arXiv subjects

Shuzhe Shi

Publications and source records attributed to Shuzhe Shi.

At least 19 recordsLinked to original sources

arXiv2026

Unveiling QCD Criticality with Cross-Rapidity Net-Baryon Cumulants

Fluctuations of conserved charges are a primary tool in the search for the QCD critical endpoint, but their beam-energy dependence is complicated by global baryon-number conservation, whose influence changes as the experimental acceptance covers different fractions of the collision system. We propose using correlations of net-baryon fluctuations between two separated rapidity windows to exploit the distinct signatures of conservation and critical dynamics. Global conservation produces a negative cross-window correlation, whereas a common long-wavelength critical fluctuation produces a positive one. We show that, within a canonical independent-source framework, the conservation-induced background and the critical signal enter additively at leading order, allowing the leading conservation term to be estimated and subtracted. The resulting yield-scaled correlator follows the nonmonotonic enhancement of an Ising-mapped equilibrium correlation length along a freeze-out trajectory passing near a hypothetical critical endpoint, while wider rapidity windows reduce the response through thermal smearing. Cross-rapidity cumulants therefore provide a rapidity-differential strategy for reducing the leading conservation background and sharpening fluctuation-based searches for QCD criticality in beam-energy-scan experiments.

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arXiv2026

Bottomonium production in an open quantum system approach with interactions from lattice quantum chromodynamic

Bottomonium production in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV is studied using a Lindblad master equation derived from potential non-relativistic quantum chromodynamics (QCD), where quantum regeneration of color-singlet states is matched to the lattice QCD imaginary potential via collapse operators. Two parametrizations of the in-medium heavy-quark potential, both constrained by lattice QCD data, are employed to compute the nuclear modification factors of $\Upsilon(1S)$, $\Upsilon(2S)$, and $\Upsilon(3S)$. The results show sensitivities to both the quantum regeneration effect and the initial condition of the density matrix. The dipole transitions in the collapse operators are found to significantly redistribute populations among different orbital angular momentum channels. It is shown that regeneration is more important when a potential with a larger imaginary part, i.e., stronger transitions between singlet and octet states, is used.

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arXiv2026

Applicability of kinetic theory in strongly coupled thermal quantum systems

In this work, we construct one-dimensional interacting lattice spinor theories with discretization in momentum space. We focus on strongly interacting Schwinger and Nambu--Jona-Lasinio models and perform ab-initio calculation of their single-particle and two-particle momentum distribution functions at finite temperature. We observe, at low temperature, high-momentum tail in single-particle and two particle distribution which reveals relative momentum in fermion-antifermion boundstates, as well as quasi-free spinor gases behavior at high temperature. The non-vanishing connected four-momentum function reveals the quantum coherence in momentum space under thermal equilibrium of the system and indicate the single particle correlation would remember more microscopic details within a thermal system. Overall, for a high-enough temperature at which the thermal kinetic energy comparable with the interaction, we observe that the two-particle correlation is subdominant compared to the single particle distributions, which indicates the applicability of kinetic theory.

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arXiv2026

Probing Pair Correlations in QCD Matter with Photon Spectra

Correlations in the phase-space distribution of partons play an important role in the initial stage of relativistic heavy-ion collisions, where the matter is dense and far from equilibrium. Photons produced in the hot medium, which predominantly originate from two-parton initial states, are sensitive to two-particle correlations in the phase-space distribution. In this work, we study how pair correlations in non-equilibrium QCD matter affect in-medium photon production. We decompose the two-particle distribution as $\mathcal F_{ab}=f_a f_b+g_{ab}$, where $g_{ab}$ is the pair correlation. Focusing on the $2\to2$ quark--antiquark annihilation and Compton channels, we compute the leading-logarithmic photon spectrum by expanding the single-particle distribution and pair correlation in a spectral basis, thereby accommodating a broad class of two-particle distributions. For a rotationally invariant medium, we find that relative-angle modes of the pair correlation generate sign-changing modifications to the photon spectrum, with magnitudes that can be comparable to the factorized contribution. Thus, photon spectra, although single-particle observables, can measure the momentum correlations of the emitting medium and therefore probe the early-time hydrodynamization.

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arXiv2026

Thermal Spectra Without Detailed Balance

A thermal spectrum is often taken as a signature that the emitted probe has reached detailed balance with the surrounding medium. We show that this interpretation is not generally valid by studying how the microscopic emission kernel determines the macroscopic spectrum. In $3+1$ dimensions, a simple thermal spectrum can be generated without probe thermalization when the relevant kernel belongs to a thermally degenerate class. A representative case is realized when the differential cross section depends on the scattering angle but carries no additional dependence on the Mandelstam variable $s$, as in low-energy Thomson scattering. Our results provide a kernel-based criterion for distinguishing genuine probe--medium exchange equilibrium from thermal spectra produced by the structure of the emission kernel itself.

