arXiv subjects

Zhe Xu

Publications and source records attributed to Zhe Xu.

At least 253 records · Page 14Linked to original sources

arXiv2013

Heavy vs. light flavor energy loss within a partonic transport model

The full space-time evolution of gluons, light and heavy quarks in ultra-relativistic heavy-ion collisions is studied within the partonic transport model Boltzmann Approach to MultiParton Scatterings (BAMPS). We discuss for light and heavy quarks the elastic and radiative energy loss with a running coupling. Radiative processes, in particular, are implemented through an improved version of the Gunion-Bertsch matrix element, which is derived from comparisons to the exact result, explicitly taking finite heavy quark masses into account. Subsequently, we present results with and without radiative processes and compare them to experimental data at LHC. A focus is put on the nuclear modification factor of charged hadrons and D mesons.

hep-ph↗

arXiv2013

Radiative parton processes in perturbative QCD - an improved version of the Gunion and Bertsch cross section from comparisons to the exact result

In this work we compare the Gunion-Bertsch approximation of the leading order perturbative QCD radiation matrix element to the exact result. To this end, we revisit the derivation of the Gunion-Bertsch approximation as well as perform extensive numerical comparisons of the Gunion-Bertsch and the exact result. We find that when employing the matrix elements to obtain rates or cross sections from phase space integration, the amplitude by Gunion and Bertsch deviates from the correct result in characteristic regions of the phase space. We propose an improved version of the Gunion-Bertsch matrix element which agrees very well with the exact result in all phase space regions.

hep-ph↗

arXiv2012

Open heavy flavor and J/psi at RHIC and LHC within a transport model

The production and space-time evolution of heavy quarks and J/psi in the quark gluon plasma is studied within the partonic transport model Boltzmann Approach to MultiParton Scatterings (BAMPS). This framework allows interactions among all partons: gluons, light quarks and heavy quarks. Heavy quarks, in particular, interact with the rest of the medium via binary scatterings with a running coupling and a more precise Debye screening which is derived from hard thermal loop calculations. We compare our results of the elliptic flow and nuclear modification factor not only to experimental data of heavy flavor electrons at RHIC, but also to LHC data of heavy flavor electrons, muons, D mesons, and non-prompt J/psi. Where no data is available yet, we make predictions for those observables. Furthermore, results on prompt J/psi elliptic flow are reported for RHIC energy within the same framework, taking the dissociation as well as regeneration of J/psi in the quark-gluon plasma into account.

hep-ph↗

arXiv2012

Open heavy flavor in Pb+Pb collisions at sqrt(s)=2.76 TeV within a transport model

The space-time evolution of open heavy flavor is studied in Pb+Pb collisions at sqrt(s)=2.76 TeV using the partonic transport model Boltzmann Approach to MultiParton Scatterings (BAMPS). An updated version of BAMPS is presented which allows interactions among all partons: gluons, light quarks, and heavy quarks. Heavy quarks, in particular, interact with the rest of the medium via binary scatterings with a running coupling and a Debye screening which is matched by comparing to hard thermal loop calculations. The lack of radiative processes in the heavy flavor sector is accounted for by scaling the binary cross section with a phenomenological factor K=3.5, which describes well the elliptic flow v_2 and nuclear modification factor R_AA at RHIC. Within this framework we calculate in a comprehensive study the v_2 and R_AA of all interesting open heavy flavor particles at the LHC: electrons, muons, D mesons, and non-prompt J/psi from B mesons. We compare to experimental data, where it is already available, or make predictions. To do this accurately next-to-leading order initial heavy quark distributions are employed which agree well with proton-proton data of heavy flavor at sqrt(s)=7 TeV.

hep-ph↗

arXiv2012

Open heavy flavor and J/psi at RHIC and LHC

The suppression and collective flow of open and hidden heavy flavor in ultra-relativistic heavy ion collisions is studied with the partonic transport model Boltzmann Approach to MultiParton Scatterings (BAMPS). Charm and bottom quarks interact with the rest of the medium via elastic scatterings with an improved Debye screening and a running coupling. Heavy flavor electron data from RHIC can only be described if the cross section is multiplied with K=3.5 to mimic the contribution from radiative processes. With the same value for K we compare to elliptic flow data of D mesons at the LHC and find a good agreement. However, the nuclear modification factor of D mesons, non-prompt J/psi, muons, and electrons at the LHC is underestimated in our calculations. In addition, we present first BAMPS results on the suppression and elliptic flow of J/psi mesons, which are treated in the same framework.

