arXiv subjects

Kai-Jia Sun

Publications and source records attributed to Kai-Jia Sun.

At least 19 recordsLinked to original sources

arXiv2026

Thermal width shift of $Δ^{++}$ in a pion gas

We compute the thermal width shift of the $Δ^{++}$ resonance induced by a pion gas within a nonrelativistic effective field theory framework. The $Δ^{++}$ self-energy is evaluated from pion-forward-scattering diagrams with intermediate proton and $Δ$ states, weighted by the thermal pion distribution. Analytical expressions for the imaginary part of the self-energy yield the temperature-dependent width correction $δΓ(T)$. The width increases with temperature, reaching approximately $6$~MeV at $T \approx 160$~MeV. When the temperature-dependent $Δ$ and nucleon masses from an NJL-model chiral restoration scenario are incorporated, the width shift becomes non-monotonic, peaking near $T \approx 140$~MeV---a consequence of the competition between collisional broadening and the shrinking $Δ\to Nπ$ phase space as the $N$--$Δ$ mass gap closes. Applying our formalism to STAR data for $Δ^{++}$ in d+Au collisions at $\sqrt{s_{NN}} = 200$~GeV, we extract a temperature $T \approx 300$~MeV at $p_t = 900$~MeV, consistent with the experimental extraction within errors. This value significantly exceeds the hadronic-phase temperature, explicitly demonstrating that the observed $Δ^{++}$ width shift is not solely of pion-gas origin---genuine hot-medium and collective-flow contributions must be substantial.

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arXiv2026

Shedding Light on (Anti-)nuclei Production with Pion-Nucleus Femtoscopy

High-energy nuclear collisions provide a unique environment for synthesizing both nuclei and antinuclei (such as $\bar{d}$ and $^4\overline{\text{He}}$) at temperatures ($k_BT\sim100$ MeV) much higher than their binding energies per nucleon of a few MeV. The underlying production mechanism, whether through statistical hadronization, nucleon coalescence, or dynamical regeneration and disintegration, remains unsettled. Here we address this question using pion-nucleus femtoscopy. By solving relativistic kinetic equations for pion-catalyzed reactions ($πNN \leftrightarrow πd$) for deuteron production and including final-state $p-$wave scatterings derived from an established effective interaction, we successfully reproduce the resonance peaks of both $π^+-p$ and $π^+-d$ femtoscopic correlations observed in $pp$ collisions at $\sqrt{s} = 13~\mathrm{TeV}$. The interplay between $Δ$ resonance and $p$-wave scatterings shifts both correlation peaks downward by about $70\text{ MeV}$ relative to vacuum $Δ$ decay. Conversely, both the nucleon coalescence model and the statistical hadronization model significantly underestimate the data and produce additional dips that are absent from the data. These results provide compelling evidence that pion-catalyzed reactions play a dominant role in the production of light (anti-)nuclei in high-energy nuclear collisions and cosmic rays.

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arXiv2026

Universal imprinting of short-range correlations in relativistic heavy-ion collisions

Protons and neutrons within atomic nuclei undergo intense and fleeting encounters driven by the strong force at short distances. These interactions generate close-proximity pairs, a phenomenon known as short-range correlations (SRCs). While the properties of SRCs have been extensively studied in cold nuclear matter, their behavior under extremely hot and dense conditions remains largely unexplored. Here, we incorporate correlated nucleon configurations into relativistic heavy-ion collisions, using the quark-gluon plasma (QGP), a state of matter present microseconds after the Big Bang, as a sensitive diagnostic tool. We find that these correlations induce substantial modifications to event-by-event geometry, which are quantitatively identified through higher-order moments of the transverse profile. Most importantly, we find a surprising linear relation between QGP geometry fluctuations and the SRC scale factor spanning systems from deuteron to lead, which reflects the universal imprinting of SRCs in relativistic heavy-ion collisions. Our findings reveal the emergence of short-range structural effects across vastly different energy scales from low-energy electron scattering to high-energy nuclear collisions.

