arXiv subjects

Chun Shen

Publications and source records attributed to Chun Shen.

At least 19 recordsLinked to original sources

arXiv2026

Revisiting the role of saturation in diffractive vector meson production

We perform a global Bayesian analysis of coherent and incoherent diffractive $\mathrm{J}/\psi$ photoproduction in $\gamma+p$ and $\gamma+\mathrm{Pb}$ collisions using a Color Glass Condensate (CGC)-based framework and ultraperipheral collision data from the Large Hadron Collider (LHC), corrected for the expected effect of electromagnetic dissociation (EMD). Using Gaussian-process emulators of the underlying CGC calculations, we infer model parameters from a combined set of HERA and LHC measurements. We find that the $\gamma+\mathrm{Pb}$ data with EMD correction substantially reduce the previously observed tension between proton and nuclear datasets, enabling a consistent simultaneous description of diffractive $\mathrm{J}/\psi$ production in $\gamma+p$ and $\gamma+\mathrm{Pb}$ collisions within the CGC framework.

hep-ph↗

arXiv2026

Nuclear structure and saturation effects from diffractive vector meson production

We study exclusive vector meson production in ultraperipheral collisions (UPCs) of a wide range of nuclei, and assess the potential of measurements to constrain the small-$x$ structure of oxygen and neon nuclei. We employ an impact-parameter-dependent color glass condensate framework incorporating JIMWLK evolution, with parameters constrained by a recent global Bayesian analysis of $\gamma+p$ and $\gamma+\mathrm{Pb}$ data. We present predictions for coherent and incoherent $\mathrm{J}/\psi$ production in $\mathrm{O}+\mathrm{O}$ and $\mathrm{Ne}+\mathrm{Ne}$ UPCs at LHC energies, and quantify theoretical uncertainties using posterior samples from the calibration. We employ several nuclear structure models and find that $t$-differential observables are sensitive to the chosen model. We further study the mass-number dependence of saturation effects through nuclear suppression factors for coherent and incoherent vector meson production. Saturation-induced suppression increases systematically with both nuclear mass number and energy. Our results provide a unified framework for the systematic study of the onset of gluon saturation and nuclear structure at high energy, accessible in future UPC measurements at the LHC and at the Electron-Ion Collider.

hep-ph↗

arXiv2026

Correlations of Feed-down Hadrons in a Thermal Model

We examine the potential impact of strong decays on the magnitude of fluctuations of net quantum numbers and integrals of balance functions based on a thermal hadron gas model. The calculations are based on a comprehensive list of known hadrons with masses up to 2.5 GeV/$c^2$ and include all decays of these hadrons with known branching fractions. The calculations are performed at vanishing baryo-chemical potential for temperatures between 140 and 200 MeV. We show that the decays feed-down substantially impact the single yield of measurable ``stable particles" as well as those of correlated densities of these species. Decays can then potentially have large and non-trivial impacts on measurements of net quantum number cumulants and balance functions. These observations are particularly important in the context of the search for the QCD critical point at the RHIC Beam Energy Scan as well as efforts to determine chemical susceptibilities near the phase transition at RHIC or LHC energies. Results obtained in this work also shed light on the importance of feed-down in measurements of balance functions in elementary p-p and nucleus-nucleus collisions.

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arXiv2026

Signatures of QCD conductivities in heavy-ion collisions

Dissipative processes are pivotal for understanding the hydrodynamic evolution of hot and dense QCD matter created in relativistic nuclear collisions. The interplay of multiple conserved charges -- net baryon, strangeness, and electric charge -- is of particular interest. We simulate the longitudinal hydrodynamic evolution with the three diffusion currents in a hydrodynamic model with a lattice-QCD-based equation of state, NEOS-4D, and estimate rapidity distributions including diffusive corrections to the phase-space distribution in the presence of multiple charges, which ensure charge conservation at particlization. We determine the response of particle yields at midrapidity to changes in the diagonal and off-diagonal conductivities. Inversely, we find that most components of the conductivity matrix can be constrained experimentally using identified particle multiplicities at different collision energies.

