arXiv subjects

Huichao Song

Publications and source records attributed to Huichao Song.

At least 19 recordsLinked to original sources

arXiv2026

PRBench: End-to-end Paper Reproduction in Physics Research

AI agents powered by large language models exhibit strong reasoning and problem-solving capabilities, enabling them to assist scientific research tasks such as formula derivation and code generation. However, whether these agents can reliably perform end-to-end reproduction from real scientific papers remains an open question. We introduce PRBench, a benchmark of 30 expert-curated tasks spanning 11 subfields of physics. Each task requires an agent to comprehend the methodology of a published paper, implement the corresponding algorithms from scratch, and produce quantitative results matching the original publication. Agents are provided only with the task instruction and paper content, and operate in a sandboxed execution environment. All tasks are contributed by domain experts from over 20 research groups at the School of Physics, Peking University, each grounded in a real published paper and validated through end-to-end reproduction with verified ground-truth results and detailed scoring rubrics. Using an agentified assessment pipeline, we evaluate a set of coding agents on PRBench and analyze their capabilities across key dimensions of scientific reasoning and execution. The best-performing agent, OpenAI Codex powered by GPT-5.3-Codex, achieves a mean overall score of 34%. All agents exhibit a zero end-to-end callback success rate, with particularly poor performance in data accuracy and code correctness. We further identify systematic failure modes, including errors in formula implementation, inability to debug numerical simulations, and fabrication of output data. Overall, PRBench provides a rigorous benchmark for evaluating progress toward autonomous scientific research.

cs.CL↗

arXiv2025

Violation of the elliptic flow scaling of $f_0(980)$ in p-Pb collisions at the LHC

We investigate the production and elliptic flow of the $f_0(980)$ in high-multiplicity p-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV using a hadronic coalescence model with the $K$ and $\bar K$ phase-space distributions provided by the Hydro-Coal-Frag hybrid model. Our results, which agree with the ALICE and CMS measurements, support the $K\bar K$ molecular interpretation of the $f_0(980)$ structure and show, however, a breakdown of the simple number-of-constituent (NC) scaling of its elliptic flow. The latter is in contrast to the deuteron elliptic flow, which exhibits a significantly better NC scaling when the same coalescence width parameter is used.

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arXiv2025

Probing the structure of $f_{0}(980)$ from the elliptic flow in p-Pb collisions at the LHC

The $f_{0}(980)$ is a light scalar meson whose internal structure remains under debate and investigation. Assuming that the $f_0(980)$ is a $K\bar K$ molecule that can only survive at the kinetic freeze-out of the evolving bulk matter, we implement the coalescence model to study its transverse momentum ($p_T$) spectra and elliptic flow ($v_2$) in high-multiplicity p-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV. Using the well-tuned kaon phase-space distributions from the Hydro-Coal-Frag model, our $K\bar{K}$ coalescence calculations with reasonable values for the $f_0(980)$ radius successfully reproduce the elliptic flow measured by CMS over the range $0 < p_{T} < 12$ GeV and also agree with the $p_T$-spectra from ALICE. These results in heavy ion collisions are consistent with the $K\bar K$ molecular picture of the $f_0(980)$. We also find that the number-of-constituent scaling of $v_2$ for the $f_0(980)$ is violated in p-Pb collisions at the LHC because most $f_0(980)$ are produced from the coalescence of kaons that have different momenta. Our study demonstrates the necessity of realistic coalescence model calculations and also explains why the CMS interpretation of the $f_0(980)$ as an ordinary $q\bar q$ meson is no longer valid by interpreting the measured $v_2$ with a simple scaling formula based on the assumption of equal momentum coalescence. The investigation also provides a novel way to explore the internal structure of light exotic hadrons that can be abundantly produced in relativistic heavy and/or light ion collisions.

