arXiv subjects

Chunjian Zhang

Publications and source records attributed to Chunjian Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Longitudinal structure of the quark-gluon plasma from differently shaped nuclei

Ultrarelativistic collisions of atomic nuclei produce the quark--gluon plasma (QGP), an extremely hot, dense state of matter. The QGP behaves like a nearly perfect fluid, so its final-state momentum distributions can be inverted to reveal its initial-state geometry. This programme has succeeded in the transverse plane but made less progress along the beam, where short-range nonflow correlations mask the longitudinal signal. Anisotropic flow has been successfully used to image the shapes of the colliding nuclei; here we use nuclear shape to image the QGP's 3D geometry---the same idea in reverse. We show in simulations that the nonflow can be removed by comparing collisions of nuclei with similar masses but different shapes. We find that nuclear deformation changes the magnitude of the elliptic flow but not its longitudinal profile, so the shape difference recovers the full longitudinal structure. The predicted two-particle decorrelation map reveals both a highly non-linear rapidity dependence inaccessible to conventional observables, and an unquantified bias in standard flow measurements themselves. Our strategy extends to the longitudinal dependence of the QGP's triangularity and size. Nuclear shape thus becomes a tool for 3D imaging of the QGP and the quantum fluctuations of the nuclear wavefunctions that seed it.

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arXiv2026

Proton-proton Femtoscopy as a Probe of Short-range Structure in High-Energy O+O Collisions

Short-range nucleon-nucleon correlations are a defining feature of the nuclear many-body wave function, yet they are invisible in the one-body density and therefore inaccessible to observables that measure a nuclear size. We show that proton-proton femtoscopy supplies the missing sub-femtometer sensitivity. In $^{16}$O+$^{16}$O collisions at $\rm \sqrt{s_{NN}}=$ 200 GeV, we compare three nuclear-structure inputs spanning mean-field, low-resolution cluster, and short-range-correlated descriptions. The $p$-$p$ correlation function separates all three, most sharply in peripheral collisions, where the \textit{ab initio} input suppresses the extracted source radius by $\sim5\%$ relative to the mean-field baseline. Under identical conditions $π^{+}$-$π^{+}$ correlations respond an order of magnitude more weakly, and the $C_{pp}/C_{π^{+}π^{+}}$ double ratio retains the full effect, pointing to the short-distance weighting of the $^{1}S_{0}$ pair rather than to an overall rescaling of the source. The signal survives the leading theoretical systematic, the choice of strong-interaction potential, which we quantify explicitly. These results identify $p$-$p$ femtoscopy as a short-distance-resolved probe of light-nucleus structure, complementary to flow observables that constrain only the low-order moments of the initial geometry.

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arXiv2026

High-order fluctuations of temperature in hot QCD matter

A new thermodynamic state function is introduced to describe the thermodynamics relevant for the mean transverse momentum fluctuations of charged particles in heavy-ion collisions, which allows us to compute the temperature fluctuations of different orders in hot quantum chromodynamics (QCD) matter for the first time. Consequently, it is found that the temperature fluctuations are suppressed remarkably as the system transitions from the hadron resonance gas (HRG) to the quark-gluon plasma (QGP) with increasing temperature or baryon chemical potential, alongside a negative skewness. This is attributed to the general fact that the heat capacity of QCD matter increases significantly in QGP in comparison to that in HRG. These predictions provide a candidate observable to discover the thermodynamic temperature fluctuations in upcoming heavy-ion collision experiments, which also paves a novel way to study QCD thermodynamics and QCD phase diagram through measurements of the mean transverse momentum fluctuations of charged particles.

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arXiv2026

Nonlinear collective flow reveals the breakdown of quadrupole--hexadecapole scaling in heavy ion collisions

Determining the role of intrinsic hexadecapole deformation ($β_4$) in nuclear structure remains a long-standing challenge. Relativistic heavy-ion collisions provide a unique opportunity to address this problem by converting the initial nuclear geometry into the collective motion of the quark--gluon plasma (QGP). Using event-by-event viscous hydrodynamic simulations of ultra-central $^{238}$U+$^{238}$U collisions at $\sqrt{s_{NN}}=193$ GeV, we investigate whether higher-order collective flow can isolate the contribution of $β_4$ and test the $β_2-β_4$ correlation. We demonstrate that information carried by the sign of $β_4$ survives the QGP evolution and is enhanced through nonlinear hydrodynamic response: the fourth-order flow harmonic acquires its topology dependence predominantly from the linear response, whereas the sensitivity of the sixth-order harmonic originates almost entirely from nonlinear mode coupling. As a consequence, the nonlinear response coefficient $ξ_{6,222}$ cleanly separates the $(β_2,β_4)$ intrinsic nuclear topologies. These results establish the sign of $β_4$ as an experimentally accessible signature of deviations from the quadrupole--hexadecapole correlation, demonstrating that higher-order collective flow provides a direct probe of nuclear multipole structure while revealing how nonlinear QGP dynamics encode subtle higher-order geometric information into final-state observables.

