arXiv subjects

X. Z. Cai

Publications and source records attributed to X. Z. Cai.

At least 127 records · Page 7Linked to original sources

arXiv2010

Azimuthal di-hadron correlations in d+Au and Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV from STAR

Yields, correlation shapes, and mean transverse momenta \pt{} of charged particles associated with intermediate to high-\pt{} trigger particles ($2.5 < \pt < 10$ \GeVc) in d+Au and Au+Au collisions at $\snn=200$ GeV are presented. For associated particles at higher $\pt \gtrsim 2.5$ \GeVc, narrow correlation peaks are seen in d+Au and Au+Au, indicating that the main production mechanism is jet fragmentation. At lower associated particle $\pt < 2$ \GeVc, a large enhancement of the near- ($\dphi \sim 0$) and away-side ($\dphi \sim π$) associated yields is found, together with a strong broadening of the away-side azimuthal distributions in Au+Au collisions compared to d+Au measurements, suggesting that other particle production mechanisms play a role. This is further supported by the observed significant softening of the away-side associated particle yield distribution at $\dphi \sim π$ in central Au+Au collisions.

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arXiv2010

An Experimental Exploration of the QCD Phase Diagram: The Search for the Critical Point and the Onset of De-confinement

The QCD phase diagram lies at the heart of what the RHIC Physics Program is all about. While RHIC has been operating very successfully at or close to its maximum energy for almost a decade, it has become clear that this collider can also be operated at lower energies down to 5 GeV without extensive upgrades. An exploration of the full region of beam energies available at the RHIC facility is imperative. The STAR detector, due to its large uniform acceptance and excellent particle identification capabilities, is uniquely positioned to carry out this program in depth and detail. The first exploratory beam energy scan (BES) run at RHIC took place in 2010 (Run 10), since several STAR upgrades, most importantly a full barrel Time of Flight detector, are now completed which add new capabilities important for the interesting physics at BES energies. In this document we discuss current proposed measurements, with estimations of the accuracy of the measurements given an assumed event count at each beam energy.

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arXiv2010

Higher Moments of Net-proton Multiplicity Distributions at RHIC

We report the first measurements of the kurtosis (κ), skewness (S) and variance (σ^2) of net-proton multiplicity (N_p - N_pbar) distributions at midrapidity for Au+Au collisions at \sqrt(s_NN) = 19.6, 62.4, and 200 GeV corresponding to baryon chemical potentials (μ_B) between 200 - 20 MeV. Our measurements of the products κσ^2 and S σ, which can be related to theoretical calculations sensitive to baryon number susceptibilities and long range correlations, are constant as functions of collision centrality. We compare these products with results from lattice QCD and various models without a critical point and study the \sqrt(s_NN) dependence of κσ^2. From the measurements at the three beam energies, we find no evidence for a critical point in the QCD phase diagram for μ_B below 200 MeV.

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arXiv2010

K*0 production in Cu+Cu and Au+Au collisions at \sqrt{s_NN} = 62.4 GeV and 200 GeV

We report on K*0 production at mid-rapidity in Au+Au and Cu+Cu collisions at \sqrt{s_{NN}} = 62.4 and 200 GeV collected by the Solenoid Tracker at RHIC (STAR) detector. The K*0 is reconstructed via the hadronic decays K*0 \to K+ pi- and \bar{K*0} \to K-pi+. Transverse momentum, pT, spectra are measured over a range of pT extending from 0.2 GeV/c to 5 GeV/c. The center of mass energy and system size dependence of the rapidity density, dN/dy, and the average transverse momentum, , are presented. The measured N(K*0)/N(K) and N(ϕ)/N(K*0) ratios favor the dominance of re-scattering of decay daughters of K*0 over the hadronic regeneration for the K*0 production. In the intermediate pT region (2.0 < pT < 4.0 GeV/c), the elliptic flow parameter, v2, and the nuclear modification factor, RCP, agree with the expectations from the quark coalescence model of particle production.

