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Isospin-symmetry violation -- kaons and beyond (ISO-BREAK 25: summary and outlook)
Authors:
Marek Gazdzicki,
Francesco Giacosa,
Katarzyna Grebieszkow,
David Blaschke,
Marcus Bleicher,
Bastian Brandt,
Wojciech Brylinski,
Tobiasz Czopowicz,
Jim Drachenberg,
Dipangkar Dutta,
Francesca Ercolessi,
Mark Gorenstein,
Linqin Huang,
Oleksii Ivanytskyi,
Nicolo Jacazio,
Joseph Kapusta,
Seweryn Kowalski,
Maciej Piotr Lewicki,
Manuel Lorenz,
Stanislaw Mrowczynski,
Vitalii Ozvenchuk,
Oleksandra Panova,
Roman Planeta,
Krzysztof Piasecki,
Milena Piotrowska
, et al. (18 additional authors not shown)
Abstract:
This report summarizes the presentations and discussions during the ISO-BREAK 25 Workshop ``Isospin symmetry violation: kaons and beyond'', which was held at Jan Kochanowski University in Kielce on October 23-25, 2025. We address the current status of the isospin-symmetry breaking discovered by NA61/SHINE in nucleus-nucleus collisions at the CERN SPS, its confirmation by other experiments and stud…
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This report summarizes the presentations and discussions during the ISO-BREAK 25 Workshop ``Isospin symmetry violation: kaons and beyond'', which was held at Jan Kochanowski University in Kielce on October 23-25, 2025. We address the current status of the isospin-symmetry breaking discovered by NA61/SHINE in nucleus-nucleus collisions at the CERN SPS, its confirmation by other experiments and studies in \ee and deep inelastic scattering. In addition, we discuss the theoretical status as well as we outline experimental and theoretical priorities towards understanding this currently unexplained phenomenon.
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Submitted 17 April, 2026;
originally announced April 2026.
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Hadron Physics Opportunities at FAIR
Authors:
J. G. Messchendorp,
F. Nerling,
P. Achenbach,
J. Aichelin,
M. Albaladejo,
L. An,
K. Aoki,
G. Appagere,
V. Baru,
M. Bashkanov,
A. Bauswein,
A. Belias,
J. Bernhard,
P. P. Bhaduri,
Ł. Bibrzycki,
D. Blaschke,
M. Bleicher,
C. Blume,
S. Bolognesi,
N. Brambilla,
E. Bratkovskaya,
I. Ciepał,
S. Collins,
V. Crede,
R. Das
, et al. (81 additional authors not shown)
Abstract:
This White Paper outlines a coordinated, decade-spanning programme of hadron and QCD studies anchored at the GSI/FAIR accelerator complex. Profiting from intense deuteron, proton and pion beams coupled with high-rate capable detectors and an international theory effort, the initiative addresses fundamental questions related to the strong interaction featuring confinement and dynamical mass generat…
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This White Paper outlines a coordinated, decade-spanning programme of hadron and QCD studies anchored at the GSI/FAIR accelerator complex. Profiting from intense deuteron, proton and pion beams coupled with high-rate capable detectors and an international theory effort, the initiative addresses fundamental questions related to the strong interaction featuring confinement and dynamical mass generation. This includes our understanding of hadron-hadron interactions and the composition of hadrons through mapping the baryon and meson spectra, including exotic states, and quantifying hadron structure. This interdisciplinary research connects topics in the fields of nuclear, heavy-ion, and (nuclear) astro (particle) physics, linking, for example, terrestrial data to constraints on neutron star structure. A phased roadmap with SIS100 accelerator start-up and envisaged detector upgrades will yield precision cross sections, transition form factors, in-medium spectral functions, and validated theory inputs. Synergies with external programmes at international accelerator facilities worldwide are anticipated. The programme is expected to deliver decisive advances in our understanding of non-perturbative (strong) QCD and astrophysics, and high-rate detector and data-science technology.
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Submitted 22 February, 2026; v1 submitted 17 December, 2025;
originally announced December 2025.