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arXiv2026

Schrödinger Generator for High-Dimensional Integration and Sampling on Quantum Many-Body States

Integration and sampling in high dimensions are among central challenges in modern science and technology, underlying applications ranging from quantum many-body physics to Bayesian inference and artificial intelligence. Although conventional Monte Carlo methods are formally scalable, their efficiency deteriorates rapidly in the presence of strong correlations or sharp features in high-dimensional configuration space. Here we introduce a new framework, termed the Schr"odinger Generator, for integration and sampling based on the explicit optimization of coordinate transformations. The method decomposes the total Jacobian into two complementary components, including an adaptive map that minimizes estimator variance by learning the marginal structure in each dimension, and a normalizing-flow-based transformation that captures non-factorizable correlations in the target distribution. A final resampling step guarantees unbiased sampling even when the learned transformation is imperfect. We demonstrate stable and scalable performance for nuclear quantum many-body states in dimensions exceeding 600. Short-range correlations among nucleons in finite nucleus are faithfully reproduced. The framework offers a physically transparent approach to high-dimensional stochastic integration and sampling, opening new possibilities for simulations of complex quantum systems.

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arXiv2026

Resonance Femtoscopy Beyond the On-Shell Approximation

The observed shift of the $Δ(1232)$ resonance peak in $π$-$p$ femtoscopic correlations challenges the conventional Breit-Wigner description of resonances in femtoscopy. We revisit the Koonin-Pratt framework by formulating femtoscopy in the momentum-space representation. By employing the T-matrix approach to disentangle on-shell and off-shell contributions, we show that the finite spatial extent of the emission source naturally induces sensitivity to off-shell scattering dynamics. Using a Friedrichs-Lee model constrained by low-energy $π$-$p$ scattering data, we numerically demonstrate that this off-shell sensitivity leads to a peak shift accompanied by a dip on the high-momentum side of the peak. The predicted high-momentum side dip is absent in the data, pointing to source properties beyond the simple Gaussian approximation.

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arXiv2026

Reconstructing rare particle source by femtoscopic correlations

Measurement of particle emission source is a fundamental objective of femtoscopy in high-energy nuclear collisions. Conventional analyses rely on Gaussian parameterizations of pair emission sources, which makes the extraction of single-particle emission sources challenging, particularly for rare particles. Here, we introduce a novel statistical reconstruction method that allows extracting information of the target source relative to a data-constrained reference source instead of the Gaussian assumption. The correlation function is expressed as an ensemble average over the single-particle-conditioned correlation kernel, defined as the particle-by-particle contribution to the correlation function conditioned by the target particles. For particles with rare yields, the particle-by-particle distribution of this kernel can be transformed into event-by-event extraction and becomes experimentally accessible, enabling a direct statistical reconstruction of the emission source of single particles, instead of inferring a pair source. We apply this method to reconstruct $J/ψ$ source via $p$-$J/ψ$ correlations, using HAL QCD-derived $NJ/ψ$ potentials in $\sqrt{s}=13.6$~TeV $pp$ collisions simulated with EPOS4HQ. The reconstructed source reproduces the key characteristics and this new approach achieves a systematic uncertainty of approximately $13\%$ based on EPOS4 simulation.

hep-ph↗

arXiv2026

Hadron polarization and equation of state at FAIR/RHIC-BES energies

The $Λ$ global polarization indicates that hot and dense matter created in non-central heavy-ion collisions carries large orbital angular momentum. However, the relation between hadronic polarization and the medium's collective rotation remains to be validated. Using the UrQMD transport model, we calculate the thermal vorticity-induced polarization of $Λ$s in Ag+Ag and Au+Au collisions from $\sqrt{s_{\rm NN}}=2.24$-$7.7$ GeV and a range of centralities. Two different equations of state used in the UrQMD simulation are compared: one resembles a hadron resonance gas, while the other is based on the chiral mean field (CMF) model, providing a more realistic description of dense nuclear matter including a chiral transition that is consistent with lattice QCD expectations. The polarization is sensitive to the equation of state and a softer EoS leads to smaller values. In addition, we show that the $Λ$ polarization in the experimental acceptance and centrality selection does not decrease for even lower beam energies. Our results indicate that the process leading to the large vorticity is a result of the large shear in the baryon current created by its stopping.