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arXiv2010

The roles of deformation and orientation in heavy-ion collisions induced by light deformed nuclei at intermediate energy

The reaction dynamics of axisymmetric deformed $^{24}$Mg + $^{24}$Mg collisions have been investigated systematically by an isospin-dependent quantum molecular dynamics (IDQMD) model. It is found that different deformations and orientations result in apparently different properties of reaction dynamics. We revealed that some observables such as nuclear stopping power ($R$), multiplicity of fragments, and elliptic flow are very sensitive to the initial deformations and orientations. There exists an eccentricity scaling of elliptic flow in central body-body collisions with different deformations. In addition, the tip-tip and body-body configurations turn out to be two extreme cases in central reaction dynamical process.

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arXiv2010

Upsilon cross section in p+p collisions at sqrt(s) = 200 GeV

We report on a measurement of the Upsilon(1S+2S+3S) -> e+e- cross section at midrapidity in p+p collisions at sqrt(s)=200 GeV. We find the cross section to be 114 +/- 38 (stat.) +23,-24 (syst.) pb. Perturbative QCD calculations at next-to-leading order in the Color Evaporation Model are in agreement with our measurement, while calculations in the Color Singlet Model underestimate it by 2 sigma. Our result is consistent with the trend seen in world data as a function of the center-of-mass energy of the collision and extends the availability of Upsilon data to RHIC energies. The dielectron continuum in the invariant mass range near the Upsilon is also studied to obtain a combined cross section of Drell-Yan plus (b b-bar) -> e+e-.

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arXiv2010

Collision system size dependence of dihadron azimuthal correlations in ultra-relativistic heavy ion collisions

The system size dependence of dihadron azimuthal correlations in ultra-relativistic heavy ion collision is simulated by a multi-phase transport model. The structure of correlation functions and yields of associated particles show clear participant path-length dependences in collision systems with a partonic phase. The splitting parameter ($D$) and Root Mean Square Width ($Δϕ_{rms}$) of away side correlation functions increase with collision system size from $^{14}$N+$^{14}$N to $^{197}$Au+$^{197}$Au collisions. The double-peak structure of away side correlation functions can only be formed in sufficient "large" collision systems. These properties provide some hints to study onset of deconfinement, which is related to the QCD phase boundary and QCD critical point, by an energy-size scan.

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arXiv2010

Dynamical dipole $γ$ radiation in heavy-ion collisions on the basis of the quantum molecular dynamics model

Dynamical dipole $γ$-ray emission in heavy-ion collisions is explored in the framework of the quantum molecular dynamics model. The studies are focused on systems of $^{40}$Ca bombarding $^{48}$Ca and its isotopes at different incident energies and impact parameters. Yields of $γ$ rays are calculated and the centroid energy and dynamical dipole emission width of the $γ$ spectra are extracted to investigate the properties of $γ$ emission. In addition, sensitivities of dynamical dipole $γ$-ray emission to the isospin and the symmetry energy coefficient of the equation of state are studied. The results show that detailed study of dynamical dipole $γ$ radiation can provide information on the equation of state and the symmetry energy around the normal nuclear density.

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arXiv2010

Neutron removal cross section as a measure of neutron skin

We study the relation between neutron removal cross section ($σ_{-N}$) and neutron skin thickness for finite neutron rich nuclei using the statistical abrasion ablation (SAA) model. Different sizes of neutron skin are obtained by adjusting the diffuseness parameter of neutrons in the Fermi distribution. It is demonstrated that there is a good linear correlation between $σ_{-N}$ and the neutron skin thickness for neutron rich nuclei. Further analysis suggests that the relative increase of neutron removal cross section could be used as a quantitative measure for the neutron skin thickness in neutron rich nuclei.

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arXiv2010

Isoscaling behavior in $^{48,40}$Ca+$^{9}$Be collisions at intermediate energy investigated by HIPSE model

The fragment production cross sections for 140 MeV/nucleon $^{48,40}$Ca$+^{9}$Be reactions have been calculated by the Heavy Ion Phase Space Exploration (HIPSE) model. Isoscaling behavior is observed. The isoscaling parameters $α$ and $β$ for both heavy and light fragments from HIPSE model calculations are in good agreement with the experiment data. Studies show that the potential parameters in the HIPSE model have very small effect on the isoscaling parameters. The effect of the excitation energy and evaporation on the isoscaling behavior has also been discussed.

