leino,
K. Lipka
, et al. (33 additional authors not shown)
Abstract:
Theoretical predictions for particle production cross sections and decays at colliders rely heavily on perturbative Quantum Chromodynamics (QCD) calculations, expressed as an expansion in powers of the strong coupling constant $α_s$. The current $\mathcal{O}(1\%)$ uncertainty of the QCD coupling evaluated at the reference Z boson mass, $α_s(m_Z) = 0.1179 \pm 0.0009$, is one of the limiting factors…
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Theoretical predictions for particle production cross sections and decays at colliders rely heavily on perturbative Quantum Chromodynamics (QCD) calculations, expressed as an expansion in powers of the strong coupling constant $α_s$. The current $\mathcal{O}(1\%)$ uncertainty of the QCD coupling evaluated at the reference Z boson mass, $α_s(m_Z) = 0.1179 \pm 0.0009$, is one of the limiting factors to more precisely describe multiple processes at current and future colliders. A reduction of this uncertainty is thus a prerequisite to perform precision tests of the Standard Model as well as searches for new physics. This report provides a comprehensive summary of the state-of-the-art, challenges, and prospects in the experimental and theoretical study of the strong coupling. The current $α_s(m_Z)$ world average is derived from a combination of seven categories of observables: (i) lattice QCD, (ii) hadronic $τ$ decays, (iii) deep-inelastic scattering and parton distribution functions fits, (iv) electroweak boson decays, hadronic final-states in (v) $e^+e^-$, (vi) e-p, and (vii) p-p collisions, and (viii) quarkonia decays and masses. We review the current status of each of these seven $α_s(m_Z)$ extraction methods, discuss novel $α_s$ determinations, and examine the averaging method used to obtain the world-average value. Each of the methods discussed provides a ``wish list'' of experimental and theoretical developments required in order to achieve the goal of a per-mille precision on $α_s(m_Z)$ within the next decade.
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Submitted 29 November, 2024; v1 submitted 15 March, 2022;
originally announced March 2022.
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NNLO+PS with MiNNLO$_{\rm PS}$: status and prospects
Authors:
Luca Buonocore,
Mauro Chiesa,
Gabriël Koole,
Daniele Lombardi,
Javier Mazzitelli,
Pier Francesco Monni,
Paolo Nason,
Emanuele Re,
Luca Rottoli,
Marius Wiesemann,
Giulia Zanderighi,
Silvia Zanoli
Abstract:
We summarize the current status and near future prospects for next-to-next-to-leading order calculations matched to parton shower based on the MiNNLO$_{\rm PS}$ method. We give a theoretical overview, illustrate selected results for $ZZ\to 4\ell$ and top-pair production processes at the LHC, and provide an outlook of the future challenges.
We summarize the current status and near future prospects for next-to-next-to-leading order calculations matched to parton shower based on the MiNNLO$_{\rm PS}$ method. We give a theoretical overview, illustrate selected results for $ZZ\to 4\ell$ and top-pair production processes at the LHC, and provide an outlook of the future challenges.
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Submitted 14 March, 2022;
originally announced March 2022.
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Top-pair production at the LHC with MiNNLO$_{\rm PS}$
Authors:
Javier Mazzitelli,
Pier Francesco Monni,
Paolo Nason,
Emanuele Re,
Marius Wiesemann,
Giulia Zanderighi
Abstract:
We consider the production of a pair of heavy quarks and illustrate the derivation of the MiNNLO method to match next-to-next-to-leading order calculations with parton showers (NNLO+PS) for this class of processes. As a first application, we construct an event generator for the fully differential simulation of hadronic top-quark pair production at NNLO+PS and discuss all details of its implementat…
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We consider the production of a pair of heavy quarks and illustrate the derivation of the MiNNLO method to match next-to-next-to-leading order calculations with parton showers (NNLO+PS) for this class of processes. As a first application, we construct an event generator for the fully differential simulation of hadronic top-quark pair production at NNLO+PS and discuss all details of its implementation in a parton shower Monte Carlo framework. We present new phenomenological results for the Large Hadron Collider obtained by including the tree-level decays of the top quarks, while accounting for spin-correlation effects. A comprehensive comparison to LHC measurements shows an excellent description of experimental data across multiple hadronic and leptonic particle-level observables. The computer code is available for download within the POWHEG-BOX.
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Submitted 22 December, 2021;
originally announced December 2021.
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Multiplicative-Accumulative matching of NLO calculations with parton showers
Authors:
Paolo Nason,
Gavin P. Salam
Abstract:
We propose a new approach for combining next-to-leading order (NLO) and parton shower (PS) calculations so as to obtain three core features: (a) applicability to general showers, as with the MCatNLO and POWHEG methods; (b) positive-weight events, as with the KrkNLO and POWHEG methods; and (c) all showering attributed to the parton shower code, as with the MCatNLO and KrkNLO methods. This is achiev…
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We propose a new approach for combining next-to-leading order (NLO) and parton shower (PS) calculations so as to obtain three core features: (a) applicability to general showers, as with the MCatNLO and POWHEG methods; (b) positive-weight events, as with the KrkNLO and POWHEG methods; and (c) all showering attributed to the parton shower code, as with the MCatNLO and KrkNLO methods. This is achieved by using multiplicative matching in phase space regions where the shower overestimates the matrix element and accumulative (additive) matching in regions where the shower underestimates the matrix element, an approach that can be viewed as a combination of the MCatNLO and KrkNLO methods.
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Submitted 5 November, 2021;
originally announced November 2021.