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arXiv2026

Anisotropic Flow of light (anti-)(hyper-)nuclei in Pb+Pb Collision at $\sqrt{s_{NN}}=5.36$ TeV

Using the coalescence model with nucleon phase-space distributions generated by the hybrid MUSIC framework, we study the elliptic flow ($v_2$) and triangular flow ($v_3$) of (anti-)protons, (anti-)deuterons, (anti-)$^3\mathrm{He}$, and ${^3_Λ\mathrm{H}}$ in Pb+Pb collisions at $\sqrt{s_{NN}} = 5.36$ TeV. We find that the simple $v_2$ scaling with the number of constituent nucleons $A$ breaks down at high transverse momentum $p_T/A > 1.5$ GeV/$c$, while an improved scaling relation holds well up to $p_T/A \approx 3$ GeV/$c$. In contrast, $v_3$ exhibits similar behavior under both scaling prescriptions, with no significant difference. We also make predictions for $v_2$ and $v_3$ of the hypertriton and find these flows are insensitive to the Lambda-deuteron ($Λ-d$) distance inside the hypertriton. Our results are compared with preliminary experimental measurements by the ALICE Collaboration and offer insight into the production mechanisms of light (anti-)(hyper-)nuclei in high-energy heavy-ion collisions.

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arXiv2025

Unfolding Baryon Number Fluctuations from Correlations of Light Nuclei Production in Heavy-Ion Collisions

Event-by-event fluctuations of the baryon number, which is mostly carried by protons and neutrons, in relativistic heavy-ion collisions provide a sensitive probe for locating the conjectured critical point in the quantum chromodynamics (QCD) phase diagram. Since current experiments have limited access to neutron fluctuations because detectors are largely insensitive to neutrons, measurements of (net-)proton fluctuations are often used as a proxy for (net-)baryon number fluctuations. Although direct measurements of neutron fluctuations are challenging, their information are encoded in the production and correlations of light nuclei, when they are formed through coalescence of nucleons at kinetic freeze-out. Here, we propose to unfold neutron fluctuations from correlations among light nuclei produced in heavy-ion collisions. Model calculations validate this approach and show that baryon number fluctuations can be unfolded up to the third order. For fourth and higher-order cumulants, however, the uncertainties become sizable, indicating that further methodological developments and refinements are required.

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arXiv2025

Effects of a first-order QCD phase transition on light nucleus production

Using an extended Polyakov-looped Nambu--Jona-Lasinio (PNJL) model to describe the baryon density fluctuations of quark matter along the isentropic trajectories corresponding to different $s/ρ_B$ values extracted from Au+Au collisions at energies $\sqrt{s_{NN}} = 7.7-200$ GeV, we investigate the effects of the first-order phase transition on the light nucleus yield ratio $N_t \times N_p/N_d^2$. The results indicate that the second-order scaled density moment $y_2$, used to quantify density fluctuations, rapidly increases to form a peak when the isentropic trajectories pass through the phase coexistence region. We extract the yield ratios $N_t\times N_p/N_d^2$ at chemical freeze-out from the isentropic trajectories at different collision energies and found significant enhancements at 19.6 GeV and 27 GeV. This is similar to the trends observed by the STAR experiment, suggesting that the enhancements in the yield ratios $N_t\times N_p/N_d^2$ observed in the STAR experiment could be explained by the density fluctuations generated in the first-order phase transition region.

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arXiv2025

Global Spin Alignment of (Anti-)$^4$Li in Non-Central Heavy-Ion Collisions

Non-central heavy-ion collisions produce hot and dense nuclear matter with significant fluid vorticity, which can induce global polarizations or alignments of particles with non-zero spins along the direction of the total orbital angular momentum. This phenomenon has been observed for hyperons and vector mesons in experiments. In the present study, we demonstrate that polarized nucleons lead to global spin alignment of the unstable nucleus $^4$Li, which can be measured through its strong decays via $^4\text{Li} \rightarrow {^3\text{He}} + p$. Assuming that $^4$Li is formed through the coalescence of polarized nucleons at kinetic freeze-out, we obtain the angular distribution of the daughter particle $^3$He in the rest frame of the polarized $^4$Li. Taking kinetically freeze-out nucleons from an isotropic and thermalized fireball of constant vorticity and including quantum corrections up to $\hbar^2$ in the coalescence calculation through the Moyal star product, we find that the angular distribution of $^3$He has a $\cos(2θ^*)$ dependence with $θ^*$ being its angle with respect to the quantization axis of $^4$Li. We also find that the $^3$He angular distribution depends on both the vorticity and the polarization of kinetically freeze-out nucleons. Future measurements on the spin alignment of $^4$Li in heavy-ion collisions thus offer a promising method to probe the spin dynamics, vortical structure, and spin-dependent equation-of-state of the nuclear matter produced in these collisions.

