-
From identical S- and P-wave pT/M spectra to maximally distinct polarizations: probing NRQCD with chi states
Authors:
Pietro Faccioli,
Carlos Lourenco,
Mariana Araujo,
Joao Seixas,
Ilse Kratschmer,
Valentin Knunz
Abstract:
A global analysis of ATLAS and CMS measurements reveals that, at mid-rapidity, the directly-produced $χ_{c1}$, $χ_{c2}$ and J/$ψ$ mesons have differential cross sections of seemingly identical shapes, when presented as a function of the mass-rescaled transverse momentum, $p_{\rm T}/M$. This identity of kinematic behaviours among S- and P-wave quarkonia is certainly not a natural expectation of non…
▽ More
A global analysis of ATLAS and CMS measurements reveals that, at mid-rapidity, the directly-produced $χ_{c1}$, $χ_{c2}$ and J/$ψ$ mesons have differential cross sections of seemingly identical shapes, when presented as a function of the mass-rescaled transverse momentum, $p_{\rm T}/M$. This identity of kinematic behaviours among S- and P-wave quarkonia is certainly not a natural expectation of non-relativistic QCD (NRQCD), where each quarkonium state is supposed to reflect a specific family of elementary production processes, of significantly different $p_{\rm T}$-differential cross sections. Remarkably, accurate kinematic cancellations among the variegated NRQCD terms (colour singlets and octets) of its factorization expansion can lead to a surprisingly good description of the data. This peculiar tuning of the NRQCD mixtures leads to a clear prediction regarding the $χ_{c1}$ and $χ_{c2}$ polarizations, the only observables not yet measured: they should be almost maximally different from one another, and from the J/$ψ$ polarization, a striking exception in the global panorama of quarkonium production. Measurements of the difference between the $χ_{c1}$, $χ_{c2}$ and J/$ψ$ polarizations, complementing the observed identity of momentum dependences, represent a decisive probe of NRQCD.
△ Less
Submitted 4 February, 2018;
originally announced February 2018.
-
Quarkonium production at the LHC: a data-driven analysis of NRQCD's predictions
Authors:
Pietro Faccioli,
Mariana Araújo,
Valentin Knünz,
Ilse Krätschmer,
Carlos Lourenço,
João Seixas
Abstract:
While non-relativistic QCD (NRQCD) foresees a variety of elementary quarkonium production mechanisms naturally leading to state-dependent kinematic patterns, the LHC cross sections and polarization measurements reveal a remarkably simple production scenario, independent of the quantum numbers and masses of the quarkonia. Surprisingly, NRQCD is able to accommodate the observed universal scenario, t…
▽ More
While non-relativistic QCD (NRQCD) foresees a variety of elementary quarkonium production mechanisms naturally leading to state-dependent kinematic patterns, the LHC cross sections and polarization measurements reveal a remarkably simple production scenario, independent of the quantum numbers and masses of the quarkonia. Surprisingly, NRQCD is able to accommodate the observed universal scenario, through a series of conspiring cancellations smoothing out its otherwise variegated hierarchy of mechanisms. This seemingly unnatural solution implies that the $χ_{c1}$ and $χ_{c2}$ polarizations, not yet measured, are strong and opposite, representing the only potential exception to a remarkably simple picture of quarkonium production. The observation of a large difference between $χ_{c2}$ and $χ_{c1}$ polarizations, which cannot be indirectly extracted from existing measurements because they mutually cancel each other in their contribution to the observed J/$ψ$ production, would be a smoking gun signal finally proving the multifaceted but mysteriously elusive structure of NRQCD. On the other hand, the measurement of two similar, small polarizations will urge improved P-wave calculations, if not a substantial revision of the NRQCD hierarchies.
△ Less
Submitted 14 February, 2017;
originally announced February 2017.