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arXiv2025

Exploring the fluid behavior in p+p collisions at $\sqrt{s}=13 \mathrm{TeV}$ with viscous anisotropic hydrodynamics

The applicability of hydrodynamics in small collision systems remains controversial due to the small size and short lifetime of the system. In this letter, we employ viscous anisotropic hydrodynamics (VAH), which incorporates large pressure anisotropies, to study the collectivity in p+p collisions at $\sqrt{s}=13 \mathrm{TeV}$.VAH provides a good description for $v_{2}\{2\}$ and $v_{3}\{2\}$ over a wide range of multiplicities and correctly reproduces the experimentally observed negative $c_{2}\{4\}$. Traditional second-order viscous hydrodynamics (VH), on the other hand, can describe the measurements, in particular the negative $c_{2}\{4\}$, only with model parameters for which the bulk of the evolution is characterized by large values of the shear Knudsen number. It also can not capture the large longitudinal/transverse pressure anisotropy during the early evolution. These demonstrate the failure of traditional viscous hydrodynamics in small collision systems and establish viscous anisotropic hydrodynamics as a more reliable framework to describe the bulk evolution and the observed anisotropic flow in p-p collisions at the LHC.

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arXiv2025

Extended applicability domain of viscous anisotropic hydrodynamics in (2+1)-D Bjorken flow with transverse expansion

We perform (2+1)-D simulations of viscous anisotropic hydrodynamics (VAH) under boost-invariant and conformal conditions. Comparing both VAH and traditional viscous hydrodynamics with kinetic theory in the relaxation-time approximation as the underlying microscopic theory, we show that VAH provides a superior description of the evolution across a wide range of opacity, effectively extending the applicability of hydrodynamic modeling. Our results demonstrate VAH's potential for describing collective flow in small systems where traditional hydrodynamics faces challenges.

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arXiv2025

Nuclear Physics Confronts Relativistic Collisions Of Isobars

High-energy collisions involving the $A=96$ isobars $^{96}$Zr and $^{96}$Ru have been performed in 2018 at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) as a means to search for the chiral magnetic effect in QCD. This would manifest itself as specific deviations from unity in the ratio of observables taken between $^{96}$Zr+$^{96}$Zr and $^{96}$Ru+$^{96}$Ru collisions. Measurements of such ratios (released at the end of 2021) indeed reveal deviations from unity, but these are primarily caused by the two collided isobars having different radial profiles and intrinsic deformations. To make progress in understanding RHIC data, nuclear physicists across the energy spectrum gathered in Heidelberg in 2022 as part of an EMMI Rapid Reaction Task Force (RRTF) to address the following question. Does the combined effort of low-energy nuclear structure physics and high-energy heavy-ion physics enable us to understand the observations made in isobar collisions at RHIC?

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arXiv2025

A "breathing'' octupole $^{208}$Pb nucleus: resolving the elliptical-to-triangular azimuthal anisotropy puzzle in ultracentral relativistic heavy ion collisions

Relativistic heavy ion collisions provide a unique opportunity to probe the nuclear structure by taking an instantaneous snapshot of the colliding nuclei and converting it into momentum anisotropies of final emitted hadrons. A long-standing puzzle of too large a ratio of the elliptical-to-triangular ($v_{2}$-to-$v_{3}$) anisotropies in ultracentral $^{208}$Pb+$^{208}$Pb collisions at the Large Hadron Collider(LHC) cannot be solved simply by hydrodynamic simulations with initial conditions containing the spherical or certain deformed shape of $^{208}$Pb. In this Letter, using the iEBE-VISHNU relativistic viscous hydrodynamic hybrid model simulations with the Trento initial condition, we show that a dynamic octupole deformation--a shape-breathing of $^{208}$Pb --could potentially solve the $v_{2}$-to-$v_{3}$ puzzle and simultaneously describe the $v_3\{4\}$ data measured in experiment. Our results highlight the unique capability of capturing transient collective properties of nuclei on yoctosecond ($10^{-24}$~s) timescales, unfeasible with low-energy nuclear reactions.

