arXiv subjects

Lu-Meng Liu

Publications and source records attributed to Lu-Meng Liu.

18 recordsLinked to original sources

arXiv2026

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

We study the impact of neutron-skin thickness on $J/\psi$ 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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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

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 $\beta_2$, 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 $\beta_2^2$ 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 $\beta_2$ of $^{238}$U.

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arXiv2025

The $\theta$-term effects on isospin asymmetric hot and dense quark matter

We investigate the impact of the CP-violating $\theta$ term on isospin symmetry breaking in quark matter and compact star properties using a two-flavor Nambu-Jona-Lasinio (NJL) model. By incorporating the $\theta$ parameter through the Kobayashi-Maskawa-'t Hooft (KMT) determinant interaction, we derive the thermodynamic potential and gap equations under finite temperature, baryon chemical potential, and isospin chemical potential. At zero temperature and baryon density, $\theta$ suppresses conventional chiral ($\sigma$) and pion ($\pi$) condensates while promoting pseudo-scalar ($\eta$) and scalar-isovector ($\delta$) condensates, thereby reducing the critical isospin chemical potential $\mu_I^{\text{crit}}$ for spontaneous symmetry breaking. For $\theta=\pi$, a first-order phase transition emerges at $\mu_I^{\text{crit}} = 0.021$ GeV, accompanied by CP symmetry restoration. Extending the investigation to finite temperature and baryon chemical potential reveals that these $\theta$-term-induced effects persist. Axion effects (modeled via $\theta\equiv a/f_a$) stiffen the equation of state (EOS) of non-strange quark stars, increasing their maximum mass and radii, in agreement with multimessenger constraints from pulsar observations and gravitational wave events. These results establish $\theta$ as a critical parameter modulating both the Quantum Chromodynamics (QCD) phase structure and compact star observables.

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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

Directly probing existence of $α$-cluster structure in $^{20}$Ne by relativistic heavy-ion collisions

Can relativistic heavy-ion collisions only probe the global shape of colliding nuclei, or their detailed internal structure as well? Taking $^{20}$Ne as an example, we attempt to directly probe its internal $α$-cluster structure, by comparing experimentally measured observables in collisions at relativistic energies from density distributions of $^{20}$Ne with and without $α$-cluster structure. Since the two density distributions give the same nucleus size and deformation, they lead to similar mid-rapidity observables. However, the $α$-cluster structure may considerably reduce the free spectator nucleon yield and enhance the spectator light nuclei yield, as a result of more compact initial phase-space distribution of nucleons inside $α$ clusters. We propose to measure the scaled yield ratio of free spectator neutrons to charged particles with mass-to-charge ratio $A/Z = 3$, 3/2, and 2 in ultra-central $^{20}$Ne+$^{20}$Ne collisions, which is found to be reduced by about $25\%$ at $\sqrt{s_\mathrm{NN}} = 7$ TeV and about $20\%$ at $\sqrt{s_\mathrm{NN}} = 200$ GeV with $α$-cluster structure in $^{20}$Ne. This scaled yield ratio thus serves as a robust and direct probe of the existence of $α$-cluster structure in $^{20}$Ne free from the uncertainty of mid-rapidity dynamics.

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arXiv2024

QCD topology and axion properties in an isotropic hot and dense medium

We study the QCD topology and axion properties at finite temperature and chemical potential in the framework of the two-flavor Nambu$-$Jona-Lasinio model. We find that the behaviors of the two lowest cumulants of the QCD topological charge distribution and axion properties are highly sensitive to the critical behavior of the chiral phase transition. In particular, the topological susceptibility and the axion mass follow the response of the chiral condensate to temperature and chemical potential, showing that both quantities decrease monotonically with the increment of temperature and/or chemical potential. However, it is important to note that the normalized fourth cumulant behaves differently depending on the temperature. At low temperatures, it is a non-monotonic function of the chemical potential, while at high temperatures, it monotonically decreases. Additionally, its value invariably approaches the asymptotic value of $b_2^{\text {inst }}=-1/12$, predicted by the dilute instanton gas model. We also observe that with the increase in chemical potential at relatively low temperatures, the axion self-coupling constant exhibits a sharp peak around the critical point, which can even be more than twice its vacuum value. After that, the self-coupling drops sharply to a much lower value than its vacuum value, eventually approaching zero in the high chemical potential limit. The finding that the axion self-coupling constant is significantly enhanced in high-density environments near the chiral phase transition could lead to the creation or enhancement of an axion Bose-Einstein condensate in compact astrophysical objects.