-
Quarkonium production at the LHC: a phenomenological analysis of surprisingly simple data patterns
Authors:
Pietro Faccioli,
Mariana Araújo,
Valentin Knünz,
Ilse Krätschmer,
Carlos Lourenço,
João Seixas
Abstract:
The LHC quarkonium production measurements reveal a startling observation: the J/$ψ$, $ψ$(2S), $χ_{c1,2}$ and $Υ$(nS) $p_{\rm T}$-differential cross sections are compatible with one universal momentum scaling pattern. Considering also the absence of strong polarizations of directly and indirectly produced S-wave mesons, we are led to the conclusion that there is currently no evidence of a dependen…
▽ More
The LHC quarkonium production measurements reveal a startling observation: the J/$ψ$, $ψ$(2S), $χ_{c1,2}$ and $Υ$(nS) $p_{\rm T}$-differential cross sections are compatible with one universal momentum scaling pattern. Considering also the absence of strong polarizations of directly and indirectly produced S-wave mesons, we are led to the conclusion that there is currently no evidence of a dependence of the partonic production mechanisms on the quantum numbers and mass of the final state. The experimental observations supporting this universal production scenario are remarkably significant, as shown by a new analysis approach, unbiased by specific theoretical calculations of partonic cross sections, which are only considered a posteriori, in comparisons with the data-driven results.
△ Less
Submitted 14 February, 2017;
originally announced February 2017.
-
Observation of the rare $B^0_s\toμ^+μ^-$ decay from the combined analysis of CMS and LHCb data
Authors:
The CMS,
LHCb Collaborations,
:,
V. Khachatryan,
A. M. Sirunyan,
A. Tumasyan,
W. Adam,
T. Bergauer,
M. Dragicevic,
J. Erö,
M. Friedl,
R. Frühwirth,
V. M. Ghete,
C. Hartl,
N. Hörmann,
J. Hrubec,
M. Jeitler,
W. Kiesenhofer,
V. Knünz,
M. Krammer,
I. Krätschmer,
D. Liko,
I. Mikulec,
D. Rabady,
B. Rahbaran
, et al. (2807 additional authors not shown)
Abstract:
A joint measurement is presented of the branching fractions $B^0_s\toμ^+μ^-$ and $B^0\toμ^+μ^-$ in proton-proton collisions at the LHC by the CMS and LHCb experiments. The data samples were collected in 2011 at a centre-of-mass energy of 7 TeV, and in 2012 at 8 TeV. The combined analysis produces the first observation of the $B^0_s\toμ^+μ^-$ decay, with a statistical significance exceeding six sta…
▽ More
A joint measurement is presented of the branching fractions $B^0_s\toμ^+μ^-$ and $B^0\toμ^+μ^-$ in proton-proton collisions at the LHC by the CMS and LHCb experiments. The data samples were collected in 2011 at a centre-of-mass energy of 7 TeV, and in 2012 at 8 TeV. The combined analysis produces the first observation of the $B^0_s\toμ^+μ^-$ decay, with a statistical significance exceeding six standard deviations, and the best measurement of its branching fraction so far. Furthermore, evidence for the $B^0\toμ^+μ^-$ decay is obtained with a statistical significance of three standard deviations. The branching fraction measurements are statistically compatible with SM predictions and impose stringent constraints on several theories beyond the SM.
△ Less
Submitted 17 August, 2015; v1 submitted 17 November, 2014;
originally announced November 2014.
-
Quarkonium production in the LHC era: a polarized perspective
Authors:
Pietro Faccioli,
Valentin Knuenz,
Carlos Lourenco,
Joao Seixas,
Hermine K. Woehri
Abstract:
Polarization measurements are usually considered as the most difficult challenge for the QCD description of quarkonium production. In fact, global data fits for the determination of the non-perturbative parameters of bound-state formation traditionally exclude polarization observables and use them as a posteriori verifications of the predictions, with perplexing results. With a change of perspecti…
▽ More
Polarization measurements are usually considered as the most difficult challenge for the QCD description of quarkonium production. In fact, global data fits for the determination of the non-perturbative parameters of bound-state formation traditionally exclude polarization observables and use them as a posteriori verifications of the predictions, with perplexing results. With a change of perspective, we move polarization data to the centre of the study, advocating that they actually provide the strongest fundamental indications about the production mechanisms, even before we explicitly consider perturbative calculations.
Considering psi(2S) and Y(3S) measurements from LHC experiments and state-of-the-art NLO short-distance calculations in the framework of non-relativistic QCD factorization (NRQCD), we perform a search for a kinematic domain where the polarizations can be correctly reproduced together with the cross sections, by systematically scanning the phase space and accurately treating the experimental uncertainties. This strategy provides a straightforward solution to the "quarkonium polarization puzzle" and reassuring signs that the theoretical framework is reliable. At the same time, the results expose unexpected hierarchies in the non-perturbative NRQCD parameters, that open new paths towards the understanding of bound-state formation in QCD.
△ Less
Submitted 16 March, 2014;
originally announced March 2014.