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arXiv2024

Exploring the Nuclear Shape Phase Transition in Ultra-Relativistic $^{129}$Xe+$^{129}$Xe Collisions at the LHC

The shape phase transition for certain isotope or isotone chains, associated with the quantum phase transition of finite nuclei, is an intriguing phenomenon in nuclear physics. A notable case is the Xe isotope chain, where the structure transits from a $γ$-soft rotor to a spherical vibrator, with the second-order shape phase transition occurring in the vicinity of $^{128-130}$Xe. In this letter, we focus on investigating the $γ$-soft deformation of $^{129}$Xe associated with the second-order shape phase transition by constructing novel correlators for ultra-relativistic $^{129}$Xe+$^{129}$Xe collisions. In particular, our iEBE-VISHNU model calculations show that the $v_2^2-[p_T]$ correlation $ρ_{2}$ and the mean transverse momentum fluctuation $Γ_{p_T}$, which were previously interpreted as the evidence for the rigid triaxial deformation of $^{129}$Xe, can also be well explained by the $γ$-soft deformation of $^{129}$Xe. We also propose two novel correlators $ρ_{4,2}$ and $ρ_{2,4}$, which carry non-trivial higher-order correlations and show unique capabilities to distinguish between the $γ$-soft and the rigid triaxial deformation of $^{129}$Xe in $^{129}$Xe+$^{129}$Xe collisions at the LHC. The present study also provides a novel way to explore the second-order shape phase transition of finite nuclei with ultra-relativistic heavy ion collisions.

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arXiv2024

The spin alignment of vector mesons with light front quarks

The global spin alignment of the vector meson has been observed in relativistic heavy ion collisions, but is still on hot debates in the theoretical community. Here we propose to apply the light front framework to explain this phenomenon since the light front form explicitly describes the hadron spin including both the quark spin and the orbital angular momentum. After applying the light front spinor, we find that the spin alignment in the polarization of vector mesons with $ρ_{00}>1/3$ can be naturally manifested and in particular, the obtained spin alignment for $ϕ$ meson is in good agreement with the experimental data. This implies that to explain the spin alignment it is important to properly include the contribution from the gluon interactions that are presented in terms of the orbital angular momentum of the hadron bound state.

hep-ph↗

arXiv2024

Exploring the compactness of $α$ cluster in $^{16}$O nuclei with relativistic $^{16}$O+$^{16}$O collisions

Probing the $α$ cluster of $^{16}$O with the relativistic $^{16}$O+$^{16}$O collisions has raised great interest in the heavy ion community. However, the effects of the $α$ cluster on the soft hadron observables vary largely among different studies. In this paper, we explain the differences by the compactness of the $α$ cluster in oxygen, using iEBE-VISHNU hydrodynamic simulations with different initial state $α$ cluster configurations. We also find several observables, such as the intensive skewness of the $[p_{\rm T}]$ correlator $Γ_{p_{\rm T}}$, the harmonic flows $v_2\{2\}$, $v_2\{4\}$, $v_3\{2\}$, and the $v_n^2-δ[p_{\rm T}]$ correlations $ρ(v_{2}^{2}, [p_{\rm T}])$, $ρ(v_{3}^{2}, [p_{\rm T}])$ in $^{16}$O+$^{16}$O collisions are sensitive to the compactness of the $α$ cluster in the colliding nuclei, which can be used to constrain the configurations of $^{16}$O in the future. Our study serves as an important step toward the quantitative exploration of the $α$ cluster configuration in the light nuclei with relativistic heavy ion collisions.

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arXiv2024

Determining the neutron skin thickness by relativistic semi-isobaric collisions

The neutron skin thickness of the benchmark nucleus $^{208}$Pb is crucial for our understanding of the equation of state of nuclear matter. In this paper, we discuss the effect of the neutron skin on the flow ratio observables in the semi-isobaric collisions $^{208}$Pb+$^{208}$Pb and $^{197}$Au+$^{197}$Au using iEBE-VISHNU hydrodynamic simulations. Our results suggest that $^{208}$Pb and $^{197}$Au should have the same magnitude of neutron skin thickness to describe the anisotropic flow ratios between the semi-isobaric systems. Our method provides an unconventional way to determine the neutron skin with the existing relativistic heavy ion collision data.