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arXiv2010

Reaction plane angle dependence of dihadron azimuthal correlations from a multiphase transport model calculation

Dihadron azimuthal angle correlations relative to the reaction plane have been investigated in Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV using a multi-phase transport model (AMPT). Such reaction plane azimuthal angle dependent correlations can shed light on path-length effect of energy loss of high transverse momentum particles propagating through the hot dense medium. The correlations vary with the trigger particle azimuthal angle with respect to the reaction plane direction, $ϕ_{s}=ϕ_{T}-Ψ_{EP}$, which is consistent with the experimental observation by the STAR collaboration. The dihadron azimuthal angle correlation functions on the away side of the trigger particle present a distinct evolution from a single peak to a broad, possibly double peak, structure when the trigger particle direction goes from in-plane to out-of-plane of the reaction plane. The away-side angular correlation functions are asymmetric with respect to the back-to-back direction in some regions of $ϕ_{s}$, which could provide insight on testing $v_{1}$ method to reconstruct the reaction plane. In addition, both the root-mean-square width ($W_{rms}$) of the away-side correlation distribution and the splitting parameter $D$ between the away-side double peaks increase slightly with $ϕ_{s}$, and the average transverse momentum of the away-side associated hadrons shows a strong $ϕ_{s}$ dependence. Our results indicate that strong parton cascade and resultant energy loss could play an important role for the appearance of a double-peak structure in the dihadron azimuthal angular correlation function on the away side of the trigger particle.

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arXiv2009

Energy Dependence of Directed Flow in Au+Au Collisions from a Multi-phase Transport Model

The directed flow of charged hadron and identified particles has been studied in the framework of a multi-phase transport (AMPT) model, for $^{197}$Au+$^{197}$Au collisions at $\sqrt{s_{NN}}=$200, 130, 62.4, 39, 17.2 and 9.2 GeV. The rapidity, centrality and energy dependence of directed flow for charged particles over a wide rapidity range are presented. AMPT model gives the correct $v_1(y)$ slope, as well as its trend as a function of energy, while it underestimates the magnitude. Within the AMPT model, the proton $v_1$ slope is found to change its sign when the energy increases to 130 GeV - a feature that is consistent with ``anti-flow''. Hadronic re-scattering is found having little effect on $v_1$ at top RHIC energies. These studies can help us to understand the collective dynamics at early times in relativistic heavy-ion collisions, and they can also be served as references for the RHIC Beam Energy Scan program.

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arXiv2009

Neutron/proton ratio of nucleon emissions as a probe of neutron skin

The dependence between neutron-to-proton yield ratio ($R_{np}$) and neutron skin thickness ($δ_{np}$) in neutron-rich projectile induced reactions is investigated within the framework of the Isospin-Dependent Quantum Molecular Dynamics (IQMD) model. The density distribution of the Droplet model is embedded in the initialization of the neutron and proton densities in the present IQMD model. By adjusting the diffuseness parameter of neutron density in the Droplet model for the projectile, the relationship between the neutron skin thickness and the corresponding $R_{np}$ in the collisions is obtained. The results show strong linear correlation between $R_{np}$ and $δ_{np}$ for neutron-rich Ca and Ni isotopes. It is suggested that $R_{np}$ may be used as an experimental observable to extract $δ_{np}$ for neutron-rich nuclei, which is very significant to the study of the nuclear structure of exotic nuclei and the equation of state (EOS) of asymmetric nuclear matter.

