arXiv subjects

Xiaofeng Luo

Publications and source records attributed to Xiaofeng Luo.

At least 55 records · Page 3Linked to original sources

arXiv2020

Centrality selection effect on higher-order cumulants of net-proton multiplicity distributions in relativistic heavy-ion collisions

We studied the centrality selection effect on cumulants (up to fourth order) and the cumulants ratios of net-proton multiplicity distributions in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 7.7, 19.6 and 200 GeV from UrQMD model. The net-proton cumulants are calculated with collision centralities by using charged particle multiplicity from different pesudorapidity ($η$) region. By comparing the results from various collision centralities, we found that the autocorrelation effects are not significant in the results with collision centralities "refmult-3" and "refmult-2", which are using mid-rapidity charged particles but excluding (anti-)protons and analysis region, respectively. Furthermore, due to the contributions of spectator protons, we observed poor centrality resolution when using charged particles at forward $η$ region at low energies. This work can serve as a baseline for centrality selection of future fluctuations analysis in relativistic heavy-ion collisions.

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arXiv2020

Light Nuclei Production in Au+Au Collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5-200 GeV from JAM model

Light nuclei production is sensitive to the baryon density fluctuations and can be used to probe the QCD phase transition in relativistic heavy-ion collisions. In this work, we studied the production of proton, deuteron, triton in central Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5, 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4 and 200 GeV from a transport model (JAM). Based on the coalescence production of light nuclei, we calculated the energy dependence of rapidity density $dN/dy$ and particle ratios ($d/p$, $t/p$, and $t/d$). More importantly, the yield ratio $N_{t} \times N_{p} / N_{d}^{2}$, which is sensitive to the neutron density fluctuations, shows a flat energy dependence and cannot describe the non-monotonic trend observed by the STAR experiment. Based on the nucleon coalescence, this work can provide constraint and reference to search for the QCD critical point and/or first order phase transition with light nuclei production in future heavy-ion collision experiments.

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arXiv2020

Chiral phase transition in a rotating sphere

We study the chiral phase transition of the two-flavor Nambu-Jona-Lasinio (NJL) model in a rotating sphere, which includes both rotation and finite size effects. We find that rotation leads to a suppression of the chiral condensate at a finite temperature, while its effects are smaller than the finite size effects. Our work can be helpful to study the effects relevant to rotation in heavy-ion collisions in a more realistic way.

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arXiv2020

A Study of the Properties of the QCD Phase Diagram in High-Energy Nuclear Collisions

With the aim of understanding the phase structure of nuclear matter created in high-energy nuclear collisions at finite baryon density, a beam energy scan program has been carried out at Relativistic Heavy Ion Collider (RHIC). In this mini-review, most recent experimental results on collectivity, criticality and heavy flavor productions will be discussed. The goal here is to establish the connection between current available data and future heavy-ion collision experiments in a high baryon density region.

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arXiv2020

Effects of resonance weak decays and hadronic re-scattering on the proton number fluctuations in Au + Au collisions at $\sqrt{s_\mathrm{NN}} = 5$ GeV from JAM model

Proton number fluctuation is sensitive observable to search for the QCD critical point in heavy-ion collisions. In this paper, we studied rapidity acceptance dependence of the proton cumulants and correlation functions in most central Au+Au collisions at $\sqrt{s_\mathrm{NN}} = 5$ GeV from a microscopic hadronic transport model (JAM). At mid-rapidity, we found the effects of resonance weak decays and hadronic re-scattering on the proton cumulants and correlation functions are small, but those effects get larger when further increasing the rapidity acceptance. On the other hand, we found the baryon number conservation is a dominant background effect on the rapidity acceptance dependence of proton number fluctuations. It leads to a strong suppression of cumulants and cumulant ratios, as well as the negative proton correlation functions. We also studied those two effects on the energy dependence of cumulant ratios of net-proton distributions in most central Au+Au collisions at $\sqrt{s_\mathrm{NN}} = 5-200$ GeV from JAM model. This work can serve as a non-critical baseline for future QCD critical point search in heavy-ion collisions at high baryon density region.

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arXiv2019

QCD phase diagram in chiral imbalance with self-consistent mean field approximation

We employ a new self-consistent mean field approximation of NJL model, which introduces a free parameter $α$ ($α$ reflects the weight of different interaction channels), to study the effects of the chiral chemical potential $μ_5$ on QCD phase structure, especially the location of the QCD critical end point (CEP). We find that, at a high temperature, the critical temperature of QCD phase transition smoothly increases with $μ_5$ at the beginning, and then decreases rapidly. At low temperature and high baryon density region, the increase of the chiral chemical potential will reduce the critical chemical potential of phase transition. The temperature of the CEP shows a non-monotonic dependence on the chiral chemical potential with a long plateau around the maximum. At $μ_5=0$, we found that the CEP will disappear when the $α$ value is larger than 0.71 and will reappear when the $μ_5$ increases. This study is important for exploring the QCD phase structure and the location of CEP in chiral imbalanced systems.

