arXiv subjects

Xiaofeng Luo

Publications and source records attributed to Xiaofeng Luo.

At least 73 records · Page 4Linked to original sources

arXiv2017

Properties and uses of factorial cumulants in relativistic heavy-ion collisions

We discuss properties and applications of factorial cumulants of various particle numbers and for their mixed channels measured by the event-by-event analysis in relativistic heavy-ion collisions. After defining the factorial cumulants for systems with multi-particle species, their properties are elucidated. The uses of the factorial cumulants in the study of critical fluctuations are discussed. We point out that factorial cumulants play useful roles in understanding fluctuation observables when they have underlying physics approximately described by the binomial distribution. As examples, we suggest novel utilization methods of the factorial cumulants in the study of the momentum cut and rapidity window dependences of fluctuation observables.

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arXiv2017

Baryon Number Fluctuations in Quasi-particle Model

Baryon number fluctuations are sensitive to the QCD phase transition and QCD critical point. According to the Feynman rules of finite-temperature field theory, we calculated various order moments and cumulants of the baryon number distributions in the quasi-particle model of quark gluon plasma. Furthermore, we compared our results with the experimental data measured by the STAR experiment at RHIC. It is found that the experimental data can be well described by the model for the colliding energies above 30 GeV and show large discrepancies at low energies. It can put new constraint on qQGP model and also provide a baseline for the QCD critical point search in heavy-ion collisions at low energies.

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arXiv2017

Identifying the presence of the critical end point in QCD phase diagram by higher order susceptibilities

For the first time, we investigate susceptibilities of dense quark matter up to $8$th order using an effective model. Generally higher order susceptibilities will have more sign changes and larger magnitude, thus should give more information about the presence and location of the conjectured QCD critical end point (CEP). Two cases are studied, one with the CEP and one being crossover phase transition throughout the QCD phase diagram. It is found that a rapid crossover transition can also give similar sign pattern as the case with the CEP and yield large signal. We propose that using several kinds of ions for collision to search for a wider range on the QCD phase diagram may help.

hep-ph↗

arXiv2017

Baryon-Strangeness Correlations in Au+Au Collisions at $\sqrt{s_\mathrm{NN}}$=7.7-200 GeV from the UrQMD model

Fluctuations and correlations of conserved charges are sensitive observables for studying the QCD phase transition and critical point in high-energy heavy-ion collisions. We have studied the centrality and energy dependence of mixed-cumulants (up to fourth order) between net-baryon and net-strangeness in Au+Au collisions at $\sqrt{s_{NN}}$= 7.7, 11.5, 19.6, 27, 39, 62.4, 200 GeV from UrQMD model. To compare with other theoretical calculations, we normalize these mixed-cumulants by various order cumulants of net-strangeness distributions. We found that the results obtained from UrQMD calculations are comparable with the results from Lattice QCD at low temperature and hadron resonance gas model. The ratios of mixed-cumulants ($R_{11}^{BS},R_{13}^{BS},R_{22}^{BS}$,~$R_{31}^{BS}$) from UrQMD calculations show weak centrality dependence. However, the mixed-cumulant ratios $R_{11}^{BS}$ and $R_{31}^{BS}$ show strong increase at low energy, while the $R_{13}^{BS}$ snd $R_{22}^{BS}$ are similar at different energies. Furthermore, we have also studied the correlations between different hadron species and their contributions to the net-baryon and net-strangeness correlations. These models studies can provide baselines for searching for the signals of QCD phase transition and critical point in heavy-ion collisions.

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arXiv2016

Susceptibilities of Conserved Charges within a Modified Nambu-Jona-Lasinio Model

We evaluate the $2$nd to $4$th order baryon, charge and strangeness susceptibilities near a chiral critical point using the Nambu--Jona--Lasinio model under different temperature and baryon chemical potential. Baryon number susceptibilities are found to be of the greatest magnitude, offering the strongest signal. Whereas the strangeness susceptibilities have the smallest divergence dominating area, owing to the large strange quark mass. We also make an attempt to compare our results with experiment data. The trend at high collision energy are found to be consistent between theory and experiment. The model calculation predicts more complex behavior at low collision energies, near the postulated critical end point.