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HL-LHC Computing Review Stage-2, Common Software Projects: Event Generators
Authors:
The HSF Physics Event Generator WG,
:,
Efe Yazgan,
Josh McFayden,
Andrea Valassi,
Simone Amoroso,
Enrico Bothmann,
Andy Buckley,
John Campbell,
Gurpreet Singh Chahal,
Taylor Childers,
Gloria Corti,
Rikkert Frederix,
Stefano Frixione,
Francesco Giuli,
Alexander Grohsjean,
Stefan Hoeche,
Phil Ilten,
Frank Krauss,
Michal Kreps,
David Lange,
Leif Lonnblad,
Zach Marshall,
Olivier Mattelaer,
Stephen Mrenna
, et al. (14 additional authors not shown)
Abstract:
This paper has been prepared by the HEP Software Foundation (HSF) Physics Event Generator Working Group (WG), as an input to the second phase of the LHCC review of High-Luminosity LHC (HL-LHC) computing, which is due to take place in November 2021. It complements previous documents prepared by the WG in the context of the first phase of the LHCC review in 2020, including in particular the WG paper…
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This paper has been prepared by the HEP Software Foundation (HSF) Physics Event Generator Working Group (WG), as an input to the second phase of the LHCC review of High-Luminosity LHC (HL-LHC) computing, which is due to take place in November 2021. It complements previous documents prepared by the WG in the context of the first phase of the LHCC review in 2020, including in particular the WG paper on the specific challenges in Monte Carlo event generator software for HL-LHC, which has since been updated and published, and which we are also submitting to the November 2021 review as an integral part of our contribution.
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Submitted 30 September, 2021;
originally announced September 2021.
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Photon and Leptons induced processes at the LHC
Authors:
Luca Buonocore,
Paolo Nason,
Francesco Tramontano,
Giulia Zanderighi
Abstract:
We study a few basic photon- and lepton-initiated processes at the LHC which can be computed using the recently developed photon and lepton parton densities. First, we consider the production of a massive scalar particle initiated by lepton-antilepton annihilation and photon-photon fusion as representative examples of searches of exotic particles. Then we study lepton-lepton scattering, since this…
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We study a few basic photon- and lepton-initiated processes at the LHC which can be computed using the recently developed photon and lepton parton densities. First, we consider the production of a massive scalar particle initiated by lepton-antilepton annihilation and photon-photon fusion as representative examples of searches of exotic particles. Then we study lepton-lepton scattering, since this Standard-Model process may be observable at the LHC. We examine these processes at leading and next-to-leading order and, using the POWHEG method, we match our calculations to parton shower programs that implement the required lepton or photon initial-states. We assess the typical size of cross-sections and their uncertainties and discuss the preferred choices for the factorization scale. These processes can also be computed starting directly from the lepto-production hadronic tensor, leading to a result where some collinear-enhanced QED corrections are missing, but all strong corrections are included. Thus, we are in the unique position to perform a comparison of results obtained via the factorization approach to a calculation that does not have strong corrections. This is particularly relevant in the case of lepton-scattering, that is more abundant at lower energies where it is affected by larger strong corrections. We thus compute this process also with the hadronic-tensor method, and compare the results with those obtained with POWHEG. Finally, for some lepton-lepton scattering processes, we compare the size of the signal to the main quark-induced background, which is double Drell-Yan production, and outline a preliminary search strategy to enhance the signal to background ratio.
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Submitted 22 September, 2021;
originally announced September 2021.
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On linear power corrections in certain collider observables
Authors:
Fabrizio Caola,
Silvia Ferrario Ravasio,
Giovanni Limatola,
Kirill Melnikov,
Paolo Nason
Abstract:
We study linear power corrections ${\cal O}(Λ_{\rm QCD}/Q)$ to certain collider observables. We present arguments that prove that such corrections cannot appear in observables that are inclusive with respect to QCD radiation, such as total cross sections as well as rapidity and transverse momentum distributions of color-neutral particles. Although our calculations are carried out in a simplified f…
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We study linear power corrections ${\cal O}(Λ_{\rm QCD}/Q)$ to certain collider observables. We present arguments that prove that such corrections cannot appear in observables that are inclusive with respect to QCD radiation, such as total cross sections as well as rapidity and transverse momentum distributions of color-neutral particles. Although our calculations are carried out in a simplified framework, our arguments and conclusions are applicable, with some reservations, to processes both at lepton and hadron colliders. We also show how an improved understanding of the origin of linear power corrections allows us to simplify their calculation. As an application, we compute the leading non-perturbative corrections to the $C$-parameter and the thrust in $e^+e^-$ annihilation in a generic three-jet configuration.
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Submitted 12 June, 2022; v1 submitted 19 August, 2021;
originally announced August 2021.
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Quantifying the economic response to COVID-19 mitigations and death rates via forecasting Purchasing Managers' Indices using Generalised Network Autoregressive models with exogenous variables
Authors:
Guy P Nason,
James L Wei
Abstract:
Knowledge of the current state of economies, how they respond to COVID-19 mitigations and indicators, and what the future might hold for them is important. We use recently-developed generalised network autoregressive (GNAR) models, using trade-determined networks, to model and forecast the Purchasing Managers' Indices for a number of countries. We use networks that link countries where the links t…
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Knowledge of the current state of economies, how they respond to COVID-19 mitigations and indicators, and what the future might hold for them is important. We use recently-developed generalised network autoregressive (GNAR) models, using trade-determined networks, to model and forecast the Purchasing Managers' Indices for a number of countries. We use networks that link countries where the links themselves, or their weights, are determined by the degree of export trade between the countries. We extend these models to include node-specific time series exogenous variables (GNARX models), using this to incorporate COVID-19 mitigation stringency indices and COVID-19 death rates into our analysis. The highly parsimonious GNAR models considerably outperform vector autoregressive models in terms of mean-squared forecasting error and our GNARX models themselves outperform GNAR ones. Further mixed frequency modelling predicts the extent to which that the UK economy will be affected by harsher, weaker or no interventions.
