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Chun Shen

Publications and source records attributed to Chun Shen.

At least 37 records · Page 2Linked to original sources

High-order anisotropic flow and nonlinear hydrodynamic responses

In this work, we study the charged hadron anisotropic flow coefficients $v_n$ (up to $n = 12$) using high-statistics event-by-event simulations of Pb+Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ and $5.36$~TeV, employing the IP-Glasma + MUSIC + UrQMD hybrid approach. The power spectra of anisotropic flow coefficients ($v_n\{2\}$ vs. $n$) are compared with the ALICE measurements from central to 50% centrality. To understand the various sources contributing to the high-order $v_n$ coefficients, we analyze the nonlinear mode coefficients for high-order anisotropic flow coefficients using different approximations and make comparisons with available measurements.

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

We extend a recent global Bayesian analysis of diffractive $\mathrm{J}/ψ$ production in $γ+p$ and $γ+\mathrm{Pb}$ 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 $γ+p$ data from HERA and the LHC, as well as $γ+\mathrm{Pb}$ 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.

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

We perform a global Bayesian analysis of diffractive $\mathrm{J}/ψ$ production in $γ+p$ and $γ+\mathrm{Pb}$ collisions within a Color Glass Condensate based framework. Using data from HERA and the LHC, we find that a simultaneous description of $γ+p$ and $γ+\mathrm{Pb}$ 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 $\mathrm{J}/ψ$ 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.

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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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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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Interplay of prompt and non-prompt photons in photon-triggered jet observables

Prompt photons are important yet challenging to observe in relativistic heavy-ion collisions, as they are produced in the early stages and traverse almost the entire QGP medium without interaction. Experimental analyses typically employ isolation cuts, in the hope to identify prompt photons. Most theoretical studies consider only events with actual prompt photons, assuming no contribution from isolated non-prompt photons to reduce computational cost. For the first time, we present a study that compares simulation results generated using inclusive (bremsstrahlung) and prompt-photon events with multiple experimental observables for both $p-p$ and $Pb-Pb$ collisions at $5.02$ TeV. Simulations are carried out using the multi-stage JETSCAPE framework tuned to describe the quenching of jets and hadrons. Isolated non-prompt photons are generated in hard photon bremsstrahlung, where the photon is radiated at a sufficient angle to the jet. Several photon triggered jet and jet substructure observables show significant contributions from inclusive photons, yielding an improvement in comparison with experimental data. Novel photon triggered jet substructure observables are also expected to show new structures, yet to be detected in experiment. This effort examines the significance of isolated non-prompt photons using parameters tuned for a simultaneous description of the leading hadron and jet spectrum, and thus provides an independent verification of the multistage evolution framework.

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Effects of hadronic reinteraction on jet fragmentation from small to large systems

We investigate the impact of the hadronic phase on jet quenching in nuclear collider experiments, an open question in heavy-ion physics. Previous studies in a simplified setup suggest that hadronic interactions could have significant effects, but a systematic analysis is needed. Using the X-SCAPE event generator with the SMASH afterburner, we study the role of hadronic rescattering on jet fragmentation hadrons. Applying this framework to $e^++e^-$ collisions, we demonstrate that even in small systems with limited particle production, hadronic interactions lead to measurable modifications in final-state hadronic and jet observables by comparing scenarios with and without afterburner rescattering.

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Four-dimensional QCD equation of state at finite chemical potentials

Exploration of the QCD phase diagram is pivotal in particle and nuclear physics. We construct a full four-dimensional equation of state of QCD with net baryon, electric charge, and strangeness by extending the NEOS model beyond the conventional two-dimensional approximation. Lattice QCD calculations based on the Taylor expansion method and the hadron resonance gas model are considered for the construction. We also develop an efficient numerical method for applying the four-dimensional equation of state to relativistic hydrodynamic simulations, which can be used for the analysis of nuclear collisions at beam energy scan energies and for different nuclear species at the BNL Relativistic Heavy Ion Collider.

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Anisotropic flow in fixed-target $^{208}$Pb+$^{20}$Ne collisions as a probe of quark-gluon plasma

The System for Measuring Overlap with Gas (SMOG2) at the LHCb detector enables the study of fixed-target ion-ion collisions at relativistic energies ($\sqrt{s_{\rm NN}}\sim100$ GeV in the centre-of-mass). With input from \textit{ab initio} calculations of the structure of $^{16}$O and $^{20}$Ne, we compute 3+1D hydrodynamic predictions for the anisotropic flow of Pb+Ne and Pb+O collisions, to be tested with upcoming LHCb data. This will allow the detailed study of quark-gluon plasma (QGP) formation as well as experimental tests of the predicted nuclear shapes. Elliptic flow ($v_2$) in Pb+Ne collisions is greatly enhanced compared to the Pb+O baseline due to the shape of $^{20}$Ne, which is deformed in a bowling-pin geometry. Owing to the large $^{208}$Pb radius, this effect is seen in a broad centrality range, a unique feature of this collision configuration. Larger elliptic flow further enhances the quadrangular flow ($v_4$) of Pb+Ne collisions via non-linear coupling, and impacts the sign of the kurtosis of the elliptic flow vector distribution ($c_2\{4\}$). Exploiting the shape of $^{20}$Ne proves thus an ideal method to investigate the formation of QGP in fixed-target experiments at LHCb, and demonstrates the power of SMOG2 as a tool to image nuclear ground states.