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arXiv2025

Data-Driven Analysis for the Bottomonium Potential in the Quark-Gluon Plasma

We present a data-driven analysis within a quantum evolutionary microscopic framework to constrain the in-medium bottomonium potential. In relativistic heavy-ion collisions, bottomonium bound states serve as invaluable probes of the quark-gluon plasma (QGP) owing to their negligible production in the QGP phase. Meanwhile, their non-relativistic nature allows a straightforward theoretical description via effective field theories such as potential models. Recent lattice QCD calculations of the bottomonium interaction potential have yielded qualitatively distinct results. These discrepancies motivate a data-driven extraction of the potential based on heavy-ion experiments. In this work, we perform a Bayesian analysis to constrain the bottomonium interaction potential. The relationship between potential parameters and observables is established by numerically solving the non-relativistic time-dependent Schr"odinger equation. By comparing these simulations with experimental measurements, our Bayesian framework provides the effective potential that is readily testable in future experiments.

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arXiv2025

Bayesian inference of the magnetic component of quark-gluon plasma

The chromo-magnetic monopoles (CMM), emergent topological excitations of non-Abelian gauge fields carrying chromo-magnetic charge, have long been postulated to play an important role in the vacuum confinement of quantum chromodynamics (QCD), the deconfinement transition at temperature $T_c\approx 160\rm MeV$, as well as the strongly coupled nature of quark-gluon plasma (QGP). While such CMMs have been found to provide solutions for challenging puzzles from heavy-ion collision measurements, they were typically introduced as model assumptions in the past. Here we show how their very existence can be determined and their abundance extracted in a data-driven way for the first time. Using the \textsc{cujet3} framework for calculations of jet energy loss and analyzing a comprehensive experimental data set for nuclear modification factor ($R_{\mathrm{AA}}$) and elliptic flow ($v_2$) of high-transverse-momentum hadrons, the fraction of CMMs in the QGP is obtained by Bayesian inference and is found to be substantial in the $1\sim 2 T_c$ region. The posterior CMM fraction is further validated by excellent agreement with additional data and is also shown to predict QGP transport properties quantitatively consistent with the state-of-the-art knowledge.

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arXiv2025

Energy loss of heavy-flavor quarks in color string medium

The paper presents preliminary estimates of heavy-flavor (HF) quark energy loss during its propagation through the non-equilibrated medium formed in minimum bias proton-proton (p+p) collisions at LHC energies. The study is inspired by the ongoing hot debates on whether tiny droplets of Quark-Gluon Plasma can be created in collisions of small systems. In this work, we model a p+p event with a fluctuating number of color strings originated from multi-pomeron exchanges. Considered longitudinal oscillations of strings dynamically initialize medium at each time step. Their varying overlaps create fluctuations in the color field energy density that governs the elastic scattering rate of HF quarks with the gluons present within the string volume. We calculate the transverse momentum dependence of the momentum loss for charm (anti-)quarks that are produced in initial hard scatterings and traverse the described environment. The simulation is performed using a developed hybrid approach on an event-by-event basis. Our results show significantly lower HF quarks energy loss compared to that obtained in the expanding hydrodynamic scenario of the new EPOS4HQ model.

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arXiv2025

Decoupling hydrodynamization from thermalization via nonlinear Boltzmann equation

The early thermalization puzzle arises from the unexpectedly early applicability of hydrodynamics in heavy-ion collisions. While hydrodynamics has traditionally been associated with the onset of local thermal equilibrium, its derivations -- whether microscopic or macroscopic -- rely instead on linearization around equilibrium. However, the linearization timescale -- the time at which a system's evolution begins to follow a linearized equation -- has not been systematically investigated. In this work, we employ the spectral nonlinear Boltzmann equation -- the lowest-order truncation of the spectral Bogoliubov--Born--Green--Kirkwood--Yvon (BBGKY) hierarchy -- to analyze the timescales of linearization and thermalization under three distinct truncation schemes. The first two truncations allow for analytic treatment via recursive spectral equations, while the third requires numerical methods for generic initial conditions. The analysis is performed for a homogeneous, massless system with a constant differential cross section. For this simplified setup, we find a robust separation: the linearization time is consistently about half the thermalization time ($\tau_{\mathrm{lin}}/\tau_{\mathrm{therm}} \approx 1/2$). This separation of timescales suggests an explanation for the early applicability of hydrodynamics and points toward a possible quantitative resolution of the early thermalization puzzle.