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Submitted 14 July, 2021;
originally announced July 2021.
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Next-to-next-to-leading order event generation for top-quark pair production
Authors:
Javier Mazzitelli,
Pier Francesco Monni,
Paolo Nason,
Emanuele Re,
Marius Wiesemann,
Giulia Zanderighi
Abstract:
The production of top-quark pairs in hadronic collisions is among the most important reactions in modern particle physics phenomenology and constitutes an instrumental avenue to study the properties of the heaviest quark observed in nature. The analysis of this process at the Large Hadron Collider relies heavily on Monte Carlo simulations of the final state events, whose accuracy is challenged by…
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The production of top-quark pairs in hadronic collisions is among the most important reactions in modern particle physics phenomenology and constitutes an instrumental avenue to study the properties of the heaviest quark observed in nature. The analysis of this process at the Large Hadron Collider relies heavily on Monte Carlo simulations of the final state events, whose accuracy is challenged by the outstanding precision of experimental measurements. In this letter we present the first matched computation of top-quark pair production at next-to-next-to-leading order in QCD with all-order radiative corrections as implemented via parton-shower simulations. Besides its intrinsic relevance for LHC phenomenology, this work also establishes an important step towards the simulation of other hadronic processes with colour charges in the final state.
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Submitted 24 February, 2022; v1 submitted 28 December, 2020;
originally announced December 2020.
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Infrared Renormalons in Kinematic Distributions for Hadron Collider Processes
Authors:
Silvia Ferrario Ravasio,
Giovanni Limatola,
Paolo Nason
Abstract:
Infrared renormalons in Quantum Chromodynamics are associated with non-perturbative corrections to short distance observables. Linear renormalons, i.e. such that the associated non-perturbative corrections scale like one inverse power of the hard scale, can affect at a non-negligible level even the very high-energy phenomena studied at the Large Hadron Collider. Using an Abelian model, we study th…
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Infrared renormalons in Quantum Chromodynamics are associated with non-perturbative corrections to short distance observables. Linear renormalons, i.e. such that the associated non-perturbative corrections scale like one inverse power of the hard scale, can affect at a non-negligible level even the very high-energy phenomena studied at the Large Hadron Collider. Using an Abelian model, we study the presence of linear renormalons in the transverse momentum distribution of a neutral vector boson $Z$ produced in hadronic collisions. We consider a process where the $Z$ transverse momentum is balanced by a sizable recoil against a coloured final state particle. One may worry that such a colour configuration, not being azimuthally symmetric, could generate unbalanced soft radiation, associated in turn with linear infrared renormalons affecting the transverse momentum distribution of the vector boson. We performed a numerical calculation of the renormalon effects for this process in the so-called large $b_0$ limit. We found no evidence of linear renormalons in the transverse momentum distribution of the $Z$ in the large transverse-momentum region, irrespective of rapidity cuts.
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Submitted 22 June, 2021; v1 submitted 28 November, 2020;
originally announced November 2020.
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An interface between the POWHEG BOX and MadGraph5_aMC@NLO
Authors:
Paolo Nason,
Carlo Oleari,
Marco Rocco,
Marco Zaro
Abstract:
In this paper we present a framework for developing POWHEG BOX generators using MadGraph5_aMC@NLO for the computation of the matrix elements. Within this framework, all the flexibility of MadGraph5_aMC@NLO for the generation of matrix elements for Standard Model processes and for several of its extensions can be exploited, as well as all features of the POWHEG BOX framework, including the possibil…
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In this paper we present a framework for developing POWHEG BOX generators using MadGraph5_aMC@NLO for the computation of the matrix elements. Within this framework, all the flexibility of MadGraph5_aMC@NLO for the generation of matrix elements for Standard Model processes and for several of its extensions can be exploited, as well as all features of the POWHEG BOX framework, including the possibility of multijet merging without a merging scale (using the so called MiNLO approach). As a proof of concept, we develop a generator for the production of a spin-0 Higgs-like boson in association with up to two jets, with CP-violating couplings.
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Submitted 14 August, 2020;
originally announced August 2020.
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Leptons in the Proton
Authors:
Luca Buonocore,
Paolo Nason,
Francesco Tramontano,
Giulia Zanderighi
Abstract:
As is the case for all light coloured Standard Model particles, also photons and charged leptons appear as constituents in ultrarelativistic hadron beams, and admit a parton density function (PDF). It has been shown recently that the photon PDF can be given in terms of the structure functions and form factors for electron-proton scattering. The same holds for lepton PDFs. In the present work we se…
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As is the case for all light coloured Standard Model particles, also photons and charged leptons appear as constituents in ultrarelativistic hadron beams, and admit a parton density function (PDF). It has been shown recently that the photon PDF can be given in terms of the structure functions and form factors for electron-proton scattering. The same holds for lepton PDFs. In the present work we set up a calculation of the lepton PDFs at next-to-leading order, using the same data input needed in the photon case. A precise knowledge of the lepton densities allows us to study lepton-initiated processes even at a hadron collider, with all possible combinations of same-charge, opposite-charge, same-flavour, different-flavour leptons and leptons-quarks, most of which cannot be realized in any other foreseeable experiment. The lepton densities in the proton are extremely small, so that their contribution to Standard Model processes is generally shadowed by processes initiated by coloured partons. We will show, however, that there are cases where these processes can be relevant, giving rise to rare Standard Model signatures and to new production channels, that can enlarge the discovery potential of New Physics at the LHC and future high energy colliders with hadrons in the initial state.