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Collision energy dependence in heavy ion collisions from nonlinear QCD evolution

We explore the effects of including the energy dependence determined from evolution equations within the color glass condensate framework on observables in ultra-relativistic heavy-ion collisions. This amounts to integrating the JIMWLK evolution equations into the IP-Glasma model, which is then coupled to viscous relativistic hydrodynamics. This methodology allows for a systematic representation of nuclei at specific Bjorken-$x$ values, which are probed at different center-of-mass energies of the collision and rapidities of final state particles. Comparing to the conventional IP-Glasma model, we find significant effects on multiplicity distributions and particle spectra, especially in smaller collision systems at the highest center of mass energies. Our results highlight the importance of incorporating nonlinear QCD evolution in the description of heavy ion collisions at varying center of mass energies, as the precise extraction of transport coefficients will be affected. This work establishes a robust framework for understanding the quark gluon plasma and nuclear structure at high energy, integrating small-$x$ physics into the initial conditions of heavy-ion collisions.

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Imaging the initial condition of heavy-ion collisions and nuclear structure across the nuclide chart

High-energy nuclear collisions encompass three key stages: the structure of the colliding nuclei informed by low-energy nuclear physics, the initial condition (IC) leading to the formation of quark-gluon plasma (QGP), and the hydrodynamic expansion and hadronization of the QGP leading to final-state hadrons observed experimentally. Recent advances in experimental and theoretical methods have ushered in a precision era, enabling an increasingly accurate understanding of these stages. However, most approaches involve simultaneously determining both QGP properties and initial conditions from a single collision system, creating complexity due to the coupled contributions of various stages to the final-state observables. To avoid this, we propose leveraging known knowledge of low-energy nuclear structure and hydrodynamic observables to constrain the IC independently. By conducting comparative studies of collisions involving isobar-like nuclei - species with similar mass numbers but different structures - we disentangle the initial condition's impacts from the QGP properties. This approach not only refines our understanding of the IC but also turns high-energy experiments into a precision tool for imaging nuclear structures, offering insights that complement traditional low-energy approaches. Opportunities for carrying out such comparative experiments at the LHC and other facilities could significantly advance both high-energy and low-energy nuclear physics. Additionally, this approach has implications for the future EIC. While the possibilities are extensive, we focus on selected proposals that could benefit both the high-energy and low-energy nuclear physics communities. Originally prepared as input for the long-range plan of U.S. nuclear physics, this white paper reflects the status as of September 2022, with a brief update on developments since then.

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Collectivity in ultra-peripheral heavy-ion and e+A collisions

We review recent theoretical progress in describing collective effects in photon+nucleus collisions. The approaches considered range from the color glass condensate where correlations are encoded in the initial state, to hydrodynamic frameworks, where a strong final state response to the initial geometry of the collision is the key ingredient to generate momentum-space correlations.

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Four-dimensional QCD equation of state with multiple chemical potentials

We construct a four-dimensional version of the equation of state (EoS) model NEOS, NEOS-4D, as a function of the temperature and chemical potentials of baryon, electric charge, and strangeness for the hot and dense QCD matter created in relativistic nuclear collisions. This EoS enables multiple conserved charge current evolution in a relativistic fluid. Input from Lattice QCD simulations and a hadron resonance gas model is considered for constructing the equation of state. We investigate its applicability to the relativistic hydrodynamic description of nuclear collisions and present a method for efficient numerical implementation.

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Probing Nuclear Structure of Heavy Ions at the Large Hadron Collider

We perform high-statistics simulations to study the impacts of nuclear structure on the ratios of anisotropic flow observables in $^{208}$Pb+$^{208}$Pb and $^{129}$Xe+$^{129}$Xe collisions at the Large Hadron Collider. Even with $40\%$ difference in atomic numbers between $^{208}$Pb and $^{129}$Xe nuclei, the ratios of anisotropic flow in the same centrality class between the two collision systems are strongly affected by the nuclear structure inputs in the initial state. The ratios of $v_2\{4\}/v_2\{2\}$ in these collisions are sensitive to the nuclear skin thickness of the colliding nuclei, providing indirect constraints on the nuclei's neutron skin. Our model predictions serve as a benchmark to compare with experimental measurements.

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Bayesian analysis of (3+1)D relativistic nuclear dynamics with the RHIC beam energy scan data

This work presents a Bayesian inference study for relativistic heavy-ion collisions in the Beam Energy Scan program at the Relativistic Heavy-Ion Collider. The theoretical model simulates event-by-event (3+1)D collision dynamics using hydrodynamics and hadronic transport theory. We analyze the model's 20-dimensional posterior distributions obtained using three model emulators with different accuracy and demonstrate the essential role of training an accurate model emulator in the Bayesian analysis. Our analysis provides robust constraints on the Quark-Gluon Plasma's transport properties and various aspects of (3+1)D relativistic nuclear dynamics. By running full model simulations with 100 parameter sets sampled from the posterior distribution, we make predictions for $p_{\rm T}$-differential observables and estimate their systematic theory uncertainty. A sensitivity analysis is performed to elucidate how individual experimental observables respond to different model parameters, providing useful physics insights into the phenomenological model for heavy-ion collisions.

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