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arXiv2024

Investigating the possibility of extracting neutron-skin thickness in nuclei by their collisions at intermediate energies

Inspired by various studies on extracting the density distributions of nuclei from their collisions at ultrarelativistic energies, in the present work we investigate the possibility of extracting the neutron-skin thickness $Δr_{np}$ in nuclei by their collisions at intermediate energies. We have analyzed the free neutron-to-proton yield ratio $n/p$ as a candidate probe at both midrapidities and forward rapidities in peripheral and central $^{124}$Sn+$^{124}$Sn collisions based on an isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, and found that the resulting $n/p$ yield ratio is more sensitive to the symmetry potential in the collision dynamics than to the initial $Δr_{np}$ in colliding nuclei in most cases. The largest effect on the $n/p$ yield ratio from the initial $Δr_{np}$ is observed for nucleons at large transverse or longitudinal momenta in central collisions at the collision energy of a few GeV/nucleon.

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arXiv2024

Deformation probes for light nuclei in their collisions at relativistic energies

We have investigated the performance of anisotropic flows $\langle v_n^2 \rangle$, transverse momentum fluctuations $\langle δp_T^2 \rangle $, and their correlations $\langle v_n^2 δp_T \rangle$ in central collisions at relativistic energies as probes of deformation parameters $β_n$ of colliding nuclei, if these nuclei are light nuclei with large $β_n$ and different configurations of $α$ clusters. The effects from higher-order $β_n$ terms are illustrated by derived relations based on the overlap of two nuclei with uniform density distributions and by dynamic simulations of collisions of heavy nuclei whose density distributions are of a deformed Woods-Saxon (WS) form. While the linear relations between $β^2_n$, $\langle v_n^2 \rangle$, and $\langle δp_T^2 \rangle$ and that between $β^3_n$ and $\langle v_n^2 δp_T \rangle$ can be violated for extremely large $β_{n}$, they are mostly valid for realistic values of $β_n$, as long as the density distribution of colliding nuclei can be described by a deformed WS form. However, these linear relations are generally not valid with more realistic density distributions of light nuclei with $α$ clusters, and the amount of deviation depends on the detailed $α$-cluster configurations. Care must be taken when one tries to extract the deformation of light nuclei, and specific probes for $α$-cluster structures in these nuclei are very much needed.

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arXiv2024

Probing configuration of $α$ clusters with spectator particles in relativistic heavy-ion collisions

We propose to use spectator particle yield ratios to probe the configuration of $α$ clusters in $^{12}$C and $^{16}$O by their collisions at RHIC and LHC energies. The idea is illustrated based on initial density distributions with various $α$-cluster configurations generated by a microscopic cluster model, and without $α$ clusters from mean-field calculations. The multifragmentation of the spectator matter produces more spectator light nuclei including $α$ clusters in collisions of nuclei with chain structure of $α$ clusters, compared to those of nuclei with a more compact structure. The yield ratio of free spectator neutrons to spectator particles with mass-to-charge ratio $A/Z=2$ scaled by their masses can be practically measured by the zero-degree calorimeter (ZDC) at RHIC and LHC, serving as a clean probe free from modeling the complicated dynamics at midrapidities.

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arXiv2024

Three-dimensional QCD phase diagram in the pNJL model

Based on the three-flavor Polyakov-looped Nambu$-$Jona-Lasinio (pNJL) model, we have studied the structure of the three-dimensional QCD phase diagram with respect to the temperature, the baryon chemical potential, and the isospin chemical potential, by investigating the interplay among the chiral quark condensate, the pion condensate, and the Polyakov loop. While the pNJL model leads to qualitatively similar structure of the normal quark phase, the pion superfluid phase, and the Sarma phase as well as their phase boundaries, when compared to the NJL model, the inclusion of the Polyakov loop enlarges considerably the areas of the pion superfluid phase and the Sarma phase, and leads to critical end points at higher temperatures. With the contribution of the gluon dynamics effectively included, the present study is expected to give a more reliable prediction of the three-dimensional QCD phase diagram compared to that in the NJL model.