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arXiv2019

Search for the QCD Critical Point by Transverse Velocity Dependence of Anti-deuteron to Deuteron Ratio

We propose the transverse velocity ($β_T$) dependence of the anti-deuteron to deuteron ratio as a new observable to search for the QCD critical point in heavy-ion collisions. The QCD critical point can attract the system evolution trajectory in the QCD phase diagram, which is known as focusing effect. To quantify this effect, we employ thermal model and hadronic transport model to simulate the dynamical particle emission along a hypothetical focusing trajectory near critical point. We found the focusing effect can lead to anomalous $β_T$ dependence of $\bar{p}/p$, $\bar{d}/d$ and $^3\overline{\text{He}}/^3\text{He}$ ratios. We examined the $β_T$ dependence of $\bar{p}/p$ and $\bar{d}/d$ ratios of central Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7 to 200 GeV measured by the STAR experiment at RHIC. Surprisingly, we only observe a negative slope in $β_T$ dependence of $\bar{d}/d$ ratio at $\sqrt{s_{NN}}$ = 19.6 GeV, which indicates the trajectory evolution has passed through the critical region. In the future, we could constrain the location of the critical point and/or width of the critical region by making precise measurements on the $β_T$ dependence of $\bar{d}/d$ ratio at different energies and rapidity.

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arXiv2019

Finite volume effects on quarkonium dissociation temperature in an impenetrable QGP sphere

The system of a quarkonium confined by an impenetrable spherical cavity filled with a hot quantum chromodynamics (QCD) medium is studied by solving the Schrödinger equation. This is the first time this issue has been raised for discussion. The Schrödinger equation with an appropriate boundary condition of a quarkonium in an impenetrable cavity filled with a hot medium is derived. The numerical results are obtained with the help of Gaussian Expansion Method. Binding energies and radii of the ground and low-excited states are obtained as a function of the medium temperature and the cavity radius. We find the behaviour of quarkonium in this cavity is different from that in infinite space. Our results show that the quarkonium dissociation temperature decreases as the cavity radius decreases and the finite volume effects on the ground state are more obvious than on the excited states. We also find that the less mass of the constituents and the bigger radius of the quarkonium lead the finite volume effects to become more obvious.

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arXiv2019

Efficiency correction for cumulants of multiplicity distributions based on track-by-track efficiency

We propose a simplified procedure for the experimental application of the efficiency correction on higher order cumulants in heavy-ion collisions. By using the track-by-track efficiency, we can eliminate possible bias arising from the average efficiencies calculated within the arbitrary binning of the phase space. Furthermore, the corrected particle spectra is no longer necessary for the average efficiency estimation and the time cost for the calculation of bootstrap statistical error can be significantly reduced.

physics.data-an↗

arXiv2019

Particle decay from statistical thermal model in high energy nucleus-nucleus collision

In high energy nucleus-nucleus collisions, it is difficult to measure the contributions of resonance strong decay and weak decay to the final measured hadrons as well as the corresponding effects on some physical observables. To provide a reference from statistical thermal model, we performed a systematic analysis for the energy dependence of particle yield and yield ratios in Au + Au collisions. We found that the primary fraction of final hadrons decreases with increasing collision energy and somehow saturates around $\sqrt{s_\textrm{NN}}$ = 10 GeV, indicating a limiting temperature in hadronic interactions. The fraction of strong or weak decay for final hadrons show a different energy dependence behavior comparing to the primarily produced hadrons. These energy dependences of various particle yield and yield ratios from strong or weak decay can provide us with baselines for many hadronic observables in high energy nucleus-nucleus collisions.

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arXiv2018

Event-by-Event Efficiency Fluctuations and Efficiency Correction for Cumulants of Superposed Multiplicity Distributions in Relativistic Heavy-ion Collision Experiments