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arXiv2026

Scaling approach to rigid and soft nuclear deformation through flow fluctuations in high-energy nuclear collisions

The nature of octupole deformation, whether static or vibrational, remains an open question in nuclear physics. Here, we propose a scaling approach to probe this ambiguity by triangular flow fluctuations using multi-particle cumulants, $c_{3,\varepsilon}\{4\}$, in relativistic $^{238}$U+$^{238}$U collisions. We demonstrate that both $|c_{3,\varepsilon}\{4\}|$ and the ratio $|c_{3,\varepsilon}\{4\}/c^2_{3,\varepsilon}\{2\}|$ scale linearly with the fourth-order moment of octupole deformation, $\langle β^4_{3,\mathrm{U}} \rangle$. Combined with the known linear relation of $c_{3,\varepsilon}\{2\}$ to $\langle β^2_{3,\mathrm{U}} \rangle$, this new relation provides a direct extraction of both the mean and variance of the octupole deformation fluctuations, finally discriminating between static and dynamic origins. This work establishes a new tool to probe the static and dynamic collective modes in high-energy nuclear collisions, advancing a significant step toward refining the initial conditions of quark-gluon plasma.

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arXiv2026

Imprints of octupole collectivity in uranium-238 on relativistic heavy-ion flow observables

Some atomic nuclei exhibit enhanced octupole collectivity, reflected in finite reflection-asymmetric multipole correlations rather than necessarily in a rigid static pear-shaped ground state. Low-energy studies indicate finite octupole strength in uranium-238, commonly interpreted as soft or vibrational in nature, in addition to its large prolate quadrupole collectivity~\cite{MCGOWAN1994569,KIBEDI:2002wxc}, in addition to its large prolate quadrupole collectivity. Here we investigate how such octupole correlations can be encoded in the initial geometry of relativistic heavy-ion collisions and mapped to final-state flow observables. Using state-of-the-art hydrodynamic calculations, we demonstrate quantitative sensitivity to octupole-induced features encoded in the initial-state geometry and suggest a modest octupole collectivity in uranium-238, confirmed by the latest high-energy experimental measurements~\cite{STAR:2025elk}. These findings provide as a complementary probe of odd-order nuclear collectivity and help constrain quark-gluon plasma initial conditions.

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arXiv2026

Nonflow Subtraction Beyond Two-Particle Correlations

Establishing collective flow in small collision systems is crucial for pinning down the minimum conditions for quark-gluon plasma (QGP) formation. In two-particle correlations, nonflow has been subtracted with good control, pushing the reach of flow measurements down to very small particle multiplicities $N$. However, the multi-particle nature of collectivity has not been established in the same $N$ regime, because the residual nonflow surviving the subevent procedure in multi-particle cumulants has never been quantified. We develop a general nonflow subtraction framework for $m$-particle cumulants, built around the approximate $1/N^{m-1}$ scaling of nonflow in the independent-source picture. Correlators containing $v_1$ serve as clean nonflow estimators, since the $p_{\rm T}$-integrated dipolar flow nearly vanishes. Using \HIJING{} as a controlled nonflow-only environment, we test the subtraction for three target observables ($\langle v_2^2\rangle$, $\langle v_2^2δp_{\rm T}\rangle$, and $c_2\{4\}$) in O+O and $d$+Au at $\sqrt{s_{\rm NN}} = 5.36$ TeV and 200 GeV. Most of the nonflow is removed, with residual fractions typically within 20--30% when converted to the two-particle level, though the best estimator differs across the three targets. We identify a multiplicity-reweighting correction, previously overlooked in two-particle correlations, that explains the long-standing undersubtraction of the naive $1/N$-scaling method; its impact grows as a power of the correlator order. The framework gives a systematic route to nonflow subtraction beyond two-particle correlations, broadening the class of multi-particle observables accessible to the small-system flow program.