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Submitted 18 August, 2020; v1 submitted 13 May, 2020;
originally announced May 2020.
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Lepton-quark collisions at the Large Hadron Collider
Authors:
Luca Buonocore,
Ulrich Haisch,
Paolo Nason,
Francesco Tramontano,
Giulia Zanderighi
Abstract:
Processes commonly studied at the Large Hadron Collider (LHC) are induced by quarks and gluons inside the protons of the LHC beams. In this letter we demonstrate that, since protons also contain leptons, it is possible to target lepton-induced processes at the LHC as well. In particular, by picking a lepton from one beam and a quark from the other beam, we present for the first time a comprehensiv…
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Processes commonly studied at the Large Hadron Collider (LHC) are induced by quarks and gluons inside the protons of the LHC beams. In this letter we demonstrate that, since protons also contain leptons, it is possible to target lepton-induced processes at the LHC as well. In particular, by picking a lepton from one beam and a quark from the other beam, we present for the first time a comprehensive analysis of resonant single leptoquark (LQ) production at a hadron collider. In the case of minimal scalar LQs, we derive novel bounds that arise from the LHC Run II considering all possible flavour combinations of an electron or a muon and an up ($u$), a down ($d$), a strange or a charm quark. For the flavour combinations with a $u$ or a $d$ quark, the obtained limits represent the most stringent constraints to date on LQs of this type. The prospects of our method at future LHC runs are also explored. Given the discovery reach of the proposed LQ signature, we argue that dedicated resonance searches in final states featuring a single light lepton and a single light-flavour jet should be added to the exotics search canon of both the ATLAS and the CMS collaboration.
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Submitted 10 November, 2020; v1 submitted 13 May, 2020;
originally announced May 2020.
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The Local Partial Autocorrelation Function and Some Applications
Authors:
Rebecca Killick,
Marina I. Knight,
Guy P. Nason,
Idris A. Eckley
Abstract:
The classical regular and partial autocorrelation functions are powerful tools for stationary time series modelling and analysis. However, it is increasingly recognized that many time series are not stationary and the use of classical global autocorrelations can give misleading answers. This article introduces two estimators of the local partial autocorrelation function and establishes their asymp…
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The classical regular and partial autocorrelation functions are powerful tools for stationary time series modelling and analysis. However, it is increasingly recognized that many time series are not stationary and the use of classical global autocorrelations can give misleading answers. This article introduces two estimators of the local partial autocorrelation function and establishes their asymptotic properties. The article then illustrates the use of these new estimators on both simulated and real time series. The examples clearly demonstrate the strong practical benefits of local estimators for time series that exhibit nonstationarities.
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Submitted 27 April, 2020;
originally announced April 2020.
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Rapidly evaluating lockdown strategies using spectral analysis: the cycles behind new daily COVID-19 cases and what happens after lockdown
Authors:
Guy P. Nason
Abstract:
Spectral analysis characterises oscillatory time series behaviours such as cycles, but accurate estimation requires reasonable numbers of observations. Current COVID-19 time series for many countries are short: pre- and post-lockdown series are shorter still. Accurate estimation of potentially interesting cycles within such series seems beyond reach. We solve the problem of obtaining accurate esti…
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Spectral analysis characterises oscillatory time series behaviours such as cycles, but accurate estimation requires reasonable numbers of observations. Current COVID-19 time series for many countries are short: pre- and post-lockdown series are shorter still. Accurate estimation of potentially interesting cycles within such series seems beyond reach. We solve the problem of obtaining accurate estimates from short time series by using recent Bayesian spectral fusion methods. Here we show that transformed new daily COVID-19 cases for many countries generally contain three cycles operating at wavelengths of around 2.7, 4.1 and 6.7 days (weekly). We show that the shorter cycles are suppressed after lockdown. The pre- and post lockdown differences suggest that the weekly effect is at least partly due to non-epidemic factors, whereas the two shorter cycles seem intrinsic to the epidemic. Unconstrained, new cases grow exponentially, but the internal cyclic structure causes periodic falls in cases. This suggests that lockdown success might only be indicated by four or more daily falls in cases. Spectral learning for epidemic time series contributes to the understanding of the epidemic process, helping evaluate interventions and assists with forecasting. Spectral fusion is a general technique that is able to fuse spectra recorded at different sampling rates, which can be applied to a wide range of time series from many disciplines.
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Submitted 16 April, 2020;
originally announced April 2020.
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MiNNLO$_{\text{PS}}$: A new method to match NNLO QCD to parton showers
Authors:
Pier Francesco Monni,
Paolo Nason,
Emanuele Re,
Marius Wiesemann,
Giulia Zanderighi
Abstract:
We present a novel method to combine QCD calculations at next-to-next-to-leading order (NNLO) with parton shower (PS) simulations, that can be applied to the production of heavy systems in hadronic collisions, such as colour singlets or a $t\bar{t}$ pair. The NNLO corrections are included by connecting the MiNLO$^\prime$ method with transverse-momentum resummation, and they are calculated at gener…
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We present a novel method to combine QCD calculations at next-to-next-to-leading order (NNLO) with parton shower (PS) simulations, that can be applied to the production of heavy systems in hadronic collisions, such as colour singlets or a $t\bar{t}$ pair. The NNLO corrections are included by connecting the MiNLO$^\prime$ method with transverse-momentum resummation, and they are calculated at generation time without any additional reweighting, making the algorithm considerably efficient. Moreover, the combination of different jet multiplicities does not require any unphysical merging scale, and the matching preserves the structure of the leading logarithmic corrections of the Monte Carlo simulation for parton showers ordered in transverse momentum. We present proof-of-concept applications to hadronic Higgs production and the Drell-Yan process at the LHC.
