Nuclear Theory·nucl-th

Wednesday·January 7, 2026

5 papers—2 primary·3 cross-listed

01
[Submitted on 6 Jan 2026]
Collision energy and system size dependence of longitudinal flow decorrelation in heavy-ion collisions at RHIC energies
Gaoguo Yan🇨🇳 · Maowu Nie🇨🇳 · Zhenyu Chen🇨🇳 · Li Yi🇨🇳 · Jiangyong Jia🇨🇳
In heavy-ion collisions, the initial collision geometry and its fluctuations drive the collective expansion of final-state hadrons in the transverse plane. However, longitudinal fluctuations induce event-plane twist and flow magnitude asymmetries, collectively known as longitudinal flow decorrelation. Using a multi-phase transport (AMPT) model, we systematically investigate the dependence of collision energy and system size of this phenomenon with Au+Au collisions at $s_{NN}$ = 19.6, 27, 54.4, 200 GeV and isobar collisions (Zr+Zr and Ru+Ru) at $s_{NN}$ = 200 GeV. The results reveal two distinct decorrelation components: $r_{n} (η)$ , which includes flow magnitude asymmetry and event-plane twist, and $R_{n} (η)$ which arises purely from event-plane twist. Both $r_{n} (η)$ and $R_{n} (η)$ decrease linearly with $η$ and exhibit a significant dependence on collision energy and the size of the system. Through the slope parameters $F_{n}$ in the linear parametrization $r_{n} (η) = 1 - 2 F_{n} η$ , we can quantify the strength of decorrelation. We further observe that both $F_{2}$ and $F_{3}$ demonstrate a pronounced power-law scaling behavior with collision energy, following the relation $F_{n} \propto l o g s_{N N}$ . These results provide valuable insights into the three-dimensional modeling of the initial stage and the evolution of relativistic heavy-ion collisions.
Subjects:
Nuclear Theory (nucl-th)
arXiv:
2601.02691 [pdf]
1 citation
02
[Submitted on 6 Jan 2026]
An Effective Upper Bound on the Pressure-to-Energy Density Ratio in Neutron Stars
Bao-Jun Cai🇨🇳 · Bao-An Li🇺🇸 · Yu-Gang Ma🇨🇳
The equation-of-state (EOS) parameter $ϕ \equiv P / ε$ , defined as the ratio of pressure to energy density, encapsulates the fundamental response of matter under extreme compression. Its value at the center of the most massive neutron star (NS), $\x \equiv P_{c} / ε_{c}$ , provides an upper bound on the maximum attainable central EOS parameter of cold visible matter. Remarkably, owing to the intrinsically nonlinear structure of the EOS in General Relativity (GR), this bound lies far below the naive Special Relativity (SR) limit of unity. In this work, we refine the theoretical upper bound on $\x$ in a self-consistent manner by incorporating, in addition to the causality constraint from SR, the mass-sphere stability condition associated with the mass evolution pattern in the vicinity of the NS center. This condition is formulated within the intrinsic and perturbative analysis of the dimensionless Tolman--Oppenheimer--Volkoff equations (IPAD-TOV) framework. The combined constraints yield an improved bound, $\x ≲ 0.385$ , which is slightly above but fully consistent with the previously derived causal-only limit, $\x ≲ 0.374$ . We further derive an improved scaling relation for NS compactness and demonstrate its robustness across a broad set of 284 realistic EOSs, including models with first-order phase transitions, exotic degrees of freedom, continuous crossover behavior, and deconfined quark cores. Within the IPAD-TOV framework, the resulting bound on $\x$ provides a new EOS-insensitive probe of the microphysics of cold superdense matter compressed by strong-field gravity in GR.
Comments:
Version accepted by Phys. Rev. D
Subjects:
Nuclear Theory (nucl-th); High Energy Astrophysical Phenomena (astro-ph.HE); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics — Phenomenology (hep-ph); Nuclear Experiment (nucl-ex)
arXiv:
2601.02980 [pdf]
PRD(2026)·3 citations
03
[Submitted on 5 Jan 2026] (cross-list from hep-ph)
Helicity Dependent Distribution Functions of the Proton and $Λ$ and $Σ^{0}$ Baryons