hep-ph↗

arXiv2012

Transition from ideal to viscous Mach cones in BAMPS

We investigate in a microscopical transport model the evolution of conical structures originating from the supersonic projectile moving through the matter of ultrarelativistic particles. Using different scenarios for the interaction between projectile and matter, and different transport properties of the matter, we study the formation and structure of Mach cones. Furthermore, the two-particle correlations for different viscosities are extracted from the numerical calculations and we compare them to an analytical approximation. In addition, by adjusting he cross section we investigate the influence of the viscosity to the structure of Mach cones.

hep-ph↗

arXiv2012

RHIC and LHC phenomena with an unified parton transport

We discuss recent applications of the partonic pQCD based cascade model BAMPS with focus on heavy-ion phenomeneology in hard and soft momentum range. The nuclear modification factor as well as elliptic flow are calculated in BAMPS for RHIC end LHC energies. These observables are also discussed within the same framework for charm and bottom quarks. Contributing to the recent jet-quenching investigations we present first preliminary results on application of jet reconstruction algorithms in BAMPS. Finally, collective effects induced by jets are investigated: we demonstrate the development of Mach cones in ideal matter as well in the highly viscous regime.

hep-ph↗

arXiv2012

Dissipative hydrodynamics for multi-component systems

Second-order dissipative hydrodynamic equations for each component of a multi-component system are derived using the entropy principle. Comparison of the solutions with kinetic transport results demonstrates validity of the obtained equations. We demonstrate how the shear viscosity of the total system can be calculated in terms of the involved cross sections and partial densities. Presence of the inter-species interactions leads to a characteristic time-dependence of the shear viscosity of the mixture, which also means that the shear viscosity of a mixture cannot be calculated using the Green-Kubo formalism the way it has been done recently. This finding is of interest for understanding of the shear viscosity of a quark-gluon-plasme extracted from comparisons of hydrodynamic simulations with experimental results from RHIC and LHC.

hep-th↗

arXiv2011

Open heavy flavor at RHIC and LHC in a partonic transport model

Heavy quarks are a unique probe to study the medium produced in ultra-relativistic heavy ion collisions. Within the partonic transport model Boltzmann approach to multi-parton scatterings (BAMPS) the production, energy loss and elliptic flow of heavy quarks are investigated in 3+1 dimensional simulations of the quark gluon plasma evolution. With only binary interactions between heavy quarks and particles from the medium calculated within perturbative QCD, the results on elliptic flow and the nuclear modification factor are not compatible with experimental data from RHIC and LHC. However, if the binary cross section is multiplied with K=4 both the elliptic flow and the nuclear modification factor are simultaneously described at RHIC and also LHC. Furthermore, preliminary results are presented that the implementation of radiative processes leads to a stronger suppression which agrees well with the measured nuclear modification factor at RHIC without the need of any K factor.

hep-ph↗

arXiv2011

Elliptic and Triangular Flow and their Correlation in Ultrarelativistic High Multiplicity Proton Proton Collisions at 14 TeV

The spatial configuration of initial partons in high-multiplicity proton-proton scatterings at 14 TeV is assumed as three randomly positioned "hot spots". The parton momentum distribution in the hot spots is calculated by HIJING2.0 with some modifications. This initial condition causes not only large eccentricity $ε_2$ but also triangularity $ε_3$ and the correlation of $ε_2-ε_3$ event-plane angles. The final elliptic flow $v_2$, triangular flow $v_3$, and the correlation of $v_2-v_3$ event-plane angles are calculated by using the parton cascade model BAMPS to simulate the space-time parton evolution. Our results show that the $v_2-v_3$ correlation is different from that of $ε_2-ε_3$. This finding indicates that translations of different Fourier components of the initial spatial asymmetry to the final flow components are not independent. A dynamical correlation between the elliptic and triangular flow appears during the collective expansion.

hep-ph↗

arXiv2011

Heavy quarks and charmonium at RHIC and LHC within a partonic transport model

Heavy quark and charmonium production as well as their space-time evolution are studied in transport simulations of heavy-ion collisions at RHIC and LHC. In the partonic transport model Boltzmann Approach of MultiParton Scatterings (BAMPS) heavy quarks can be produced in initial hard parton scatterings or during the evolution of the quark-gluon plasma. Subsequently, they interact with the medium via binary scatterings with a running coupling and a more precise Debye screening which is derived from hard thermal loop calculations, participate in the flow and lose energy. We present results of the elliptic flow and nuclear modification factor of heavy quarks and compare them to available data. Furthermore, preliminary results on J/psi suppression at forward and mid-rapidity are reported for central and non-central collisions at RHIC. For this, we study cold nuclear matter effects and the dissociation as well as regeneration of J/psi in the quark-gluon plasma.

hep-ph↗

arXiv2011

Elliptic flow and energy loss of heavy quarks in ultra-relativistic heavy ion collisions