hep-ph↗

arXiv2016

Effects of Nuclear Potential on the Cumulants of Net-Proton and Net-Baryon Multiplicity Distributions in Au+Au Collisions at $\sqrt{s_{\text{NN}}} = 5\,\text{GeV}$

We analyze the rapidity and transverse momentum dependence for the cumulants of the net-proton and net-baryon distributions in Au+Au collisions at $\sqrt{s_{\text{NN}}} = 5\,\text{GeV}$ with a microscopic hadronic transport (JAM) model. To study the effects of mean field potential and softening of equation of state (EoS) on the fluctuations of net-proton (baryon) in heavy-ion collisions, the calculations are performed with two different modes. The softening of EoS is realized in the model by implementing the attractive orbit in the two-body scattering to introduce a reduction pressure of the system. By comparing the results from the two modes with the results from default cascade, we find the mean field potential and softening of EoS have strong impacts on the rapidity distributions ($\text{d}N/\text{d}y$) and the shape of the net-proton (baryon) multiplicity distributions. The net-proton (baryon) cumulants and their ratios calculated from all of the three modes are with similar trends and show significant suppression with respect to unity, which can be explained by the presence of baryon number conservations. It indicates that the effects of mean field potential and softening of EoS might be not the ingredients that are responsible to the observed strong enhancement in the most central Au+Au collisions at 7.7 GeV measured by the STAR experiment at RHIC.

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arXiv2016

Error Estimation for Moments Analysis in Heavy-Ion Collision Experiments

Fluctuations of conserved quantities are predicted to be sensitive to the correlation length and connected to the thermodynamic susceptibility. Thus, moments of net-baryon, net-charge and net-strangeness have been extensively studied theoretically and experimentally to explore phase structure and bulk properties of QCD matters created in heavy ion collision experiment. As the moments analysis is statistics hungry study, the error estimation is crucial to extract physics information from the limited experimental data. In this paper, we will derive the limit distributions and error formula based on Delta theorem in statistics for various order moments used in the experimental data analysis. The Monte Carlo simulation is also applied to test the error formula.

physics.data-an↗

arXiv2016

Cumulants of Net-Proton, Net-Kaon and Net-Charge Multiplicity Distributions in Au+Au Collisions at RHIC BES Energies from UrQMD Model

Fluctuations of conserved quantities are sensitive observables to probe the signature of QCD phase transition and critical point in heavy-ion collisions. With the UrQMD model, we have studied the centrality and energy dependence of various order cumulants and cumulant ratios (up to fourth order) of net-proton,net-charge and net-kaon multiplicity distributions in Au+Au collisions at $\sqrt{s_{NN}}$= 7.7, 11.5, 19.6, 27, 39, 62.4, 200 GeV. The model results show that the production mechanism of the particles and anti-particles have significant impacts on the cumulants of net-particles multiplicity distributions and show strong energy dependence. We also made comparisons between model calculations and experimental data measured in the first phase of the beam energy scan (BES) program by the STAR experiment at RHIC. The comparisons indicate that the baryon conservation effect strongly suppress the cumulants of net-proton distributions at low energies and the non-monotonic energy dependence for the net-proton {\KV} at the most central Au+Au collisions measured by the STAR experiment can not be described by the UrQMD model. Since there has no physics of QCD phase transition and QCD critical point implemented in the UrQMD, the model results provide us baselines and qualitative estimates about the non-critical background contributions to the fluctuations observables in heavy-ion collisions.

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arXiv2016

Importance of separated efficiencies between positively and negatively charged particles for cumulant calculations

We show the importance of separated efficiency corrections between positively and negatively charged particles for cumulant calculations by Monte Carlo toy models and analytical calculations. Our results indicate that Sσ in published net-proton results from the STAR experiment will be suppressed about 5 to 10% in central collisions, and 10 to 20% in peripheral collisions at the beam energy of \sqrt s_{NN} = 62.4 and 200 GeV if the separated efficiencies are used to efficiency correction.