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arXiv2025

From Hyperons to Hypernuclei: A New Route to Unravel Proton Spin Polarization

Ultra-relativistic nuclear collisions create the quark-gluon plasma (QGP) known as the hottest, least viscous, and most vortical fluid ever produced in terrestrial laboratories. Its vortical structure has been uncovered through the spin polarization of Lambda ($Λ$) hyperons, attributed to the spin-orbit coupling that transfers the system's orbital angular momentum to the quark spin, which is then inherited by hadrons via quark recombination or coalescence. However, $Λ$ polarization reflects primarily the strange-quark component, leaving the spin dynamics of the up and down quarks largely unexplored. Although the proton is an ideal probe, its stability makes direct measurements experimentally challenging. Here, we propose to unravel proton spin polarization via hypertriton ($^3_Λ\text{H}$) measurements, exploiting the fact that spin information is preserved when polarized nucleons and $Λ$ coalesce to form hypertriton. We show that, over a broad range of collision energies, the polarizations of proton, $Λ$, and hypertriton are related by a simple linear scaling law. Since both $Λ$ and hypertriton polarizations can be measured via their self-analyzing weak decays, this linear relation provides a practical experimental avenue for accessing spin polarizations of protons and neutrons-the dominant baryonic degrees of freedom in nuclear collisions.

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arXiv2025

Investigating $^{238}$U Deformation via Dilepton Production in Relativistic Heavy-Ion Collisions

Due to their weak coupling to the strongly interacting matter produced in relativistic heavy-ion collisions, dileptons serve as a sensitive probe of the initial geometry of the colliding nuclei. In this study, we investigate the influence of initial nuclear quadrupole deformation, characterized by the parameter $β_2$, on dilepton production in $U+U$ collisions at $\sqrt{s_{NN}}=196$ GeV. The analysis is varried out using a modified multiphase transport model in which partonic interactions are described by the Nambu-Jona-Lasinio model. We observe a clear linear dependence of dilepton yields on $β_2^2$ in both the low-mass region (LMR, $<1 GeV/c^2$) and intermediate-mass region (IMR, $1-3 GeV/c^2$) of the dilepton spectrum for the most central collisions. Also, dilepton production in the IMR region exhibits a stronger sensitivity to nuclear deformation than in the LMR, reflecting the dominance of earlier partonic processes in this mass range. These results suggest that precise measurements of dilepton yields in relativistic heavy-ion collisions can provide a viable means to determine the deformation parameter $β_2$ of $^{238}$U.

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arXiv2025

Alpha clustering in warm and dense nuclear matter from heavy-ion collisions

Although light nuclear clusters are known to affect the properties of warm and dilute nuclear matter, their role in warm and dense nuclear matter remains unclear due to the lack of experimental evidence for their modifications by the Mott effect in such an environment. To address this issue, we resort to intermediate-energy heavy-ion collisions, where light clusters are mainly produced in the transiently formed warm and dense matter. A kinetic approach, which includes dynamically the formation and dissociation of light clusters, is employed to deduce the strength of the Mott effects and the $α$-particle fraction in warm and dense nuclear matter from the light-nuclei yields measured by the FOPI Collaboration in central Au$+$Au collisions at energies of $0.25A$ to $0.6A~\rm GeV$. We find an unexpectedly abundant $α$ clustering in this environment, which will have profound implications for modeling the nuclear equation of state and describing supernovae and neutron star mergers.