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arXiv2023

Measuring deformed neutron skin with free spectator nucleons in relativistic heavy-ion collisions

The neutron skin in deformed nuclei is generally not uniformly distributed but has an angular distribution, depending on both the spin-dependent nuclear interaction and the nuclear symmetry energy. To extract the information of the deformed neutron skin, we have explored the possibility of using free spectator nucleons in central tip-tip and body-body collisions at top RHIC energy with four typical deformed nuclei. The density distributions of neutrons and protons are consistently obtained from the Skyrme-Hartree-Fock-Bogolyubov calculation, and the angular distribution of the neutron skin can be varied by adjusting the strength of the nuclear spin-orbit coupling. With the information of spectator nucleons obtained based on a Monte-Carlo Glauber model, the free spectator nucleons are generated from a multifragmentation process. By investigating the results from different systems and with different collision configurations, we found that although it is difficult to probe the deformed neutron skin in $^{96}$Zr and $^{238}$U by their collisions, it is promising to extract the polar angular distributions of the neutron skin in $^{96}$Ru and $^{197}$Au by comparing the yield ratios of free spectator neutrons to protons in their central tip-tip and body-body collisions. The proposed observables can be measured by dedicated zero-degree calorimeters in heavy-ion collision experiments that have been carried out in recent years by RHIC.

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arXiv2023

Collision geometry effect on free spectator nucleons in relativistic heavy-ion collisions

Based on the deformed nucleon distributions obtained from the constrained Skyrme-Hartree-Fock-Bogolyubov calculation using different nuclear symmetry energies, we have investigated the effects of the neutron skin and the collision geometry on the yield of free spectator nucleons as well as the yield ratio $N_n/N_p$ of free spectator neutrons to protons in collisions of deformed nuclei at RHIC energies. We found that tip-tip (body-body) collisions with prolate (oblate) nuclei lead to fewest free spectator nucleons, compared to other collision configurations. While the $N_n/N_p$ ratio is sensitive to the average neutron-skin thickness of colliding nuclei and the symmetry energy, it is affected by the polar angular distribution of the neutron skin in different collision configurations. We also found that the collision geometry effect can be as large as 50% the symmetry energy effect in some collision systems. Due to the particular deformed neutron skin in $^{238}$U and $^{96}$Zr, the symmetry energy effect on the $N_n/N_p$ ratio is enhanced in tip-tip $^{238}$U+$^{238}$U collisions and body-body $^{96}$Zr+$^{96}$Zr collisions compared to other collision orientations in the same collision system. Our study may shed light on probing deformed neutron skin by selecting desired configurations in high-energy collisions with deformed nuclei.

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arXiv2022

Probing neutron-skin thickness with free spectator neutrons in ultracentral high-energy isobaric collisions

We show that the yield ratio of free spectator neutrons produced in high-energy $^{96}$Zr+$^{96}$Zr to $^{96}$Ru+$^{96}$Ru collisions is a clean probe of the neutron-skin thickness of colliding nuclei and the slope parameter $L$ of the symmetry energy. The idea is demonstrated based on the proton and neutron density distributions via a state-of-the-art Skyrme-Hartree-Fock-Bogolyubov calculation. Among spectator nucleons given by the Glauber model, free spectator neutrons include those from direct production that survive from clusterization as well as those from deexcitation of heavy clusters described by the popular GEMINI model. More free neutrons are produced in collisions of $^{96}$Zr nucleus due to its larger neutron skin, compared to those produced in collisions of $^{96}$Ru nucleus with a smaller neutron skin. The difference of the free spectator neutron yield is further increased with the increasing difference of the neutron-skin thickness between $^{96}$Zr and $^{96}$Ru with a larger $L$ value, and the increase in ultracentral collisions is particularly insensitive to model details and experimental uncertainties. Since the production of free spectator neutrons is not affected by the complicated dynamics in the mid-rapidity region, the ratio of their multiplicities in ultracentral isobaric collisions is a robust observable for constraining the neutron skin and $L$ value.