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arXiv2016

Exploring the QCD Phase Structure with Beam Energy Scan in Heavy-ion Collisions

Beam energy scan programs in heavy-ion collisions aim to explore the QCD phase structure at high baryon density. Sensitive observables are applied to probe the signatures of the QCD phase transition and critical point in heavy-ion collisions at RHIC and SPS. Intriguing structures, such as dip, peak and oscillation, have been observed in the energy dependence of various observables. In this paper, an overview is given and corresponding physics implications will be discussed for the experimental highlights from the beam energy scan programs at the STAR, PHENIX and NA61/SHINE experiments. Furthermore, the beam energy scan phase II at RHIC (2019-2020) and other future experimental facilities for studying the physics at low energies will be also discussed.

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arXiv2015

Energy Dependence of Moments of Net-Proton and Net-Charge Multiplicity Distributions at STAR

We present the energy dependence of moments of net-proton and net-charge multiplicity distributions in Au+Au collisions measured by the STAR experiment in the first phase of the Beam Energy Scan (BES) at the Relativistic Heavy Ion Collider (RHIC). By using the time of flight detector for particle identification, the upper transverse momentum ($p_{T}$) limit for proton and anti-proton is extended from 0.8 GeV/c up to 2 GeV/c. The $p_{T}$ and rapidity acceptance dependence study for the moments of net-proton distribution show that the larger the acceptance is, the greater the deviation from unity. The most pronounced structure is found in the energy dependence of {\KV} of net-proton distributions from the $0\sim5%$ most central collisions within $0.4<p_{T}<2$ GeV/c and at mid-rapidity $|y|<0.5$. At energies above 39 GeV, the values of {\KV} are close to unity and for energies below 39 GeV, it shows significant deviation below unity around 19.6 and 27 GeV, then a large increase above unity is observed at 7.7 GeV.

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arXiv2015

Fluctuations of Conserved Quantities in High Energy Nuclear Collisions at RHIC

Fluctuations of conserved quantities in heavy-ion collisions are used to probe the phase transition and the QCD critical point for the strongly interacting hot and dense nuclear matter. The STAR experiment has carried out moment analysis of net-proton (proxy for net-baryon (B)), net-kaon (proxy for net-strangeness (S)), and net-charge (Q). These measurements are important for understanding the quantum chromodynamics phase diagram. We present the analysis techniques used in the moment analysis by the STAR experiment and discuss the moments of net-proton and net-charge distributions from the first phase of the Beam Energy Scan program at the Relativistic Heavy Ion Collider.

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arXiv2014

Baseline for the cumulants of net-proton distributions at STAR

We present a systematic comparison between the recently measured cumulants of the net-proton distributions by STAR for 0-5% central Au+Au collisions at $\sqrt{s_{NN}}$=7.7-200 GeV and two kinds of possible baseline measures, the Poisson and Binomial baselines. These baseline measures are assuming that the proton and anti-proton distributions independently follow Poisson statistics or Binomial statistics. The higher order cumulant net-proton data are observed to deviate from all the baseline measures studied at 19.6 and 27 GeV. We also compare the net-proton with net-baryon fluctuations in UrQMD and AMPT model, and convert the net-proton fluctuations to net-baryon fluctuations in AMPT model by using a set of formula.

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arXiv2013

Volume Fluctuation and Autocorrelation Effects in the Moment Analysis of Net-proton Multiplicity Distributions in Heavy-Ion Collisions

Moments (Variance ($σ^2$), Skewness($S$), Kurtosis($κ$)) of multiplicity distributions of conserved quantities, such as net-baryon,net-charge and net-strangeness, are predicted to be sensitive to the correlation length of the system and connected to the thermodynamic susceptibilities computed in Lattice QCD and Hadron Resonance Gas (HRG) model. In this paper, we present several measurement artifacts that could lead to volume fluctuation and auto-correlation effects in the moment analysis of net-proton multiplicity distributions in heavy-ion collisions using the UrQMD model. We discuss methods to overcome these artifacts so that the extracted moments could be used to obtain physical conclusions. In addition we present methods to properly estimate the statistical errors in moment analysis.