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Submitted 10 January, 2022; v1 submitted 19 August, 2019;
originally announced August 2019.
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Addendum to: A Theoretical Study of Top-Mass Measurements at the LHC Using NLO+PS Generators of Increasing Accuracy
Authors:
Silvia Ferrario Ravasio,
Tomas Jezo,
Paolo Nason,
Carlo Oleari
Abstract:
This paper is a follow-up of Ref.~\cite{Ravasio:2018lzi}, where we studied the impact of next-to-leading order calculations merged with parton shower generators (NLO+PS) of increasing accuracy in the extraction of the top mass at hadron colliders. Here we examined results obtained with the older (fortran-based) shower generators Pythia6.4 and Herwig6.5. Our findings are in line with what we found…
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This paper is a follow-up of Ref.~\cite{Ravasio:2018lzi}, where we studied the impact of next-to-leading order calculations merged with parton shower generators (NLO+PS) of increasing accuracy in the extraction of the top mass at hadron colliders. Here we examined results obtained with the older (fortran-based) shower generators Pythia6.4 and Herwig6.5. Our findings are in line with what we found in our previous paper with the new, c++-based, generators Pythia8.2 and Herwig7.1.
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Submitted 21 June, 2019;
originally announced June 2019.
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Standard Model Physics at the HL-LHC and HE-LHC
Authors:
P. Azzi,
S. Farry,
P. Nason,
A. Tricoli,
D. Zeppenfeld,
R. Abdul Khalek,
J. Alimena,
N. Andari,
L. Aperio Bella,
A. J. Armbruster,
J. Baglio,
S. Bailey,
E. Bakos,
A. Bakshi,
C. Baldenegro,
F. Balli,
A. Barker,
W. Barter,
J. de Blas,
F. Blekman,
D. Bloch,
A. Bodek,
M. Boonekamp,
E. Boos,
J. D. Bossio Sola
, et al. (201 additional authors not shown)
Abstract:
The successful operation of the Large Hadron Collider (LHC) and the excellent performance of the ATLAS, CMS, LHCb and ALICE detectors in Run-1 and Run-2 with $pp$ collisions at center-of-mass energies of 7, 8 and 13 TeV as well as the giant leap in precision calculations and modeling of fundamental interactions at hadron colliders have allowed an extraordinary breadth of physics studies including…
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The successful operation of the Large Hadron Collider (LHC) and the excellent performance of the ATLAS, CMS, LHCb and ALICE detectors in Run-1 and Run-2 with $pp$ collisions at center-of-mass energies of 7, 8 and 13 TeV as well as the giant leap in precision calculations and modeling of fundamental interactions at hadron colliders have allowed an extraordinary breadth of physics studies including precision measurements of a variety physics processes. The LHC results have so far confirmed the validity of the Standard Model of particle physics up to unprecedented energy scales and with great precision in the sectors of strong and electroweak interactions as well as flavour physics, for instance in top quark physics. The upgrade of the LHC to a High Luminosity phase (HL-LHC) at 14 TeV center-of-mass energy with 3 ab$^{-1}$ of integrated luminosity will probe the Standard Model with even greater precision and will extend the sensitivity to possible anomalies in the Standard Model, thanks to a ten-fold larger data set, upgraded detectors and expected improvements in the theoretical understanding. This document summarises the physics reach of the HL-LHC in the realm of strong and electroweak interactions and top quark physics, and provides a glimpse of the potential of a possible further upgrade of the LHC to a 27 TeV $pp$ collider, the High-Energy LHC (HE-LHC), assumed to accumulate an integrated luminosity of 15 ab$^{-1}$.
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Submitted 20 December, 2019; v1 submitted 11 February, 2019;
originally announced February 2019.
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Monte Carlo event generators for high energy particle physics event simulation
Authors:
Andy Buckley,
Frank Krauss,
Simon Plätzer,
Michael Seymour,
Simone Alioli,
Jeppe Andersen,
Johannes Bellm,
Jon Butterworth,
Mrinal Dasgupta,
Claude Duhr,
Stefano Frixione,
Stefan Gieseke,
Keith Hamilton,
Gavin Hesketh,
Stefan Hoeche,
Hannes Jung,
Wolfgang Kilian,
Leif Lönnblad,
Fabio Maltoni,
Michelangelo Mangano,
Stephen Mrenna,
Zoltán Nagy,
Paolo Nason,
Emily Nurse,
Thorsten Ohl
, et al. (18 additional authors not shown)
Abstract:
Monte Carlo event generators (MCEGs) are the indispensable workhorses of particle physics, bridging the gap between theoretical ideas and first-principles calculations on the one hand, and the complex detector signatures and data of the experimental community on the other hand. All collider physics experiments are dependent on simulated events by MCEG codes such as Herwig, Pythia, Sherpa, POWHEG,…
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Monte Carlo event generators (MCEGs) are the indispensable workhorses of particle physics, bridging the gap between theoretical ideas and first-principles calculations on the one hand, and the complex detector signatures and data of the experimental community on the other hand. All collider physics experiments are dependent on simulated events by MCEG codes such as Herwig, Pythia, Sherpa, POWHEG, and MG5_aMC@NLO to design and tune their detectors and analysis strategies. The development of MCEGs is overwhelmingly driven by a vibrant community of academics at European Universities, who also train the next generations of particle phenomenologists. The new challenges posed by possible future collider-based experiments and the fact that the first analyses at Run II of the LHC are now frequently limited by theory uncertainties urge the community to invest into further theoretical and technical improvements of these essential tools. In this short contribution to the European Strategy Update, we briefly review the state of the art, and the further developments that will be needed to meet the challenges of the next generation.