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arXiv2025

New constraints on equation of state of hot QCD matter

The longitudinal structure of the quark-gluon plasma(QGP) remains a key challenge in heavy-ion physics. In this Letter, we propose a novel observable, event-by-event mean transverse momentum fluctuations Var$_{\langle p_{T} \rangle}$, which is sensitive to the local pressure gradients and serves as a probe of longitudinal dynamics in the initial state of QGP. We demonstrate that the covariance of averaged transverse momentum at two rapidities $\mathrm{Cov}_{\langle p_T \rangle}(\eta_1, \eta_2)$ and its associated decorrelation measures, $R_{p_T}(\eta_1, \eta_2)$ and $r_{p_T}(\eta, \eta_{\mathrm{ref}})$, exhibit strong sensitivity to the stiffness of equation of state (EoS) of QGP, while showing negligible dependence on the QGP transport coefficients. This distinctive behavior, revealed through state-of-the-art (3+1)-dimensional hydrodynamic simulations, establishes a powerful approach for constraining the EoS of QCD matter. In the meantime, our results provide new insights into the longitudinal structure of the QGP and its properties under high baryon density.

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arXiv2025

Electromagnetic Radiation from Baryon-Rich Matter in Heavy-Ion Collisions

We perform a study of electromagnetic radiation in heavy-ion collisions at Relativistic Heavy Ion Collider (RHIC) Beam Energy Scan (BES) and SPS energies using the iEBE-MUSIC framework, which includes 3D dynamical Monte Carlo Glauber initial conditions, MUSIC (3+1)D viscous relativistic hydrodynamics, and the UrQMD hadronic afterburner. The multistage modeling has been calibrated to hadronic data at RHIC-BES energies using a Bayesian analysis. Integrating the thermal photon emission rates with the medium evolution, we study the direct photon yield and elliptic flow and how they vary with collision energy and emission source. We compare with results obtained by the STAR and PHENIX Collaborations. We employ next-to-leading order thermal QCD dilepton emission rates to compute dilepton invariant mass spectra and extract the effective temperature of the quark-gluon plasma at different collision energies.

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arXiv2025

Impact of QCD Energy Evolution on Observables in Heavy-Ion Collisions

We study how the inclusion of energy dependence as dictated by quantum chromodynamic (QCD) small-$x$ evolution equations affects key observables in ultra-relativistic heavy-ion collisions. Specifically, we incorporate JIMWLK evolution into the IP-Glasma framework, which serves as the initial condition for a simulation pipeline that includes viscous relativistic hydrodynamics and a hadronic afterburner. This approach enables a consistent modeling of highly energetic nuclei across varying Bjorken-$x$ values, which are relevant for different collision energies and rapidity regions. In comparison to the standard IP-Glasma setup without small-$x$ evolution, we observe pronounced changes in particle multiplicities and spectral distributions, especially in smaller systems and at the highest available energies. We further explore effects on anisotropic flow observables and correlations between mean transverse momentum and elliptic flow. Our findings underscore the critical role of nonlinear QCD evolution in accurately modeling the early stages of heavy-ion collisions, as well as its implications for extracting transport properties of the quark-gluon plasma.

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arXiv2025

Neutron Skin from Conserved Charge Measurements at Collider Experiments

We propose a novel method for measuring the neutron skin of heavy nuclei using collider experiments. Specifically, we demonstrate that the neutron skin thickness of the lead nucleus can be extracted in $p$+$^{208}$Pb collisions by analyzing a double ratio: The ratio of net electric charge to net baryon number measured near the lead-going rapidity, taken for high-multiplicity events and divided by the same ratio for low-multiplicity events. We compute the expected sensitivity of the double ratio to the neutron skin within a comprehensive (3+1)D relativistic hydrodynamic framework that incorporates multiple conserved charge currents and a charge-dependent lattice-QCD-based equation of state. We provide predictions for both $p$+$^{208}$Pb collisions at ${\sqrt{s_{\mathrm{NN}}}=72}$~GeV and $\sqrt{s_{\mathrm{NN}}}=5.02$~TeV, corresponding to the center of mass energies realized in the SMOG2 fixed-target setup at LHCb and the LHC collider mode, respectively.