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arXiv2024

Exploring the partonic collectivity in small systems at the LHC

Using the Hydro-Coal-Frag model that combines hydrodynamics at low $p_{\rm T}$, quark coalescence at intermediate $p_{\rm T}$, and the LBT transport model at high $p_{\rm T}$, we study the spectra and elliptic flow of identified hadrons in high multiplicity p--Pb and p--p collisions at the Large Hadron Collider (LHC). In p--Pb collisions, the Hydro-Coal-Frag model gives a good description of the differential elliptic flow over the $p_{\rm T}$ range from 0 to 6 GeV and the approximate number of constituent quark (NCQ) scaling at intermediate $p_{\rm T}$. Although Hydro-Coal-Frag model can also roughly describe the elliptic flow in high multiplicity p--p collisions with the quark coalescence process, the larger contribution from the string fragmentations leads to a notable violation of the NCQ scaling of $v_2$ at intermediate $p_{\rm T}$ as observed in the experiment. Comparison runs of the Hydro-Frag model without the coalescence process demonstrate that regardless the parameter adjustments, the Hydro-Frag model cannot simultaneously describe the $p_{\rm T}$ spectra and the elliptic flow of identified hadrons in either p--Pb collisions or p--p collisions. The calculations in this paper thus provide support for the existence of partonic degrees of freedom and the possible formation of the QGP in the small systems created at the LHC.

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arXiv2023

Dynamical critical fluctuations near the QCD critical point with hydrodynamic cooling rate

Within the model A in the Hohenberg's dynamical universality classification, we investigate the critical slowing down effects on the critical fluctuations driven by the expanding quark-gluon plasma, using a trajectory and cooling rate obtained from hydrodynamics. We numerically solved the Langevin dynamics of the non-conserved order parameter field and find that, compared with commonly used Hubble-like expansion, the cooling rate of a realistic hydrodynamic system is pretty large and the associated critical slowing down effects strongly suppress the higher-order cumulants of the order parameter field ({\it e.g.,} $C_4$). Furthermore, for an evolving system that approaches the critical point, such critical slowing down suppression overcomes the enhancement of the critical fluctuations, which indicates that the largest fluctuations of the order parameter field ({\it i.e.,} $C_2$) do not necessarily associate with the evolving trajectory closest to the critical point.

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arXiv2023

Collective flow and the fluid behavior in p/d/$^3$He+Au collisions at $\sqrt{s_{NN}} = 200$ GeV

By varying the intrinsic initial geometry, the p/d/$^3$He+Au collisions at the Relativistic Heavy Ion Collider (RHIC) provide a unique opportunity to understand the collective behavior and probe the possible sub-nucleon fluctuations in small systems. In this paper, we employ the hybrid model iEBE-VISHNU with Trento initial conditions to study the collective flow and the fluid behavior in p/d/$^3$He+Au collisions. With fine-tuned parameters, iEBE-VISHNU can describe the $v_2(p_T)$ and $v_3(p_T)$ data from the PHENIX and STAR collaborations. However, for some certain parameter sets with initial sub-nucleon fluctuation, the hydrodynamic simulations have already beyond their limits with the average Knudsen number $\langle K_n \rangle$ obviously larger than one. Our calculations demonstrate that, for a meaningful evaluation of the fluid behavior in the small systems, model simulations should also pay attention to the validity range of hydrodynamics.