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arXiv2025

Atmospheric Antideuteron Flux Within a Dynamical Coalescence Approach

Cosmic antideuterons are considered as one of the most promising tools for the indirect detection of dark matter due to their ultra-low astrophysical backgrounds. Currently only upper limits on the antideuteron flux exist, but advancements in experimental detection technology may soon lead to positive signals. A major source of background is the production of secondary antideuterons through collisions of cosmic rays with the surrounding medium. In this study, antideuteron production is modeled using a multiphase transport model (AMPT) coupled with a dynamical coalescence model. By applying a widely used leaky box model and incorporating specific processes, we present a new theoretical baseline for atmospheric secondary antideuteron flux, including a tertiary contribution, from primary cosmic rays interacting with Earth's atmosphere. Our results indicate that the atmospheric antideuteron flux are within the range of various existing calculations and remain well below the upper limits set by the Balloon-borne Experiment with a Superconducting Spectrometer (BESS). The atmospheric antideuteron is found to dominate the antideuteron background at kinetic energies below $0.26 $ GeV/n.

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arXiv2025

Deciphering Hypertriton and Antihypertriton Spins from Their Global Polarizations in Heavy-Ion Collisions

Understanding the properties of hypernuclei is crucial for constraining the nature of hyperon-nucleon ($Y\text{-}N$) interactions, which plays a key role in determining the inner structure of compact stars. The lightest hypernuclei and antihypernuclei are the hypertriton ($^3_Λ\text{H}$), which consists of a pair of nucleons and a $Λ$ hyperon, and its antinucleus (${^3_{\barΛ}}\overline{\rm H}$). Significant knowledge has recently been acquired regarding the mass, lifetime, and binding energy of $^3_Λ\text{H}$. However, its exact spin, whether $\frac{1}{2}$ or $\frac{3}{2}$, remains undetermined in both experimental and theoretical studies. Here, we present a novel method of using the hypertriton global polarization in heavy-ion collisions to decipher not only its total spin but also its internal spin structure. This method is based on the finding that its three different spin structures exhibit distinct beam energy dependence of its global polarization when it is produced in these collisions from the coalescence of proton, neutron and $Λ$. Future observations of the hypertriton and antihypertriton global polarizations thus provide the opportunity to unveil the spin structures of hypertriton and antihypertriton and their production mechanisms in heavy-ion collisions.

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arXiv2024

Effects of chiral symmetry restoration on dilepton production in heavy ion collisions

Because of their weak interactions with the strongly interacting matter produced in relativistic heavy-ion collisions, dileptons provide an ideal probe of the early dynamics of these collisions. Here, we study dilepton production using a partonic transport model that is based on an extended Nambu-Jona-Lasinio (NJL) model. In this model, the in-medium quark masses decrease with increasing temperature as a result of the restoration of chiral symmetry. We find that the extracted temperature from dileptons of intermediate masses agrees well with the temperature of the partonic matter, suggesting that dilepton production can be used as a thermometer for the produced partonic matter. Our results also indicate that the extracted in-medium quark masses decrease with increasing dilepton temperature, implying that dilepton production can further serve as a probe of chiral symmetry restoration in high energy heavy-ion collisions.

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arXiv2024

Probing the internal structures of $pΩ$ and $ΩΩ$ with their production at the LHC

The strange dibaryons $pΩ$ ($^5\rm{S}_2$) and $ΩΩ$ ($^1\rm{S}_0$) are likely bound, existing either in molecular states like the deuteron or as more exotic compact six-quark states. Here, we investigate the production of these two dibaryons in Pb+Pb collisions at $\sqrt{s_{NN}}$=2.76 TeV at the CERN Large Hadron Collider (LHC) within a covariant coalescence model, which employs a blast-wave-like parametrization for the phase-space configurations of constituent particles at freeze-out. For the molecular states, the $pΩ$ and $ΩΩ$ are produced via $p$-$Ω$ and $Ω$-$Ω$ coalescence, respectively, while for the six-quark states, they are formed through $uudsss$ and $ssssss$ coalescence. We find that the yield ratio $N_{pΩ}/N_Ω$ and $N_{ΩΩ}/N_Ω$ have a distinct centrality dependence between the molecular and multi-quark states, thus offering a promising way for distinguishing the two states. Our results suggest that the measurements of $pΩ$ and $ΩΩ$ production in relativistic heavy-ion collisions can shed light on their internal structures.

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