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Submitted 5 February, 2019;
originally announced February 2019.
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Renormalons and the Top Quark Mass Measurement
Authors:
Paolo Nason
Abstract:
I illustrate a recent work on the large-order behaviour of the perturbative expansion (and the related power-suppressed ambiguities) arising from infrared renormalons, in the context of top mass measurements in open-top production processes.
I illustrate a recent work on the large-order behaviour of the perturbative expansion (and the related power-suppressed ambiguities) arising from infrared renormalons, in the context of top mass measurements in open-top production processes.
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Submitted 15 January, 2019;
originally announced January 2019.
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All-orders behaviour and renormalons in top-mass observables
Authors:
Silvia Ferrario Ravasio,
Paolo Nason,
Carlo Oleari
Abstract:
We study a simplified model of top production and decay, consisting in a virtual vector boson $W^*$ decaying into a massive-massless $t$-$\bar{b}$ quark-antiquark pair. The top has a finite width and further decays into a stable vector boson $W$ and a $b$ quark. We then consider the emission or the virtual exchange of one gluon, with all possible light-quark loop insertions. These are the dominant…
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We study a simplified model of top production and decay, consisting in a virtual vector boson $W^*$ decaying into a massive-massless $t$-$\bar{b}$ quark-antiquark pair. The top has a finite width and further decays into a stable vector boson $W$ and a $b$ quark. We then consider the emission or the virtual exchange of one gluon, with all possible light-quark loop insertions. These are the dominant diagrams in the limit of an infinite number of light flavours. We devise a procedure to compute this process fully, by analytic and numerical methods, and for any infrared-safe final-state observables. We examine the results at arbitrary orders in perturbation theory, and assess the factorial growth associated with renormalons. We look for renormalon effects leading to corrections of order $Λ_{\rm QCD}$, that we dub `linear' renormalons, in the inclusive cross section (with and without selection cuts), in the mass of the reconstructed-top system, and in the average energy of the final-state $W$ boson, considering both the pole and the $\overline{\rm MS}$ scheme for the top mass. We find that the total cross section without cuts, if expressed in terms of the $\overline{\rm MS}$ mass, does not exhibit linear renormalons, but, as soon as selection cuts are introduced, jets-related linear renormalons arise in any mass scheme. In addition, we show that the reconstructed mass is affected by linear renormalons in any scheme and that the average energy of the $W$ boson (that we consider as a simplified example of leptonic observable), in any mass scheme, has a renormalon in the narrow-width limit, that is however screened at large orders for finite top widths, provided the top mass is in the $\overline{\rm MS}$ scheme.
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Submitted 25 October, 2018;
originally announced October 2018.
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The Top Quark Mass at the LHC
Authors:
Paolo Nason
Abstract:
I briefly discuss some theoretical aspects of top mass measurements at the LHC. In particular, I illustrate a recent theoretical study performed using next-to-leading order (NLO) calculations interfaced to shower generators (NLO+PS) of increasing accuracy, interfaced to both Pythia8 and Herwig7 Monte Carlo generators.
I briefly discuss some theoretical aspects of top mass measurements at the LHC. In particular, I illustrate a recent theoretical study performed using next-to-leading order (NLO) calculations interfaced to shower generators (NLO+PS) of increasing accuracy, interfaced to both Pythia8 and Herwig7 Monte Carlo generators.
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Submitted 15 January, 2018;
originally announced January 2018.
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A theoretical study of top-mass measurements at the LHC using NLO+PS generators of increasing accuracy
Authors:
Silvia Ferrario Ravasio,
Tomas Jezo,
Paolo Nason,
Carlo Oleari
Abstract:
In this paper we study the theoretical uncertainties in the determination of the top-quark mass using next-to-leading-order (NLO) generators interfaced to parton showers (PS) that have different levels of accuracy. Specifically we consider three generators: one that implements NLO corrections in the production dynamics, one that includes also NLO corrections in top decay in the narrow width approx…
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In this paper we study the theoretical uncertainties in the determination of the top-quark mass using next-to-leading-order (NLO) generators interfaced to parton showers (PS) that have different levels of accuracy. Specifically we consider three generators: one that implements NLO corrections in the production dynamics, one that includes also NLO corrections in top decay in the narrow width approximation, and one that implements NLO corrections for both production and decay including finite-width and interference effects. Since our aim is to provide an assessment of the uncertainties of purely theoretical origin, we consider simplified top-mass related observables that are broadly related to those effectively used by experiments, eventually modelling experimental resolution effects with simple smearing procedures. We estimate the differences in the value of the extracted top mass that would occur due to the use of the three different NLO generators, to the variation of scales, to the choice of parton distribution functions and to the matching procedure. Furthermore, we also consider differences due to the shower and to the modelling of non-perturbative effects by interfacing our NLO generators to both Pythia8.2 and Herwig7.1, with various settings. We find very different results depending upon the adopted shower model. While with Pythia8.2 we find moderate differences between the different NLO+PS generators, with Herwig7.1 we find very large ones. Furthermore, the differences between Pythia8.2 and Herwig7.1 generators are also remarkably large.
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Submitted 3 July, 2018; v1 submitted 11 January, 2018;
originally announced January 2018.