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arXiv2022

Free spectator nucleons in ultracentral relativistic heavy-ion collisions as a probe of neutron skin

Besides the yield ratio of free spectator neutrons produced in ultracentral $^{96}$Zr+$^{96}$Zr to $^{96}$Ru+$^{96}$Ru collisions, we propose that the yield ratio $N_n/N_p$ of free spectator neutrons to protons in a single collision system at RHIC and LHC can be a more sensitive probe of the neutron-skin thickness $Δr_{\mathrm{np}}$ and the slope parameter $L$ of the symmetry energy. The idea is demonstrated based on the proton and neutron density distributions of colliding nuclei obtained from Skyrme-Hartree-Fock-Bogolyubov calculations, and a Glauber model that provides information of spectator matter. The final spectator particles are produced from direct emission, clusterization by a minimum spanning tree algorithm or a Wigner function approach, and deexcitation of heavy clusters by GEMINI. A larger $Δr_{\mathrm{np}}$ associated with a larger $L$ value increases the isospin asymmetry of spectator matter and thus leads to a larger $N_n/N_p$, especially in ultracentral collisions where the multiplicity of free nucleons are free from the uncertainties of cluster formation and deexcitation. We have further shown that the double ratio of $N_n/N_p$ in isobaric collision systems or in collisions by isotopes helps to cancel the detecting efficiency for protons. Effects from nuclear deformation and electromagnetic excitation are studied, and they are found to be subdominant compared to the expected sensitivity to $Δr_{\mathrm{np}}$.

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arXiv2021

Observation of different scenarios in different temperatures in small and large collision systems

We used the modified Hagedron function and analyzed the experimental data measured by the BRAHMS, STAR, PHENIX and ALICE Collaborations in Copper-Copper, Gold-Gold, deuteron-Gold, Lead-Lead, proton-Lead and proton-proton collisions, and extracted the related parameters (kinetic freeze-out temperature, transverse flow velocity, kinetic freeze-out volume, mean transverse momentum and initial temperature) from the transverse momentum spectra of the particles (non-strange and strange particles). We observed that all the above parameters decrease from central to peripheral collisions, except transverse flow velocity which remains unchanged from central to peripheral collisions. The kinetic freeze-out temperature depends on the cross-section interaction of the particle such that larger cross-section of the particle corresponds to smaller T0, and reveals the two kinetic freeze-out scenario, while the initial temperature depends on the mass of the particle and it increase with the particle mass. The transverse flow velocity and mean transverse momentum depends on the mass of the particle and the former decrease while the later increase with the particle mass. In addition, the kinetic kinetic freeze-out volume also decrease with particle mass which reveals the volume differential freeze-out scenario and indicates different freeze-out surfaces for different particles. We also extracted the entropy index-parameter n and the parameter N0, and the former remains almost unchanged while the later decrease from central to peripheral collisions. Furthermore, the kinetic freeze-out temperature, transverse flow velocity, kinetic freeze-out volume, initial temperature, mean transverse momentum and the parameter N0 at LHC are larger than that of RHIC, and they show their dependence on the collision cross-section as well as on collision energy at RHIC and LHC.

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arXiv2021

Isospin effect on quark matter instabilities

We have studied the mechanical and chemical instabilities as well as the liquid-gas-like phase transition in isospin asymmetric quark matter based on the NJL and the pNJL model. Areas of the mechanical instability region and the liquid-gas coexistence region are seen to be enlarged with a larger quark matter symmetry energy or in the presence of strange quarks. Our study shows that the light cluster yield ratio observed in relativistic heavy-ion collisions may not be affected much by the uncertainties of the isospin effect, and favors a smooth hadron-quark phase transition in compact stars as well as their mergers.

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arXiv2021

3-dimensional QCD phase diagram with pion condensate in the NJL model

With the isovector coupling constants adjusted to reproduce the physical pion mass and lattice QCD results in baryon-free quark matter, we have carried out rigourous calculations for the pion condensate in the 3-flavor Nambu-Jona-Lasinio model, and studied the 3-dimensional QCD phase diagram. With the increasing isospin chemical potential $μ_I$, we have observed two nonzero solutions of the pion condensate at finite baryon chemical potentials $μ_B$, representing respectively the pion superfluid phase and the Sarma phase, and their appearance and disappearance correspond to a second-order (first-order) phase transition at higher (lower) temperatures $T$ and lower (higher) $μ_B$. Calculations by assuming equal constituent mass of $u$ and $d$ quarks would lead to large errors of the QCD phase diagram within $μ_B \in (500, 900)$ MeV, and affect the position of the critical end point.

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