We performed systematic studies on the effects of event-by-event efficiency fluctuations on efficiency correction for cumulant analysis in relativistic heavy-ion collision experiments. Experimentally, particle efficiencies of events measured under different experimental conditions should be different. For fluctuation measurements, the final event-by-event multiplicity distributions should be the superposed distributions of various type of events measured under different conditions. We demonstrate efficiency fluctuation effects using numerical simulation, in which we construct an event ensemble consisting of events with two different efficiencies. By using the mean particle efficiencies, we find that the efficiency corrected cumulants show large deviations from the original inputs when the discrepancy between the two efficiencies is large. We further studied the effects of efficiency fluctuations for the cumulants of net-proton distributions by implementing the UrQMD events of Au+Au collisions at $\sqrt{s_{NN}}=7.7$ GeV in a realistic STAR detector acceptance. We consider the unequal efficiency in two sides of the Time Projection Chamber (TPC), multiplicity dependent efficiency, and the event-by-event variations of the collision vertex position along the longitudinal direction ($V_\mathrm{z}$). When the efficiencies fluctuate dramatically within the studied event sample, the effects of efficiency fluctuations have significant impacts on the efficiency corrections of cumulants with the mean efficiency. We find that this effect can be effectively suppressed by binning the entire event ensemble into various sub-event samples, in which the efficiency variations are relatively small. The final efficiency corrected cumulants can be calculated from the weighted average of the corrected factorial moments of the sub-event samples with the mean efficiency.

physics.data-an↗

arXiv2018

Beam energy dependence of squeeze-out effect on the directed and elliptic flow in Au+Au collisions at high baryon density region

We present a detailed analysis of the beam energy dependence of the mechanisms for the generation of directed and elliptic flows in Au+Au collisions focusing on the role of hadronic rescattering and spectator shadowing within a microscopic transport model JAM with different equation of state. A systematic study of the beam energy dependence is performed for Au+Au collisions at $\sqrt{s_{NN}} =2.3 - 62.4$ GeV. The transition of the dynamical origin of the directed flow is observed. We find that the initial Glauber type nucleon-nucleon collisions generate negative $v_1$ for nucleons at midrapidity due to the presence of spectator matter, and this negative nucleon $v_1$ is turned to be positive by the meson-baryon interactions at the beam energy region of $\sqrt{s_{NN}} < 30$ GeV. In contrast, above 30 GeV there is no spectator shadowing at midrapidity, and initial nucleon-nucleon collisions do not generate directed flow, but subsequent rescatterings among produced particles generate negative $v_1$ for nucleons. It is demonstrated that negative pion-directed flows are mostly generated by the interaction with the spectator matter. It is also shown that the squeeze-out effect is largely suppressed in the case of softening, which leads to the enhancement of elliptic flow around $\sqrt{s_{NN}}=5-7$ GeV. The elliptic flow at midrapidity above 10 GeV is not influenced by the squeeze-out due to spectator matter, while its effect is seen at the forward rapidity range of $y/y_\mathrm{c.m.}>0.5$, which decreases as beam energy increases.

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arXiv2017

Effects of mean-field and the softening of equation of state on elliptic flow in Au+Au collision at $\sqrt{s_{NN}}$ = 5 GeV from JAM model

We perform a systematic study of elliptic flow ($v_2$) in Au+Au collision at $\sqrt{s_{NN}}=5$ GeV by using a microscopic transport model JAM. The centrality, pseudorapidity, transverse momentum and beam energy dependence of $v_2$ for charged as well as identified hadrons are studied. We investigate the effects of both the hadronic mean-field and the softening of equation of state (EoS) on elliptic flow. The softening of EoS is realized by imposing attractive orbits in two body scattering, which can reduce the pressure of the system. We found that the softening of EoS leads to the enhancement of $v_2$, while the hadronic mean-field suppresses $v_2$ relative to the cascade mode. It indicates that elliptic flow at high baryon density regions is highly sensitive to the EoS and the enhancement of $v_2$ may probe the signature of a first-order phase transition in heavy-ion collisions at beam energies of a strong baryon stopping region.

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arXiv2017

Search for the QCD Critical Point with Fluctuations of Conserved Quantities in Relativistic Heavy-Ion Collisions at RHIC : An Overview

Fluctuations of conserved quantities, such as baryon, electric charge and strangeness number, are sensitive observables in relativistic heavy-ion collisions to probe the QCD phase transition and search for the QCD critical point. In this paper, we review the experimental measurements of the cumulants (up to fourth order) of event-by-event net-proton (proxy for net-baryon), net-charge and net-kaon (proxy for net-strangeness) multiplicity distributions in Au+Au collisions at $\sqrt{s_{NN}}=7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, 200$ GeV from the first phase of beam energy scan program at the Relativistic Heavy-Ion Collider (RHIC). We also summarize the data analysis methods of suppressing the volume fluctuations, auto-correlations and the unified description of efficiency correction and error estimation. Based on theoretical and model calculations, we will discuss the characteristic signatures of critical point as well as backgrounds for the fluctuation observables in heavy-ion collisions. The physics implications and the future second phase of the beam energy scan (2019-2020) at RHIC will be also discussed.