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The Top Mass in Hadronic Collisions
Authors:
Paolo Nason
Abstract:
I discuss theoretical issues related to the top mass measurements in hadronic collisions.
I discuss theoretical issues related to the top mass measurements in hadronic collisions.
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Submitted 19 January, 2018; v1 submitted 7 December, 2017;
originally announced December 2017.
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Heavy quark radiation in NLO+PS POWHEG generators
Authors:
L. Buonocore,
P. Nason,
F. Tramontano
Abstract:
In this paper we deal with radiation from heavy quarks in the context of next-to-leading order calculations matched to parton shower generators. A new algorithm for radiation from massive quarks is presented that has considerable advantages over the one previously employed. We implement the algorithm in the framework of the ${\tt POWHEG-BOX}$, and compare it with the previous one in the case of th…
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In this paper we deal with radiation from heavy quarks in the context of next-to-leading order calculations matched to parton shower generators. A new algorithm for radiation from massive quarks is presented that has considerable advantages over the one previously employed. We implement the algorithm in the framework of the ${\tt POWHEG-BOX}$, and compare it with the previous one in the case of the ${\tt hvq}$ generator for bottom production in hadronic collisions, and in the case of the ${\tt bb4l}$ generator for top production and decay.
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Submitted 9 February, 2018; v1 submitted 16 November, 2017;
originally announced November 2017.
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The Photon Content of the Proton
Authors:
Aneesh V. Manohar,
Paolo Nason,
Gavin P. Salam,
Giulia Zanderighi
Abstract:
The photon PDF of the proton is needed for precision comparisons of LHC cross sections with theoretical predictions. In a recent paper, we showed how the photon PDF could be determined in terms of the electromagnetic proton structure functions $F_2$ and $F_L$ measured in electron-proton scattering experiments, and gave an explicit formula for the PDF including all terms up to next-to-leading order…
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The photon PDF of the proton is needed for precision comparisons of LHC cross sections with theoretical predictions. In a recent paper, we showed how the photon PDF could be determined in terms of the electromagnetic proton structure functions $F_2$ and $F_L$ measured in electron-proton scattering experiments, and gave an explicit formula for the PDF including all terms up to next-to-leading order. In this paper we give details of the derivation. We obtain the photon PDF using the factorisation theorem and applying it to suitable BSM hard scattering processes. We also obtain the same PDF in a process-independent manner using the usual definition of PDFs in terms of light-cone Fourier transforms of products of operators. We show how our method gives an exact representation for the photon PDF in terms of $F_2$ and $F_L$, valid to all orders in QED and QCD, and including all non-perturbative corrections. This representation is then used to give an explicit formula for the photon PDF to one order higher than our previous result. We also generalise our results to obtain formulæ for the polarised photon PDF, as well as the photon TMDPDF. Using our formula, we derive the $P_{γi}$ subset of DGLAP splitting functions to order $αα_s$ and $α^2$, which agree with known results. We give a detailed explanation of the approach that we follow to determine a photon PDF and its uncertainty within the above framework.
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Submitted 18 August, 2017; v1 submitted 3 August, 2017;
originally announced August 2017.
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An NLO+PS generator for $t\bar{t}$ and $Wt$ production and decay including non-resonant and interference effects
Authors:
Tomáš Ježo,
Jonas M. Lindert,
Paolo Nason,
Carlo Oleari,
Stefano Pozzorini
Abstract:
We present a Monte Carlo generator that implements significant theoretical improvements in the simulation of top-quark pair production and decay at the LHC. Spin correlations and off-shell effects in top-decay chains are described in terms of exact matrix elements for $p p \to \ell^+ν_{\scriptscriptstyle\ell}\, l^-\barν_{\scriptscriptstyle l} b \bar{b}$ at NLO QCD, where the leptons $\ell$ and…
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We present a Monte Carlo generator that implements significant theoretical improvements in the simulation of top-quark pair production and decay at the LHC. Spin correlations and off-shell effects in top-decay chains are described in terms of exact matrix elements for $p p \to \ell^+ν_{\scriptscriptstyle\ell}\, l^-\barν_{\scriptscriptstyle l} b \bar{b}$ at NLO QCD, where the leptons $\ell$ and $l$ belong to different families, and $b$ quarks are massive. Thus, the contributions from $t\bar{t}$ and $Wt$ single-top production as well as their quantum interference are fully included. Matrix elements are matched to the Pythia8 parton shower using a recently proposed method that allows for a consistent treatment of resonances in the POWHEG framework. These theoretical improvements are especially important for the interpretation of precision measurements of the top-quark mass, for single-top analyses in the $Wt$ channel, and for $t\bar{t}$ and $Wt$ backgrounds in the presence of jet vetoes or cuts that enhance off-shell effects. The new generator is based on a process-independent interface of the OpenLoops amplitude generator with the POWHEGBOX framework.
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Submitted 15 July, 2016;
originally announced July 2016.
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How bright is the proton? A precise determination of the photon parton distribution function
Authors:
Aneesh Manohar,
Paolo Nason,
Gavin P. Salam,
Giulia Zanderighi
Abstract:
It has become apparent in recent years that it is important, notably for a range of physics studies at the Large Hadron Collider, to have accurate knowledge on the distribution of photons in the proton. We show how the photon parton distribution function (PDF) can be determined in a model-independent manner, using electron-proton ($ep$) scattering data, in effect viewing the $ep\to e+X$ process as…
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It has become apparent in recent years that it is important, notably for a range of physics studies at the Large Hadron Collider, to have accurate knowledge on the distribution of photons in the proton. We show how the photon parton distribution function (PDF) can be determined in a model-independent manner, using electron-proton ($ep$) scattering data, in effect viewing the $ep\to e+X$ process as an electron scattering off the photon field of the proton. To this end, we consider an imaginary, beyond Standard Model process with a flavour changing photon-lepton vertex. We write its cross section in two ways, one in terms of proton structure functions, the other in terms of a photon distribution. Requiring their equivalence yields the photon distribution as an integral over proton structure functions. As a result of the good precision of $ep$ data, we constrain the photon PDF at the level of 1-2% over a wide range of momentum fractions.