Yang Yu🇨🇳 · Peng Cheng🇨🇳 · Hui-Yu Xing🇨🇳 · Daniele Binosi🇮🇹 · Craig D. Roberts🇨🇳
Using continuum Schwinger function methods, a coherent set of predictions for proton, $Λ$ and $Σ^{0}$ distribution functions (DFs) has been made available -- both helicity dependent and unpolarised. The results and comparisons between them reveal impacts of diquark correlations and SU $(3)$ -flavour symmetry breaking, some of which are highlighted in this contribution. For instance: in-proton ratios of helicity-dependent/unpolarised valence-quark DFs are presented; it is highlighted that, were it not for the presence of axialvector diquarks in the $Σ^{0}$ , the valence strange quark would carry none of the $Σ^{0}$ spin; and the sign and size of polarised gluon DFs is discussed -- at a scale typical of modern measurements, gluon partons carry roughly 40% of each octet baryon's spin.
Comments:
9 pages, 1 figure. Contribution to the proceedings of the 26th International Symposium on Spin Physics (SPIN25) -- A Century of Spin, 2025 September 22-26, Qingdao Haitian Hotel, Qingdao, Shandong Province, China
Subjects:
High Energy Physics — Phenomenology (hep-ph); High Energy Physics — Experiment (hep-ex); High Energy Physics — Lattice (hep-lat); Nuclear Experiment (nucl-ex); Nuclear Theory (nucl-th)
arXiv:
2601.02587 [pdf]
0 citations
04
[Submitted on 6 Jan 2026] (cross-list from gr-qc)
How Lorentz boosts reshape relaxation spectra
Lorenzo Gavassino🇬🇧
In relativity, relaxation processes are often assumed to undergo time dilation under Lorentz boosts. We show that this intuition fails generically. Due to relativity of simultaneity, Lorentz boosts can split a single relaxation mode into a continuum of excitations, with a width set by the maximal signal propagation speed. Focusing on linearized relativistic (kinetic or rheological) theories with an Onsager-type symmetry, we derive rigorous bounds on relaxation spectra in arbitrary inertial frames, expressed solely in terms of rest-frame spectral data at zero wavenumber. As a consequence, non-hydrodynamic gaps, maximal relaxation rates, and the convergence radii of hydrodynamic modes obey nontrivial Lorentz-covariant constraints. These results provide a unified framework for understanding how relativity constrains relaxation dynamics in many-body systems.
Comments:
6 pages and 2 figures (main text) + 6 pages and 3 figures (supplementary material), published in PRL (see https://journals.aps.org/prl/abstract/10.1103/d2xr-rj7g)
Subjects:
General Relativity and Quantum Cosmology (gr-qc); High Energy Physics — Theory (hep-th); Nuclear Theory (nucl-th)
arXiv:
2601.03081 [pdf]
PRL(2026)·11 citations
05
[Submitted on 6 Jan 2026] (cross-list from hep-ph)
Anatomy of Roper Resonance
Igor Strakovsky (GWU)🇺🇸
Sixty years ago, the first excited state of a proton/neutron was ``born.'' During this time, we learned a lot about it, specifically - how unique this case is: a single resonance with two pole positions on different Riemann sheets. Let me present a brief history to remind readers how development progressed. Sure, history is sometimes something that never happened, described by those who were never there...
Comments:
6 pages, 3 figures, 1 table; It is a great pleasure to dedicate this article to Dave Roper on his ninetieth birthday
Subjects:
High Energy Physics — Phenomenology (hep-ph); High Energy Physics — Experiment (hep-ex); Nuclear Experiment (nucl-ex); Nuclear Theory (nucl-th)
arXiv:
2601.03102 [pdf]
Acta Phys.Polon.B(2026)·0 citations

Affiliations

first authorsco-authorsvia INSPIRE

Collision energy and system size dependence of longitudinal flow decorrelation in heavy-ion collisions at RHIC energies

An Effective Upper Bound on the Pressure-to-Energy Density Ratio in Neutron Stars

Helicity Dependent Distribution Functions of the Proton and Λ and Σ0 Baryons

How Lorentz boosts reshape relaxation spectra

Anatomy of Roper Resonance

Affiliations

Helicity Dependent Distribution Functions of the Proton and $Λ$ and $Σ^{0}$ Baryons