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arXiv2025

Learning Informed Prior Distributions with Normalizing Flows for Bayesian Analysis

We investigate the use of normalizing flow (NF) models as flexible priors in Bayesian inference via Markov Chain Monte Carlo (MCMC) sampling for iterative Bayesian calibration. Trained on posteriors from previous analyses, these models can be used as informative priors that capture non-trivial distributions and correlations in subsequent inference tasks. We compare different training strategies and loss functions, finding that training based on Kullback-Leibler (KL) divergence and unsupervised learning consistently yield the most accurate reproductions of reference distributions. We apply such a sequential Bayesian workflow to a high-energy nuclear physics problem; MCMC with NF-based priors reproduces the results of one-shot joint inference well, provided the target distributions are unimodal. In cases with pronounced multi-modality or dataset tension, distortions may arise, underscoring the need for caution in multi-stage Bayesian inference. A comparison between the pocoMC MCMC sampler and the standard emcee sampler further demonstrates the importance of advanced and robust algorithms for exploring the posterior space. Overall, our results establish NF-based priors as a practical and efficient tool for sequential Bayesian inference in high-dimensional parameter spaces.

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arXiv2025

Nuclear Suppression in Diffractive Vector Meson Production within the Color Glass Condensate Framework

We extend a recent global Bayesian analysis of diffractive $\mathrm{J}/\psi$ production in $\gamma+p$ and $\gamma+\mathrm{Pb}$ collisions within the color glass condensate (CGC) framework to investigate potential modifications of the nucleon structure inside nuclei. To this end, we perform fits that allow the effective nucleon structure parameters in Pb nuclei to differ from those of free protons. This approach directly addresses the question of whether the proton's spatial gluon distribution at intermediate to large $x$ is modified in the nuclear environment. We compare results obtained with shared and independent nucleon structure parameters and assess the impact on the simultaneous description of $\gamma+p$ data from HERA and the LHC, as well as $\gamma+\mathrm{Pb}$ data from the LHC. Our findings show that there is no hint of difference in the nucleon structure beyond those already present in the CGC when embedding nucleons inside a nuclear environment.

hep-ph↗

arXiv2025

Electromagnetic radiation from Quark-Gluon Plasma at finite baryon density

Using the Bayesian calibrated iEBE-MUSIC framework, we compute the production of electromagnetic radiation from hot hadronic matter at finite baryon density. Results for thermal photon and thermal dilepton yields are obtained by folding in-medium emission rates with posterior-sampled backgrounds evolved hydrodynamically. We consider different photon sources and analyze the collision-energy dependence of the thermal-to-prompt photon ratio. The sensitivity of the dilepton spectra to the pre-equilibrium stage is explored by considering different initialization procedures. Finally, we examine the impact on dilepton spectra of choosing parameter sets stemming from different Bayesian data analyses.

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arXiv2025

Testing hydrodynamic response to initial-state geometry in Pb+$d^\uparrow$ collisions

Deuterons with different polarization states have distinct shapes for their wavefunctions. This offers a unique opportunity to experimentally control the initial-state collision geometry with the polarization of the light-ion targets in relativistic heavy-ion experiments. We study the charged hadron elliptic flow coefficients with respect to the polarization angle of deuterons in Pb + polarized deuteron collisions using a hydrodynamics + hadronic transport model. Hydrodynamic response to initial-state geometry predicts a distinct sign of $v_2$ correlated with the deuteron's polarization states, providing a clean test case for elucidating the collective origin in small collision systems.

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arXiv2025

Event-by-event vortex rings in fixed-target p+Ar collisions

We present event-by-event simulations for central asymmetric p+Ar collisions at $\sqrt{s_\mathrm{NN}} = 68$ GeV to investigate the formation and evolution of vortex-ring structures from the early-stage longitudinal flow velocity profile. Our predictions for their imprints on Lambda hyperon's polarization observables are complementary to those presented in Ref. [Phys.Rev.C 110 (2024) 5, 054908] and can be explored in the future fixed-target collisions at the Large Hadron Collider beauty (LHCb) experiment.