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arXiv2023

Probing the nuclear deformation with three-particle asymmetric cumulant in RHIC isobar runs

$^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr collisions at $\sqrt{s_{_{\rm NN}}}=200$ GeV provide unique opportunities to study the geometry and fluctuations raised from the deformation of the colliding nuclei. Using iEBE-VISHNU hybrid model, we predict ${\rm ac}_{2}\{3\}$ ratios between these two collision systems and demonstrate that the ratios of ${\rm ac}_{2}\{3\}$, as well as the ratios of the involving flow harmonics and event-plane correlations, are sensitive to quadrupole and octupole deformations, which could provide strong constrains on the shape differences between $^{96}$Ru and $^{96}$Zr. We also study the nonlinear response coefficients $χ_{4,22}$, which show insensitivity to the deformation effect.

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arXiv2023

High energy nuclear physics meets Machine Learning

Though being seemingly disparate and with relatively new intersection, high energy nuclear physics and machine learning have already begun to merge and yield interesting results during the last few years. It's worthy to raise the profile of utilizing this novel mindset from machine learning in high energy nuclear physics, to help more interested readers see the breadth of activities around this intersection. The aim of this mini-review is to introduce to the community the current status and report an overview of applying machine learning for high energy nuclear physics, to present from different aspects and examples how scientific questions involved in high energy nuclear physics can be tackled using machine learning.

hep-ph↗

arXiv2022

Scales in light-nuclei production near the QCD critical point

Based on the coalescence model, we analyse the light-nuclei production near the critical point by expanding the phase-space distribution function $f(\mathbf{r},\mathbf{p})$ in terms of the phase-space cumulants $\sim \langle r^m p^m\rangle_c$. We show that the dominant contribution of the phase-space distribution to the yield of light nuclei is determined by the second-order phase-space cumulants. Here, we identify the fireball size, the homogeneity length, and the effective temperature, which are encoded in the second-order phase-space cumulants, as the relevant scales in explaining the yield of light nuclei. These scales are typically much larger than the correlation length of the critical fluctuations created in the rapid expansion of the heavy-ion systems, so we need to eliminate this dominant contribution of the relevant scales in order to isolate the critical contribution from the yield of light nuclei. We find that the second-order phase-space cumulants appeared in the yields of light-nuclei with different mass numbers share a similar structure. This property allows us to construct ratios of light-nuclei yields in appropriate combinations so that the effect of the relevant scales of the light-nuclei yield cancels, which isolates the critical effects.

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arXiv2022

Examination of background effects on light-nuclei yield ratio in relativistic heavy-ion collisions

The light-nuclei yield ratio is one of the candidates to probe the critical fluctuations of hot QCD matter. In this paper, we investigate the \textit{background effects}, namely the non-critical effects coming from the non-trivial thermal background, on the light-nuclei production within the framework of the coalescence model. Specifically, we analyze the impact of the equilibrium phase-space distribution function of nucleons, $f(\mathbf{r},\mathbf{p})$, on the light-nuclei yield ratio $N_tN_p/N_d^2$, where $N_t$, $N_p$, and $N_d$ denote triton, proton, and deuteron yields. By considering the characteristic function of the phase-space distribution, we systematically expand the yield of light nuclei of $A$-constituent nucleons, $N_A$, in terms of the \textit{phase-space cumulants}, $\langle\mathbf{r}^n\mathbf{p}^m\rangle_c$. We find that the cumulants up to the second-order are canceled out in the generalized ratio $N_p^{B-A} N_B^{A-1}/N_A^{B-1}$. This means that the dominant background effects including the fireball size, the kinetic freeze-out temperature, and the coordinate--momentum correlations caused by the radial expansion play an insignificant role in the yield ratio, which supports the yield ratio as a useful tool for the critical-point search. We also show several examples of background phase-space distributions for the qualitative illustration. The higher-order cumulants, which correspond to the non-Gaussian shape of the phase-space profile, play an important role in the variation of the yield ratio particularly for the smaller fireball sizes. Qualitatively, the spatial structure of the background decreases the yield ratio, and the azimuthal anisotropy $v_n$ increases it. The higher order of the azimuthal anisotropy causes a larger effect on the yield ratio. These results call for the comprehensive future studies of the yield ratio using sophisticated dynamical models.

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