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arXiv2009

Viscosity and dilepton production of a chemically equilibrating quark-gluon plasma at finite baryon density

By considering the effect of shear viscosity we have investigated the evolution of a chemically equilibrating quark-gluon plasma at finite baryon density. Based on the evolution of the system we have performed a complete calculation for the dilepton production from the following processes: $q\bar{q}{\to}l\bar{l}$, $q\bar{q}{\to}gl\bar{l}$, Compton-like scattering ($qg{\to}ql\bar{l}$, $\bar{q}g{\to}{\bar{q}}l\bar{l}$), gluon fusion $g\bar{g}{\to}c\bar{c}$, annihilation $q\bar{q}{\to}c\bar{c}$ as well as the multiple scattering of quarks. We have found that quark-antiquark annihilation, Compton-like scatterring, gluon fusion, and multiple scattering of quarks give important contributions. Moreover, we have also found that the dilepton yield is an increasing function of the initial quark chemical potential, and the increase of the quark phase lifetime because of the viscosity also obviously raises the dilepton yield.

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arXiv2009

Pion Interferometry in Au+Au and Cu+Cu Collisions at $\sqrt{s_{\rm{NN}}}$ = 62.4 and 200 GeV

We present a systematic analysis of two-pion interferometry in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 62.4 GeV and Cu+Cu collisions at $\sqrt{s_{\rm{NN}}}$ = 62.4 and 200 GeV using the STAR detector at RHIC. The multiplicity and transverse momentum dependences of the extracted correlation lengths (radii) are studied. The scaling with charged particle multiplicity of the apparent system volume at final interaction is studied for the RHIC energy domain. The multiplicity scaling of the measured correlation radii is found to be independent of colliding system and collision energy.

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arXiv2009

Breaking of the number-of-constituent-quark scaling for identified-particle elliptic flow as a signal of phase change in low-energy data taken at the BNL Relativistic Heavy Ion Collider (RHIC)

We argue that measurements of identified-particle elliptic flow in a wide energy range could shed light on the possible phase change in high-energy heavy ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC). When the hadronization process is dominated by quark coalescence, the number-of-constituent-quark (NCQ) scaling for the identified-particle elliptic flow can serve as a probe for studying the strong interacting partonic matter. In the upcoming RHIC low-energy runs, the NCQ scaling behavior may be broken because of the change of the effective degrees of freedom of the hot dense matter, which corresponds to the transition from the dominant partonic phase to the dominant hadronic phase. A multiphase transport model is used to present the dependence of NCQ scaling behavior on the different hadronization mechanisms.

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arXiv2009

Isospin dependence of projectile-like fragment production at intermediate energies

The cross sections of fragments produced in 140 $A$ MeV $^{40,48}$Ca + $^9$Be and $^{58,64}$Ni + $^9$Be reactions are calculated by the statistical abration-ablation(SAA) model and compared to the experimental results measured at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. The fragment isotopic and isotonic cross section distributions of $^{40}$Ca and $^{48}$Ca, $^{58}$Ni and $^{64}$Ni, $^{40}$Ca and $^{58}$Ni, and $^{48}$Ca and $^{64}$Ni are compared and the isospin dependence of the projectile fragmentation is studied. It is found that the isospin dependence decreases and disappears in the central collisions. The shapes of the fragment isotopic and isotonic cross section distributions are found to be very similar for symmetric projectile nuclei. The shapes of the fragment isotopic and isotonic distributions of different asymmetric projectiles produced in peripheral reactions are found very similar. The similarity of the distributions are related to the similar proton and neutron density distributions inside the nucleus in framework of the SAA model.

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arXiv2008

Parton Distributions at Hadronization from Bulk Dense Matter Produced at RHIC

We present an analysis of $Ω$, $Ξ$, $Λ$ and $ϕ$ spectra from Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV in terms of distributions of effective constituent quarks at hadronization. Consistency in quark ratios derived from various hadron spectra provides clear evidence for hadron formation dynamics as suggested by quark coalescence or recombination models. We argue that the constituent quark distribution reflects properties of the effective partonic degrees of freedom at hadronization. Experimental data indicate that strange quarks have a transverse momentum distribution flatter than that of up/down quarks consistent with hydrodynamic expansion in partonic phase prior to hadronization. After the AMPT model is tuned to reproduce the strange and up/down quark distributions, the model can describe the measured spectra of hyperons and $ϕ$ mesons very well where hadrons are formed through dynamical coalescence.

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