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Submitted 16 December, 2016; v1 submitted 14 July, 2016;
originally announced July 2016.
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Physics at a 100 TeV pp collider: Standard Model processes
Authors:
M. L. Mangano,
G. Zanderighi,
J. A. Aguilar Saavedra,
S. Alekhin,
S. Badger,
C. W. Bauer,
T. Becher,
V. Bertone,
M. Bonvini,
S. Boselli,
E. Bothmann,
R. Boughezal,
M. Cacciari,
C. M. Carloni Calame,
F. Caola,
J. M. Campbell,
S. Carrazza,
M. Chiesa,
L. Cieri,
F. Cimaglia,
F. Febres Cordero,
P. Ferrarese,
D. D'Enterria,
G. Ferrera,
X. Garcia i Tormo
, et al. (51 additional authors not shown)
Abstract:
This report summarises the properties of Standard Model processes at the 100 TeV pp collider. We document the production rates and typical distributions for a number of benchmark Standard Model processes, and discuss new dynamical phenomena arising at the highest energies available at this collider. We discuss the intrinsic physics interest in the measurement of these Standard Model processes, as…
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This report summarises the properties of Standard Model processes at the 100 TeV pp collider. We document the production rates and typical distributions for a number of benchmark Standard Model processes, and discuss new dynamical phenomena arising at the highest energies available at this collider. We discuss the intrinsic physics interest in the measurement of these Standard Model processes, as well as their role as backgrounds for New Physics searches.
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Submitted 6 July, 2016;
originally announced July 2016.
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Precision Studies of Observables in pp->W->l nu and pp->gamma,Z->l+l- processes at the LHC
Authors:
S. Alioli,
A. B. Arbuzov,
D. Yu. Bardin,
L. Barze,
C. Bernaciak,
S. G. Bondarenko,
C. Carloni Calame,
M. Chiesa,
S. Dittmaier,
G. Ferrera,
D. de Florian,
M. Grazzini,
S. Hoeche,
A. Huss,
S. Jadach,
L. V. Kalinovskaya,
A. Karlberg,
F. Krauss,
Y. Li,
H. Martinez,
G. Montagna,
A. Mueck,
P. Nason,
O. Nicrosini,
F. Petriello
, et al. (11 additional authors not shown)
Abstract:
This report was prepared in the context of the LPCC "Electroweak Precision Measurements at the LHC WG" and summarizes the activity of a subgroup dedicated to the systematic comparison of public Monte Carlo codes, which describe the Drell-Yan processes at hadron colliders, in particular at the CERN Large Hadron Collider (LHC). This work represents an important step towards the definition of an accu…
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This report was prepared in the context of the LPCC "Electroweak Precision Measurements at the LHC WG" and summarizes the activity of a subgroup dedicated to the systematic comparison of public Monte Carlo codes, which describe the Drell-Yan processes at hadron colliders, in particular at the CERN Large Hadron Collider (LHC). This work represents an important step towards the definition of an accurate simulation framework necessary for very high-precision measurements of electroweak (EW) observables such as the $W$ boson mass and the weak mixing angle. All the codes considered in this report share at least next-to-leading-order (NLO) accuracy in the prediction of the total cross sections in an expansion either in the strong or in the EW coupling constant. The NLO fixed-order predictions have been scrutinized at the technical level, using exactly the same inputs, setup and perturbative accuracy, in order to quantify the level of agreement of different implementations of the same calculation. A dedicated comparison, again at the technical level, of three codes that reach next-to-next-to-leading-order (NNLO) accuracy in quantum chromodynamics (QCD) for the total cross section has also been performed. These fixed-order results are a well-defined reference that allows a classification of the impact of higher-order sets of radiative corrections. Several examples of higher-order effects due to the strong or the EW interaction are discussed in this common framework. Also the combination of QCD and EW corrections is discussed, together with the ambiguities that affect the final result, due to the choice of a specific combination recipe.
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Submitted 7 June, 2016;
originally announced June 2016.
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On the ultimate uncertainty of the top quark pole mass
Authors:
M. Beneke,
P. Marquard,
P. Nason,
M. Steinhauser
Abstract:
We combine the known asymptotic behaviour of the QCD perturbation series expansion, which relates the pole mass of a heavy quark to the MSbar mass, with the exact series coefficients up to the four-loop order to determine the ultimate uncertainty of the top-quark pole mass due to the renormalon divergence. We perform extensive tests of our procedure by varying the number of colours and flavours, a…
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We combine the known asymptotic behaviour of the QCD perturbation series expansion, which relates the pole mass of a heavy quark to the MSbar mass, with the exact series coefficients up to the four-loop order to determine the ultimate uncertainty of the top-quark pole mass due to the renormalon divergence. We perform extensive tests of our procedure by varying the number of colours and flavours, as well as the scale of the strong coupling and the MSbar mass. Including an estimate of the internal bottom and charm quark mass effect, we conclude that this uncertainty is around 110 MeV. We further estimate the additional contribution to the mass relation from the five-loop correction and beyond to be around 300 MeV.
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Submitted 9 June, 2017; v1 submitted 11 May, 2016;
originally announced May 2016.