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arXiv2026

Probing the neutron-skin thickness through $J/ψ$ photoproduction in ultra-peripheral collisions

We study the impact of neutron-skin thickness on $J/ψ$ photoproduction in ultra-peripheral $^{208}\mathrm{Pb}+{}^{208}\mathrm{Pb}$ collisions. Within the Color Glass Condensate framework, we calculate coherent and incoherent cross sections and examine their dependence on the momentum transfer $|t|$ for different neutron-skin thicknesses. We find a clear imprint of the neutron skin on the $|t|$ spectra: a larger neutron skin leads to a smoother and more extended color-density profile, suppressing the coherent cross section at large $|t|$ while enhancing the incoherent cross section through increased event-by-event configurational fluctuations in the nuclear periphery. We further show that the ratio of incoherent to coherent integrated cross sections provides a particularly sensitive and robust observable, with reduced theoretical uncertainties. These results establish diffractive vector-meson photoproduction in ultra-peripheral collisions as a powerful tomographic tool to constrain the neutron-skin thickness and the transverse gluon distribution at the LHC and future Electron-Ion Colliders.

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arXiv2026

Disentangling nuclear structure through multiparticle azimuthal correlations in high-energy isobar collisions

Event-by-event fluctuations in the amplitudes of flow harmonics offer a novel approach to probing the initial-state characteristics in heavy-ion collisions. In this study, we conduct a systematic investigation of correlations among various flow harmonics utilizing multiparticle cumulants in $^{96}$Ru+$^{96}$Ru and $^{96}$Zr+$^{96}$Zr collisions at $\sqrtsnn =$ 200 GeV within the framework of a multiphase transport model. Correlated nuclear density distributions specific to the isobar systems are incorporated to evaluate the sensitivity of selected observables to variations in nuclear deformation and neutron skin thickness. The analysis reveals that multiparticle azimuthal correlations are responsive to these nuclear structure features, predominantly in the most central collision events. Furthermore, the examined correlations exhibit shallow dependence on the assumed shear viscosity values. These findings provide a quantitative evaluation of the extent to which multiparticle flow observables can discern nuclear structure effects in isobar collisions and offer valuable guidance for future detailed dynamical investigations and experimental measurements.

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arXiv2026

Selected highlights from STAR experiment

In this paper, we review recent highlights in heavy-ion collisions and proton-proton collisions at top energies from STAR experiment at the Relativistic Heavy Ion Collider (RHIC) with key contributions from Chinese groups, including the Quark-Gluon Plasma (QGP) bulk properties, electromagnetic probes, heavy flavor and jets, antimatter hyper-nucleus, nuclear structure, global polarization, and nucleon spin structure. These data serve as important ingredients in the physics of Quantum Chromodynamics (QCD).

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arXiv2025

Evidence for modest octupole deformation in $^{238}$U from high-energy heavy-ion collisions

We present a novel ``imaging-by-smashing" approach for probing nuclear deformation in high-energy heavy-ion collisions. By analyzing anisotropic-flow ($v_n$) and mean transverse momentum ($\left[p_T\right]$)-based observables in collisions of highly deformed $^{238} \mathrm{U}$ nucleus and nearly spherical $^{197} \mathrm{Au}$ nucleus, we extract the deformation parameters of $^{238}$U. The key observables include the variances $\left\langle v_n^2\right\rangle,\left\langle\left(δp_T\right)^2\right\rangle$, and the covariance $\left\langle v_n^2 δp_T\right\rangle$. Ratios of these observables between $^{238}$U+$^{238}$U and $^{197}$Au+$^{197}$Au collisions largely cancel final-state effects, thereby isolating the influence of nuclear deformation. We further report the first experimental indication of octupole deformation in $^{238}$U via $v_3$-based observables~\cite{2025rot}. The extracted deformation parameters, comparing with state-of-the-art hydrodynamic model calculations, are consistent with low-energy nuclear structure data. These results establish high-energy collisions as a powerful probe of nuclear shapes on femtosecond timescales.

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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 $\mathrm{Cov}_{\langle p_T \rangle}(η_1, η_2)$ and its associated decorrelation measures, $R_{p_T}(η_1, η_2)$ and $r_{p_T}(η, η_{\mathrm{ref}})$, 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

Symmetric-asymmetric collision comparison: disentangling nuclear structure and subnucleonic structure effects for small system flow

Previous flow measurements in small collision systems were mostly based on highly asymmetric collisions ($p$+Pb, $p$+Au, $d$+Au, $^{3}$He+Au), where both nuclear structure and subnucleonic fluctuations are important. Comparing these asymmetric systems with the newly available symmetric $^{16}$O+$^{16}$O collisions at RHIC and LHC provides a unique opportunity to disentangle these two contributions. Using Glauber models incorporating both nucleon and quark-level substructure, we analyze multiplicity distributions and initial-state estimators: eccentricities $\varepsilon_n$ for anisotropic flow $v_n$ and inverse transverse size $d_{\perp}$ for radial flow. We find that subnucleonic fluctuations impact O+O collisions differently from asymmetric systems, creating specific patterns in flow observables that enable disentangling the competing contributions. Such experimental comparisons will reduce uncertainties in the initial conditions and improve our understanding of the properties of the QGP-like medium produced in small systems.