The space-time propagation of heavy quarks in ultra-relativistic heavy ion collisions is studied within the partonic transport model Boltzmann Approach of MultiParton Scatterings (BAMPS). In this model heavy quarks interact with the partonic medium via binary scatterings. The cross sections for these interactions are calculated with leading order perturbative QCD, but feature a more precise Debye screening derived within the hard thermal loop approximation and obey the running of the coupling. Within this framework the elliptic flow and the nuclear modification factor of heavy quarks are computed for RHIC and LHC energies and compared to available experimental data. It is found that binary scatterings alone cannot reproduce the data and, therefore, radiative corrections have to be taken into account.

hep-ph↗

arXiv2011

Jet quenching and elliptic flow at RHIC and LHC within a pQCD-based partonic transport model

Fully dynamic simulations of heavy ion collisions at RHIC and at LHC energies within the perturbative QCD-based partonic transport model BAMPS (Boltzmann Approach to Multi-Parton Scatterings) are presented, focusing on the simultaneous investigation of jet quenching and elliptic flow. The model features inelastic 2 <-> 3 processes based on the Gunion-Bertsch matrix element and has recently been extended to include light quark degrees of freedom, allowing for direct comparison to hadronic data on the nuclear modification factor via a fragmentation scheme for high-pT partons. The nuclear modification factor of neutral pions in central Au+Au collisions at RHIC energy is compared to experimental data. Furthermore first results on the nuclear modification factor and the integrated elliptic flow of charged hadrons in Pb+Pb collisions at LHC are presented and compared to recent ALICE data. These investigations are complemented by a study on the suppression of D-mesons at LHC based on elastic interactions with the medium.

hep-ph↗

arXiv2011

Extraction of shear viscosity in stationary states of relativistic particle systems

Starting from a classical picture of shear viscosity we construct a stationary velocity gradient in a microscopic parton cascade. Employing the Navier-Stokes ansatz we extract the shear viscosity coefficient $η$. For elastic isotropic scatterings we find an excellent agreement with the analytic values. This confirms the applicability of this method. Furthermore for both elastic and inelastic scatterings with pQCD based cross sections we extract the shear viscosity coefficient $η$ for a pure gluonic system and find a good agreement with already published calculations.

hep-th↗

arXiv2011

Dissipative hydrodynamics for relativistic multi-component systems

Second-order dissipative hydrodynamic equations for each component of a multi-component system are derived using the entropy principle. The shear viscosity of the whole system, appearing in the equation summed-up over all components, is related to the partial shear pressures and cannot be considered as an external parameter. We demonstrate that it is essential to solve hydrodynamic equations for each component, instead of treating a mixture as an effective one-component system with a free parameter $η/s$. Thus, extractions of the $η/s$ value of the QGP at RHIC and LHC have to be reexamined.

hep-ph↗

arXiv2010

Jets and flow within a pQCD-based partonic transport model

We present fully dynamic simulations of heavy ion collisions at RHIC energies within the perturbative QCD-based partonic transport model BAMPS, focusing on the simultaneous investigation of jet-quenching and elliptic flow. The model consistently features elastic and inelastic 2 -> 3 processes, the latter being based on the Gunion-Bertsch matrix element. We discuss first attempts to extend the model to include light quark degrees of freedom and study the energy loss of high energy gluons and quarks in a static partonic medium. The difference between gluons and quarks in inelastic processes is found to be weaker than expected from color factors, due to a self-quenching effect associated with a cut-off modeling the LPM effect.

hep-ph↗

arXiv2010

Study of magnetic alloy cores for HIRFL-CSRm compressor cavity

For selecting the properly magnetic alloy (MA) material to load the RF compression cavity, the measurement of the MA cores which is produced by Liyuan Company has been carried out at IMP. We measured 4 kinds of MA core materials, type V1, V2, A1 and A2. And we mainly focus on the permeability, quality factor (Q value) and shunt impedance of the MA core. The MA cores which have higher permeability, lower Q value and higher shunt impedance will be selected to load RF compression cavity. According to the results of measurement, the type V1, V2 and A2 material will be chosen as candidate to load RF cavity.

physics.acc-ph↗

arXiv2010

Heavy quarks at RHIC and LHC within a partonic transport model

Production and space-time evolution of heavy quarks in central and non-central heavy-ion collisions at RHIC and LHC are studied with the partonic transport model Boltzmann Approach of MultiParton Scatterings (BAMPS). In addition to the initially created heavy quarks in hard parton scatterings during nucleon-nucleon collisions, secondary heavy quark production in the quark-gluon plasma is investigated and the sensitivity on various parameters is estimated. In BAMPS heavy quarks scatter with particles of the medium via elastic collisions, whose cross section is calculated with the running coupling and a more precise implementation of Debye screening. In this framework, we compute the elliptic flow and nuclear modification factor of heavy quarks and compare it to the experimental data.

hep-ph↗