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arXiv2025

Nuclear suppression in diffractive vector meson production within the color glass condensate framework

We perform a global Bayesian analysis of diffractive $\mathrm{J}/\psi$ production in $\gamma+p$ and $\gamma+\mathrm{Pb}$ collisions within a Color Glass Condensate based framework. Using data from HERA and the LHC, we find that a simultaneous description of $\gamma+p$ and $\gamma+\mathrm{Pb}$ observables is challenging. Introducing a global $K$-factor to account for theoretical uncertainties improves the agreement with data and enhances the framework's predictive power. We present predictions for integrated $\mathrm{J}/\psi$ cross sections at different photon-nucleus energies and study their $A$-dependence relative to a no-saturation baseline, quantifying nuclear suppression and providing insights into the onset of saturation effects.

hep-ph↗

arXiv2025

Perturbative high-energy evolution in the IP-Glasma initial state

We include the perturbative JIMWLK energy evolution into the IP-Glasma initial state description used to simulate the early-time dynamics in heavy ion collisions. By numerically solving the JIMWLK equation on an event-by-event basis, we obtain the energy (Bjorken-$x$) dependent structure of the colliding nuclei. Combining the initial state with hydrodynamic simulations, this enables us to predict how observables evolve when moving from RHIC to LHC energies.

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arXiv2025

A Resummed Hydrodynamic Description of Relativistic Heavy-ion Collisions

We introduce a resummed hydrodynamic scheme for evolving the viscous stress tensors in relativistic viscous hydrodynamics, based on which the necessary non-linear causality conditions can be imposed. When the magnitudes of the shear and bulk viscous stress tensors are small relative to the ideal part energy-momentum tensor, this new resummed scheme reduces to the standard second-order relativistic hydrodynamic theories. Nontrivial nonlinear corrections from high-order gradient terms retain the sizes of shear and bulk viscous stress tensors within tunable maximum allowed values. We perform event-by-event simulations for Pb+Pb and p+Pb collisions at 5.02 TeV to quantify the theoretical uncertainties from this resummed scheme on final-state flow observables.

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arXiv2025

Global Bayesian Analysis of $\mathrm{J}/\psi$ Photoproduction on Proton and Lead Targets

We perform a global Bayesian analysis of diffractive $\mathrm{J}/\psi$ production in $\gamma+p$ and $\gamma+\mathrm{Pb}$ collisions using a color glass condensate (CGC) based calculation framework. As past calculations have shown that CGC-based models typically overpredict the $\mathrm{J}/\psi$ production in $\gamma+\mathrm{Pb}$ collisions at high center of mass energy, we address the question of whether it is possible to describe coherent and incoherent diffractive $\mathrm{J}/\psi$ data from $\gamma+p$ collisions at HERA and the LHC, and from $\gamma+\mathrm{Pb}$ collisions at the LHC simultaneously. Our results indicate that a simultaneous description of $\gamma+p$ and $\gamma+\mathrm{Pb}$ data is challenging, with results improving when an overall $K$-factor -- scaling $\gamma+p$ and $\gamma+\mathrm{Pb}$ cross sections to absorb model uncertainties -- is introduced.

hep-ph↗

arXiv2025

Bayesian Model Selection and Uncertainty Propagation for Beam Energy Scan Heavy-Ion Collisions

We apply the Bayesian model selection method (based on the Bayes factor) to optimize $\sqrt{s_\mathrm{NN}}$-dependence in the phenomenological parameters of the (3+1)-dimensional hybrid framework for describing relativistic heavy-ion collisions within the Beam Energy Scan program at the Relativistic Heavy-Ion Collider. The effects of various experimental measurements on the posterior distribution are investigated. We also make model predictions for longitudinal flow decorrelation, rapidity-dependent anisotropic flow and identified particle $v_0(p_\mathrm{T})$ in Au+Au collisions, as well as anisotropic flow coefficients in small systems. Systematic uncertainties in the model predictions are estimated using the variance of the simulation results with a few parameter sets sampled from the posterior distributions.

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