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arXiv2025

Collective motion in the massive Schwinger model via Tensor Network

We simulate the real-time dynamics of a massive Schwinger model using the Time-Evolving Block Decimation tensor network algorithm. Starting from a non-equilibrium initial state with localized energy excitation on top of vacuum, we track the subsequent evolution to investigate two distinct physical phenomena. First, by analyzing the system's energy-momentum tensor, we show that this system exhibits hydrodynamic behavior analogous to Bjorken flow at large coupling-to-mass ratio, a signature that diminishes as the coupling weakens, or mass increases. Second, by examining the evolution of the electric field and charge density, we observe the signal of spontaneous parity symmetry breaking phase transition in a dynamical system. The parity-restored regime is marked by ''string breaking'' and efficient charge screening, while the parity-broken regime displays stable propagation of nearly free charges and persistent electric fields connecting them.

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arXiv2025

Onset of Bjorken Flow in Quantum Evolution of the Massive Schwinger Model

The onset of hydrodynamics in the hot medium created in relativistic heavy-ion collisions is a crucial theoretical question. A first-principle simulation requires a real-time, non-perturbative calculation of the quantum system. In this Letter, we perform such simulations using the tensor network method, which enables large-scale quantum many-body simulations by retaining only the most essential quantum states for collective behaviors. We focus on the massive Schwinger model, a low-dimensional analog of quantum chromodynamics (QCD), as they share important properties such as confinement and chiral symmetry breaking. Starting from an initial state that puts a localized excitation atop the vacuum and mimics the energy deposition from colliding nuclei, we observe hydrodynamic behavior consistent with Bjorken flow in all relevant degrees of freedom: energy density, fluid velocity, and bulk pressure. The time scale for hydrodynamic onset aligns with the thermalization time of the quantum distribution function.

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arXiv2025

Attractor of hydrodynamic attractors

Understanding how hydrodynamics emerges rapidly in the medium produced by relativistic heavy-ion collisions remains a key theoretical challenge. While the attractor solution -- manifesting as a non-thermal fixed point during the early evolution stage -- offers a potential explanation, it does not fully account for how far-from-equilibrium systems quickly approach near-equilibrium states. In this Letter, we demonstrate that the attractor in a higher-order hydrodynamic theory converges to the same solution as a second-order theory before reaching the Navier--Stokes limit. This finding suggests that commonly used second-order hydrodynamic equations, provided they incorporate the correct transport coefficients, are adequate to approximate the system's behavior starting from an intermediate time -- even when using a higher-order theory that would be more suitable for describing the far-from-equilibrium evolution.

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arXiv2025

Assessing background effects in search of the chiral vortical effect in relativistic heavy-ion collisions

The search for the Chiral Vortical Effect (CVE) in relativistic heavy-ion collisions is carried out by measuring azimuthal correlators for baryon pairs such as $\Lambda$ and protons. Experimental results from the ALICE collaboration show significant separations in these observables, however, the interpretation remains unclear. It is believed that background contributions from baryon production mechanisms may play an important role. Using three phenomenological models, the Blast Wave, AMPT, and AVFD+UrQMD, we systematically investigate the background effects in Pb--Pb collisions at \snn = 5.02 TeV. We demonstrate that local baryon conservation, as well as hadronic annihilation processes, can significantly influence the correlators. The feed-down contribution from secondary protons is also estimated. Our study provides a foundation for disentangling background mechanisms and further facilitates the search for the CVE.

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arXiv2025

Spectral BBGKY: a scalable scheme for nonlinear Boltzmann and correlation kinetics

The Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy provides a time-reversal-symmetric framework for describing the nonequilibrium evolution of many-body systems. Despite the success of Boltzmann-based numerical approaches, systematically extending beyond this lowest-order truncation to the full nonlinear BBGKY hierarchy remains a major challenge. Moreover, even at the Boltzmann level, accurately treating the nonlinear collision term still presents significant difficulties. Here we propose the spectral BBGKY hierarchy, an analytically equivalent and numerically tractable reformulation of the conventional BBGKY hierarchy. The spectral formulation reduces the original 6n-dimensional phase-space problem to the evolution of spectral coefficients over the 3n-dimensional coordinate space. We also develop an analytic scheme for computing the collision integrals, which achieves high accuracy and removes the need for ensemble averaging over repeated stochastic evolutions from the same initial state. The scheme evaluates the full eight-fold integral exactly for massless particles, and reduces it to a three-fold one for massive particles. The validity of the spectral BBGKY hierarchy is verified through conservation law analysis, comparison with an analytical solution, convergence tests, and analysis of spectral coefficient leakage. At minimal truncation, the spectral BBGKY yields a spectral nonlinear Boltzmann equation that captures full dynamics with a computational cost comparable to that of linearized approaches. When extended to higher-order truncations, the spectral BBGKY hierarchy provides a flexible framework for studying multiparticle correlations. This framework advances our ability to investigate the early thermalization puzzle in relativistic heavy-ion collisions and to elucidate the applicability of hydrodynamics at remarkably early stages of quark-gluon plasma evolution.

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arXiv2025

Thermalization from quantum entanglement: jet simulations in the massive Schwinger model

We investigate the emergence of thermalization in a quantum field-theoretic model mimicking the production of jets in QCD -- the massive Schwinger model coupled to external sources. Specifically, we compute the expectation values of local operators as functions of time and compare them to their thermal counterparts, quantify the overlap between the evolving density matrix and the thermal one, and compare the dynamics of the energy-momentum tensor to predictions from relativistic hydrodynamics. Through these studies, we find that the system approaches thermalization at late times and elucidate the mechanisms by which quantum entanglement drives thermalization in closed field-theoretic systems. Our results show how thermodynamic behavior emerges in real time from unitary quantum dynamics.

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arXiv2025

Optimal Observables for the Chiral Magnetic Effect from Machine Learning

The detection of the Chiral Magnetic Effect (CME) in relativistic heavy-ion collisions remains challenging due to substantial background contributions that obscure the expected signal. In this Letter, we present a novel machine learning approach for constructing optimized observables that significantly enhance CME detection capabilities. By parameterizing generic observables constructed from flow harmonics and optimizing them to maximize the signal-to-background ratio, we systematically develop CME-sensitive measures that outperform conventional methods. Using simulated data from the Anomalous Viscous Fluid Dynamics framework, our machine learning observables demonstrate up to 90\% higher sensitivity to CME signals compared to traditional $\gamma$ and $\delta$ correlators, while maintaining minimal background contamination. The constructed observables provide physical insight into optimal CME detection strategies, and offer a promising path forward for experimental searches of CME at RHIC and the LHC.

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arXiv2025

Leading order, next-to-leading order, and non-perturbative parton collision kernels: Effects on the jet substructure

As an important signature of the quark-gluon plasma (QGP), a high-precision energy-loss model is essential to independently verify the QGP properties extracted from soft particles. In this work, we optimize the energy loss modeling in MARTINI by introducing the formation time of the parton shower in the initial hard scattering, which is essential for a simultaneous description of the hadron and jet $R_{AA}$. Based on this, we study the phenomenological influence of the higher order collision kernels -- the up-to-NLO one evaluated by EQCD and the non-perturbative (NP) one computed in lattice QCD -- on the energy loss of the hard parton, compared to the LO kernel. The hadron and jet $R_{AA}$ are calculated with AMY rates using the three kernels and the optimized parameter sets for the running coupling. The results exhibit remarkable similarities in their overall values as well as in $p_T$ and centrality dependence. Sizable differences in the jet substructure are observed between different soft radiation rates.

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arXiv2025

Towards constraining QCD phase transitions in neutron star interiors: Bayesian Inference with TOV linear response analysis

The potential hadron-to-quark phase transition in neutron stars has not been fully understood as the property of cold, dense, and strongly interacting matter cannot be theoretically described by the first-principle perturbative calculations, nor have they been systematically measured through terrestrial low-to-intermediate energy heavy-ion experiments. Given the Tolman--Oppenheimer--Volkoff (TOV) equations, the equation of state (EoS) of the neutron star (NS) matter can be constrained by the observations of NS mass, radius, and tidal deformability. However, large observational uncertainties and the limited number of observations currently make it challenging to strictly reconstruct the EoS, especially to identify interesting features such as a strong first-order phase transition. In this work, we study the dependency of reconstruction quality of the phase transition on the number of NS observations of mass and radius as well as their uncertainty, based on a fiducial EoS. We conquer this challenging problem by constructing a neural network, which allows one to parametrize the EoS with minimum model-dependency, and by devising an algorithm of parameter optimization based on the analytical linear response analysis of the TOV equations. This work may pave the way for the understanding of the phase transition features in NSs using future $x$-ray and gravitational wave measurements.

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