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Isospin kaon anomaly and its consequences
Authors:
Francesco Giacosa,
Martin Rohrmoser
Abstract:
Isospin symmetry is well fulfilled in the QCD vacuum, as evidenced by small mass differences of isospin partners and suppressed isospin-violating decays. Recently, the NA61/SHINE collaboration reported an unexpectedly large isospin-violating charged-to-neutral kaon ratio in Ar-Sc heavy-ion collisions (HIC).Using a quark recombination approach, we introduce a function of kaon multiplicities that re…
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Isospin symmetry is well fulfilled in the QCD vacuum, as evidenced by small mass differences of isospin partners and suppressed isospin-violating decays. Recently, the NA61/SHINE collaboration reported an unexpectedly large isospin-violating charged-to-neutral kaon ratio in Ar-Sc heavy-ion collisions (HIC).Using a quark recombination approach, we introduce a function of kaon multiplicities that reduces to unity in the isospin-symmetric limit independently of the scattering energy and type of nuclei. Using this quantity, we show that nucleus-nucleus collisions violate isospin sizably (at the $6.4σ$--level), while proton-proton data on kaon multiplicities do not. We predict other isospin-violating enhancements in HIC, such as the proton-to-neutron ratio $p/n \sim 1.2$ and the hyperon ratio $Σ^{+}/Σ^{-}\sim1.4$. Finally, we extend the approach to antiquarks in the initial state, useful for e.g. pion-nucleus scattering reactions.
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Submitted 30 September, 2025; v1 submitted 2 April, 2025;
originally announced April 2025.
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Evidence of isospin-symmetry violation in high-energy collisions of atomic nuclei: Theoretical and Phenomenological considerations
Authors:
Wojciech Brylinski,
Marek Gazdzicki,
Francesco Giacosa,
Mark Gorenstein,
Roman Poberezhnyuk,
Subhasis Samanta
Abstract:
Recently, the NA61/SHINE collaboration at the CERN SPS reported evidence of isospin-symmetry violation in high-energy nuclear collisions [Nature Commun. 16, 2849 (2025)]. The effect was observed in the relative yields of charged and neutral kaons and cannot be explained by known sources of isospin symmetry breaking. In this work, we extend the theoretical and phenomenological aspects of that study…
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Recently, the NA61/SHINE collaboration at the CERN SPS reported evidence of isospin-symmetry violation in high-energy nuclear collisions [Nature Commun. 16, 2849 (2025)]. The effect was observed in the relative yields of charged and neutral kaons and cannot be explained by known sources of isospin symmetry breaking. In this work, we extend the theoretical and phenomenological aspects of that study. We discuss the historical background and introduce the concepts of isospin transformations and symmetry. Importantly, we relate isospin symmetry to the QCD flavor symmetry, and we present both conceptual and analytical proofs demonstrating the equality of the mean multiplicities of charged and neutral kaons for an initial ensemble of colliding systems that is invariant under charge-symmetry transformation.
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Submitted 2 March, 2026; v1 submitted 12 December, 2023;
originally announced December 2023.
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Evidence of isospin-symmetry violation in high-energy collisions of atomic nuclei
Authors:
NA61/SHINE Collaboration,
:,
H. Adhikary,
P. Adrich,
K. K. Allison,
N. Amin,
E. V. Andronov,
I. -C. Arsene,
M. Bajda,
Y. Balkova,
D. Battaglia,
A. Bazgir,
S. Bhosale,
M. Bielewicz,
A. Blondel,
M. Bogomilov,
Y. Bondar,
A. Brandin,
W. Brylinski,
J. Brzychczyk,
M. Buryakov,
A. F. Camino,
M. Cirkovic,
M. Csanád,
J. Cybowska
, et al. (133 additional authors not shown)
Abstract:
Strong interactions preserve an approximate isospin symmetry between up ($u$) and down ($d$) quarks, part of the more general flavor symmetry. In the case of $K$ meson production, if this isospin symmetry were exact, it would result in equal numbers of charged ($K^+$ and $K^-$) and neutral ($K^0$ and $\overline K^{\,0}$) mesons in the final state. Here, we report results on the relative abundance…
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Strong interactions preserve an approximate isospin symmetry between up ($u$) and down ($d$) quarks, part of the more general flavor symmetry. In the case of $K$ meson production, if this isospin symmetry were exact, it would result in equal numbers of charged ($K^+$ and $K^-$) and neutral ($K^0$ and $\overline K^{\,0}$) mesons in the final state. Here, we report results on the relative abundance of charged over neutral $K$ meson production in argon and scandium nuclei collisions at a center-of-mass energy of 11.9 GeV per nucleon pair. We find that the production of $K^+$ and $K^-$ mesons at mid-rapidity is $(18.4\pm 6.1)\%$ higher than that of the neutral $K$ mesons. Although with large uncertainties, earlier data on nucleus-nucleus collisions in the collision center-of-mass energy range $2.6 < \sqrt{s_{NN}} < 200$~\GeV are consistent with the present result. Using well-established models for hadron production, we demonstrate that known isospin-symmetry breaking effects and the initial nuclei containing more neutrons than protons lead only to a small (few percent) deviation of the charged-to-neutral kaon ratio from unity at high energies. Thus, they cannot explain the measurements. The significance of the flavor-symmetry violation beyond the known effects is 4.7$σ$ when the compilation of world data with uncertainties quoted by the experiments is used. New systematic, high-precision measurements and theoretical efforts are needed to establish the origin of the observed large isospin-symmetry breaking.