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arXiv2013

Beam Energy Dependence of Higher Moments of Net-proton Multiplicity Distributions in Heavy-ion Collisions at RHIC

We present various order moments (Variance ($σ^2$), Skewness($S$), Kurtosis($κ$)) of net-proton multiplicity distributions in Au+Au collisions from the first phase of RHIC beam energy scan ({\sNN}=7.7$-$200 GeV). The measurements are carried out by the STAR detector at mid-rapidity ($|y|<0.5$) and within transverse momentum range $0.4<p_{T}<0.8$ {\gevc}. The product of moments ({\SD} and \KV) are predicted to be sensitive to the correlation length and connected to ratios of baryon number susceptibility. We observe deviations below Poisson expectations in the most central collisions for all of the energies. These results are compared with a transport model to understand the effects not related to critical physics. We also discuss the error estimation methods and the techniques to suppress centrality resolution effect in the moment analysis.

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arXiv2012

Search for the QCD Critical Point by Higher Moments of Net-proton Multiplicity Distributions at STAR

We present the measurements of the higher moments of the net-proton multiplicity distributions for Au+Au collisions from the first phase of the beam energy scan program at RHIC. The measurements are carried out at midrapidity ($|y|<0.5$) and within the transverse momentum range $0.4<p_{T}<0.8$ {\gevc} by STAR. The moment products, $\KV$ and $\SD$, which are related to the ratios of various order baryon number susceptibilities and sensitive to the correlation length, are applied to search for the QCD critical point. We report the centrality and energy dependence of the moment products ({\KV} and {\SD}), results are compared with Poisson expectations. The experiment data are also compared with transport model calculation to understand the non-critical physics effects.

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arXiv2011

Search for the QCD Critical Point by Higher Moments of Net-proton Multiplicity Distributions at RHIC

Higher moments of net-proton multiplicity distributions are applied to search for the QCD critical point. In this paper, we will present measurements for kurtosis ($κ$), skewness ($S$) and variance $σ^2$) of net-proton multiplicity distributions at the mid-rapidity ($|y|<0.5$) and transverse momentum range $0.4<p_{T}<0.8$ {\gevc} for Au+Au collisions at {\sNN} = 7.7, 11.5, 39, 62.4 and 200 GeV, Cu+Cu collisions at {\sNN} = 22.4, 62.4 and 200 GeV, $d$+Au collisions at {\sNN} = 200 GeV and p+p collisions at {\sNN} = 62.4 and 200 GeV. The moment products $κσ^2$ and $S σ$ of net-proton distributions, which are related to volume independent baryon number susceptibility ratios, are consistent with Lattice QCD and Hadron Resonance Gas (HRG) model calculations at high energies ({\sNN} = 62.4 and 200 GeV). Deviations of $κσ^2$ and $Sσ$ for the Au+Au collisions at low energies ({\sNN} = 7.7, 11.5 and 39 GeV) from HRG model calculations are also observed.

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arXiv2011

Probing the QCD Critical Point with Higher Moments of Net-proton Multiplicity Distributions

Higher moments of event-by-event net-proton multiplicity distributions are applied to search for the QCD critical point in the heavy ion collisions. It has been demonstrated that higher moments as well as moment products are sensitive to the correlation length and directly connected to the thermodynamic susceptibilities computed in the Lattice QCD and Hadron Resonance Gas (HRG) model. In this paper, we will present measurements for kurtosis ($κ$), skewness ($S$) and variance ($σ^{2}$) of net-proton multiplicity distributions at the mid-rapidity ($|y|<0.5$) and $0.4<p_{T}<0.8$ GeV/$c$ for Au+Au collisions at $\sqrt{s_{NN}}$=19.6, 39, 62.4, 130 and 200 GeV, Cu+Cu collisions at $\sqrt{s_{NN}}$=22.4, 62.4 and 200 GeV, d+Au collisions at $\sqrt{s_{NN}}$=200 GeV and p+p collisions at $\sqrt{s_{NN}}$=62.4 and 200 GeV. The moment products $κσ^{2}$ and $S σ$ of net-proton distributions, which are related to volume independent baryon number susceptibility ratio, are compared to the Lattice QCD and HRG model calculations. The $κσ^{2}$ and $S σ$ of net-proton distributions are consistent with Lattice QCD and HRG model calculations at high energy, which support the thermalization of the colliding system. Deviations of $κσ^{2}$ and $S σ$ for the Au+Au collisions at low energies from HRG model calculations are also observed.

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