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arXiv2025

Imaging shapes of ground-state uranium-238 nuclei in high-energy nuclear collisions at RHIC

The shape and orientation of colliding nuclei play a crucial role in determining the initial conditions of the quark-gluon plasma (QGP), which influence key observables such as anisotropic and radial flow. In these proceedings, we present the measurements of $v_2$, $p_{\rm T}$ fluctuations and $v_2-p_{\rm T}$ correlations in $^{238}$U + $^{238}$U and $^{197}$Au + $^{197}$Au collisions at center of mass energies $\sqrt{s_{\rm NN}}=$ 193 and 200 GeV, respectively. Our results reveal significant differences in these observables between the two systems, particularly in the most central collisions. Comparisons with hydrodynamic model calculations indicate a large deformation in the ground states of $^{238}$U nuclei, consistent with previous low-energy experiments. However, data also imply a small deviation from axial symmetry of $^{238}$U [1]. Our work introduces a novel approach for imaging nuclear shapes, enhances the modeling of QGP initial conditions, and sheds light on nuclear structure evolution across different energy scales. The potential applications of this method for other nuclear species are discussed.

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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

Validation and extrapolation of atomic mass with physics-informed fully connected neural network

Machine learning offers a powerful framework for validating and predicting atomic mass. We compare three improved neural network methods for representation and extrapolation for atomic mass prediction. The powerful method, adopting a macroscopic-microscopic approach and treating complex nuclear effects as output labels, achieves superior accuracy in AME2020, yielding a much lower root-mean-square deviation of 0.122 MeV in the test set, significantly lower than alternative methods. It also exhibits a better extrapolation performance when predicting AME2020 from AME2016, with a root-mean-square deviation of 0.191 MeV. We further conduct sensitivity analyses against the model inputs to verify interpretable alignment beyond statistical metrics. Incorporating theoretical predictions of magic numbers and masses, our fully connected neural networks reproduce key nuclear phenomena including nucleon pairing correlation and magic number effects. The extrapolation capability of the framework is discussed and the accuracy of predicting new mass measurements for isotope chains has also been tested.

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arXiv2025

$Ab$-$initio$ nucleon-nucleon correlations and their impact on high energy $^{16}$O+$^{16}$O collisions

Investigating nucleon-nucleon correlations inherent to the strong nuclear force is one of the core goals in nuclear physics research. We showcase the unique opportunities offered by collisions of $^{16}$O nuclei at high-energy facilities to reveal detailed many-body properties of the nuclear ground state. We interface existing knowledge about the geometry of $^{16}$O coming from \textit{ab-initio} calculations of nuclear structure with transport simulations of high-energy $^{16}$O+$^{16}$O collisions. Bulk observables in these processes, such as the elliptic flow or the fluctuations of the mean transverse momentum, are found to depend significantly on the input nuclear model and to be sensitive to realistic clustering and short-range repulsive correlations, effectively opening a new avenue to probe these features experimentally. This finding demonstrates collisions of oxygen nuclei as a tool to elucidate initial conditions of small collision systems while fostering connections with effective field theories of nuclei rooted in quantum chromodynamics (QCD).

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arXiv2025

Energy dependence of transverse momentum fluctuations in Au+Au collisions from a multiphase transport model

Event-by-event mean transverse momentum fluctuations ($\langle p_\mathrm{T}\rangle$) serve as a sensitive probe of initial state overlap geometry and energy density fluctuations in relativistic heavy-ion collisions. We present a systematic investigation of $\langle p_\mathrm{T}\rangle$ fluctuations in \auau collisions at $\mathrm{\sqrt{s_{NN}}} =$3.0-19.6 GeV, examining their centrality and energy dependence with the framework of an improved multiphase transport (AMPT) model. The centrality dependence of the $p_\mathrm{T}$ cumulants up to fourth order deviates significantly from simple powering-law scaling. Scaled cumulants are performed, with variances aligning well with the trends observed in the experimental data. Employing a two-subevent method, short-range correlations are slightly suppressed compared to the standard approach. Furthermore, baryons exhibit more pronounced $\langle p_\mathrm{T}\rangle$ fluctuations than mesons, potentially attributable to the effect of radial flow. These results provide referenced insights into the role of initial state fluctuations across different energies in heavy-ion collisions.

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