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arXiv2017

Proton Cumulants and Correlation Functions in Au + Au Collisions at $\sqrt{s_\mathrm{NN}}$=7.7-200 GeV from UrQMD Model

We studied the acceptance dependence of proton cumulants (up to fourth order) and correlation functions in 0--5\% most central Au+Au collisions at $\sqrt{s_\text{NN}}$=7.7, 11.5, 19.6, 27, 39, 62.4 and 200 GeV from UrQMD model. We found that high order proton cumulants show suppressions at large acceptance. By decomposing the proton cumulants into linear combination of multi-proton correlation functions, we observed the two-proton correlation functions always show negative values due to the effects of baryon number conservations. The three and four-proton correlation functions are close to zero and show negligible acceptance dependence. We further observed that the proton cumulants and correlation functions follow similar trends and show a scaling behavior when plotting the results versus mean number of protons. The comparisons between experimental data and the UrQMD calculations show that the non-monotonic energy dependence of proton correlation functions measured by STAR experiment cannot be described by the UrQMD model. The UrQMD calculations can provide us baselines for the experimental studies of the proton cumulants and correlation functions. Finally, we propose to measure the rapidity dependence of the reduced proton correlation functions to search for the QCD critical point in heavy-ion collisions.

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arXiv2017

Unified Description of Efficiency Correction and Error Estimation for Moments of Conserved Quantities in Heavy-Ion Collisions

We provide a unified description of efficiency correction and error estimation for moments of conserved quantifies in heavy-ion collisions. Moments and cumulants are expressed in terms of the factorial moments, which can be easily corrected for the efficiency effect. By deriving the covariance between factorial moments, one can obtain the general error formula for the efficiency corrected moments based on the error propagation derived from the Delta theorem. The skellam distribution based Monto Carlo simulation is used to test the Delta theorem and Bootstrap error estimation methods. The statistical errors calculated from the two methods can well reflect the statistical fluctuations of the efficiency corrected moments.

physics.data-an↗

arXiv2017

Enhancement of elliptic flow can signal a first order phase transition in high energy heavy ion collisions

The beam energy dependence of the elliptic flow,$v_2$, is studied in mid-central Au+Au collisions in the energy range of $3\leq \sqrt{s_{NN}} \leq 30$ GeV within the microscopic transport model JAM. The results of three different modes of JAM are compared; cascade-,hadronic mean field-, and a new mode with modified equations of state, with a first order phase transition (1.O.P.T.) and with a crossover transition. The standard hadronic mean field suppresses $v_2$, while the inclusion of the effects of a 1.O.P.T. (and also of a crossover transition) does enhance $v_2$ at $\sqrt{s_{NN}}<30$ GeV. The enhancement or suppression of the scaled energy flow, dubbed "elliptic flow"is understood as being due to out of plane- flow, i.e. $v_2<0$, dubbed out of plane - "squeeze-out", which occurs predominantly in the early, compression stage. Subsequently, the in-plane flow dominates, in the expansion stage, $v_2 > 0$. The directed flow, dubbed "bounce- off", is an independent measure of the pressure, which quickly builds up the transverse momentum transfer in the reaction plane. When the spectator matter leaves the participant fireball region, where the highest compression occurs, a hard expansion leads to larger $v_2$. A combined analysis of the three transverse flow coefficients, radial $v_0$-, directed $v_1$- and elliptic $v_2$- flow, in the beam energy range of $3\leq\sqrt{s_{NN}}\leq10$ GeV, distinguishes the different compression and expansion scenarios: a characteristic dependence on the early stage equation of state is observed. The enhancement of both the elliptic and the transverse radial flow and the simultaneous collapse of the directed flow of nucleons offers a clear signature if 1.O.P.T. is realized at the highest baryon densities created in high energy heavy-ion collisions.

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arXiv2017

Cumulants of event-by-event net-strangeness distributions in Au+Au collisions at $\sqrt{s_\mathrm{NN}}$=7.7-200 GeV from UrQMD model

Fluctuations of conserved quantities, such as baryon, electric charge and strangeness number, are sensitive observables in heavy-ion collisions to search for the QCD phase transition and critical point. In this paper, we performed a systematical analysis on the various cumulants and cumulant ratios of event-by-event net-strangeness distributions in Au+Au collisions at $\sqrt{s_{NN}}$=7.7, 11.5, 19.6, 27, 39, 62.4 and 200 GeV from UrQMD model. We performed a systematical study on the contributions from various strange baryons and mesons to the net-strangeness fluctuations. The results demonstrate that the cumulants and cumulant ratios of net-strangeness distributions extracted from different strange particles show very different centrality and energy dependence behavior. By comparing with the net-kaon fluctuations, we found that the strange baryons play an important role in the fluctuations of net-strangeness. This study can provide useful baselines to study the QCD phase transition and search for the QCD critical point by using the fluctuations of net-strangeness in heavy-ion collisions experiment. It can help us to understand non-critical physics contributions to the fluctuations of net-strangeness.

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