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Submitted 16 May, 2025; v1 submitted 11 December, 2023;
originally announced December 2023.
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Strong Interaction Physics at the Luminosity Frontier with 22 GeV Electrons at Jefferson Lab
Authors:
A. Accardi,
P. Achenbach,
D. Adhikari,
A. Afanasev,
C. S. Akondi,
N. Akopov,
M. Albaladejo,
H. Albataineh,
M. Albrecht,
B. Almeida-Zamora,
M. Amaryan,
D. Androić,
W. Armstrong,
D. S. Armstrong,
M. Arratia,
J. Arrington,
A. Asaturyan,
A. Austregesilo,
H. Avagyan,
T. Averett,
C. Ayerbe Gayoso,
A. Bacchetta,
A. B. Balantekin,
N. Baltzell,
L. Barion
, et al. (419 additional authors not shown)
Abstract:
This document presents the initial scientific case for upgrading the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab (JLab) to 22 GeV. It is the result of a community effort, incorporating insights from a series of workshops conducted between March 2022 and April 2023. With a track record of over 25 years in delivering the world's most intense and precise multi-GeV electron…
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This document presents the initial scientific case for upgrading the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab (JLab) to 22 GeV. It is the result of a community effort, incorporating insights from a series of workshops conducted between March 2022 and April 2023. With a track record of over 25 years in delivering the world's most intense and precise multi-GeV electron beams, CEBAF's potential for a higher energy upgrade presents a unique opportunity for an innovative nuclear physics program, which seamlessly integrates a rich historical background with a promising future. The proposed physics program encompass a diverse range of investigations centered around the nonperturbative dynamics inherent in hadron structure and the exploration of strongly interacting systems. It builds upon the exceptional capabilities of CEBAF in high-luminosity operations, the availability of existing or planned Hall equipment, and recent advancements in accelerator technology. The proposed program cover various scientific topics, including Hadron Spectroscopy, Partonic Structure and Spin, Hadronization and Transverse Momentum, Spatial Structure, Mechanical Properties, Form Factors and Emergent Hadron Mass, Hadron-Quark Transition, and Nuclear Dynamics at Extreme Conditions, as well as QCD Confinement and Fundamental Symmetries. Each topic highlights the key measurements achievable at a 22 GeV CEBAF accelerator. Furthermore, this document outlines the significant physics outcomes and unique aspects of these programs that distinguish them from other existing or planned facilities. In summary, this document provides an exciting rationale for the energy upgrade of CEBAF to 22 GeV, outlining the transformative scientific potential that lies within reach, and the remarkable opportunities it offers for advancing our understanding of hadron physics and related fundamental phenomena.
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Submitted 24 August, 2023; v1 submitted 13 June, 2023;
originally announced June 2023.
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Cancellation of the sigma meson in thermal models
Authors:
Wojciech Broniowski,
Francesco Giacosa,
Viktor Begun
Abstract:
The by now well-established scalar-isoscalar resonance $f_{0}(500)$ (the $σ$ meson) seems potentially relevant in the evaluation of thermodynamic quantities of a hadronic gas, since its mass is low. However, we recall that its contribution to isospin-averaged observables is, to a surprising accuracy, canceled by the repulsion from the pion-pion scalar-isotensor channel. As a result, in practice on…
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The by now well-established scalar-isoscalar resonance $f_{0}(500)$ (the $σ$ meson) seems potentially relevant in the evaluation of thermodynamic quantities of a hadronic gas, since its mass is low. However, we recall that its contribution to isospin-averaged observables is, to a surprising accuracy, canceled by the repulsion from the pion-pion scalar-isotensor channel. As a result, in practice one should not incorporate $f_0(500)$ in standard hadronic resonance-gas models for studies of isospin averaged quantities. In our analysis we use the formalism of the virial expansion, which allows one to calculate the thermal properties of an interacting hadron gas in terms of derivatives of the scattering phase shifts, hence in a model-independent way directly from experimentally accessible quantities. A similar cancellation mechanism occurs for the scalar kaonic interactions between the $I=1/2$ channel (containing the alleged $K_{0}^{\ast}(800)$ or the $κ$ meson) and the $I=3/2$ channel.
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Submitted 14 September, 2015; v1 submitted 3 June, 2015;
originally announced June 2015.