nucl-th digest · Jan 7, 2026

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

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

[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

Wednesday·January 7, 2026

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

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