Boltzmann Approach To MultiParton Scatterings Research Articles

Electrical conductivity of hot and dense QCD matter created in heavy-ion collisions: A color string percolation approach

Recently, transport coefficients viz. shear viscosity, electrical conductivity etc. of strongly interacting matter produced in heavy-ion collisions have drawn considerable interest. We study the normalised electrical conductivity ($\sigma_{\rm el}$/T) of hot QCD matter as a function of temperature (T) using the Color String Percolation Model (CSPM). We also study the temperature dependence of shear viscosity and its ratio with electrical conductivity for the QCD matter. We compare CSPM estimations with various existing results and lattice Quantum Chromodynamics (lQCD) predictions with (2+1) dynamical flavours. We find that $\sigma_{\rm el}$/T in CSPM has a very weak dependence on the temperature. We compare CSPM results with those obtained in Boltzmann Approach to Multi-Parton Scatterings (BAMPS) model. A good agreement is found between CSPM results and predictions of BAMPS with fixed strong coupling constant.

The different energy loss mechanisms of inclusive and b-tagged reconstructed jets within ultra-relativistic heavy-ion collisions

The phenomenon of jet quenching provides essential information about the properties of hot and dense matter created in ultra-relativistic heavy-ion collisions. Recent results from experiments at the Large Hadron Collider (LHC) show evidence for an unexpectedly similar suppression of both light and heavy flavor jets. Furthermore, the role of radiative energy loss of heavy quarks is still under active discussion within the theoretical community. By employing the parton cascade Boltzmann Approach to Multi-Parton Scatterings (BAMPS), which numerically solves the 3+1 D Boltzmann equation both for light and heavy flavor partons, we calculate the nuclear modification factor of inclusive and b-tagged reconstructed jets in 0–10% central sLHC=2.76 A TeV Pb + Pb collisions. Based on perturbative QCD cross sections we find a suppression of both light and heavy flavor jets. While the inclusive jets are slightly too strong suppressed within Bamps in comparison with data, both elastic + radiative and only elastic interactions lead to a realistic b-tagged jet suppression. To further investigate light and heavy flavor energy loss we predict the R dependence of inclusive and b-tagged jet suppression. Furthermore, we propose the medium modification of b-tagged jet shapes as an observable for discriminating between different heavy quark energy loss scenarios.

Non-thermal photon production in the quark–gluon plasma

We have implemented leading order photon production processes in nonequilibrium partonic transport simulations. These include Compton scattering, quark-antiquark annihilation and bremsstrahlung. We use BAMPS (Boltzmann Approach To Multi-Parton Scatterings), a numerical code which solves the 3+1d ultrarelativistic Boltzmann equation for massless on-shell quarks and gluons with Monte-Carlo methods. It allows us to study photon production microscopically, including strong nonequilibrium effects. BAMPS is applicable for the whole evolution of the quark-gluon plasma (QGP) in RHIC and LHC heavy-ion collisions, where the photon production is influenced by the chemical and thermal non-equilibrium state in the early phase. Highly energetic quark- and gluon jets will convert into or radiate photons, effects we include by default. We show results for photon spectra from the QGP and investigate its role for the elliptic flow of photons. The yield is smaller than results from other groups, the reason is the slow quark chemical equilibration. Photons induced by jet-like particles show very different momentum anisotropies compared to a hydrodynamically flowing medium.

Nonequilibrium photon production in partonic transport simulations

We discuss the implementation of leading-order photon production in nonequilibrium partonic transport simulations. In this framework photons are produced by microscopic scatterings, where we include the exact matrix elements of Compton scattering, quark-antiquark annihilation, and bremsstrahlung processes. We show how the hard-thermal loop inspired screening of propagators has to be modified such that the microscopic production rate agrees well with the analytically known resummed leading-order rate. We model the complete quark-gluon plasma phase of heavy-ion collisions by using the partonic transport approach called the Boltzmann approach to multiparton scatterings (BAMPS), which solves the ultrarelativistic Boltzmann equation with Monte Carlo methods. We show photon spectra and elliptic flow of photons from BAMPS and discuss nonequilibrium effects. Due to the slow quark chemical equilibration in BAMPS, the yield is lower than the results from other groups; in turn we see a strong effect from scatterings of energetic jet-like partons with the medium. This nonequilibrium photon production can dominate the thermal emission, such that the spectra are harder and the photonic elliptic flow of the quark-gluon plasma becomes negative.

Elastic and radiative heavy quark interactions in ultra-relativistic heavy-ion collisions

Elastic and radiative heavy quark interactions with light partons are studied with the partonic transport model named the Boltzmann approach to multiparton scatterings (BAMPSs). After calculating the cross section of radiative processes for finite masses in the improved Gunion–Bertsch approximation and verifying this calculation by comparing to the exact result, we study elastic and radiative heavy quark energy loss in a static medium of quarks and gluons. Furthermore, the full 3 + 1D space–time evolution of gluons, light quarks, and heavy quarks in ultra-relativistic heavy-ion collisions at the BNL Relativistic Heavy-Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC) are calculated with BAMPS including elastic and radiative heavy flavor interactions. Treating light and heavy particles on the same footing in the same framework, we find that the experimentally measured nuclear modification factor of charged hadrons and D mesons at the LHC can be simultaneously described. In addition, we calculate the heavy flavor evolution with an improved screening procedure from hard-thermal-loop calculations and confront the results with experimental data of the nuclear modification factor and the elliptic flow of heavy flavor particles at the RHIC and the LHC.

Elastic vs. radiative heavy quark energy loss within a 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.

Elliptic flow and nuclear modification factor within a partonic transport model

Abstract The partonic transport approach BAMPS (Boltzmann Approach to Multi-Parton Scatterings) aims to describe microscopically the partonic phase of ultra-relativistic heavy-ion collisions by perturbative quantum chromodynamics. To this end, it numerically solves the 3+1D Boltzmann equation by allowing interactions among all parton species: both elastic 2 → 2 and inelastic 2 ↔ 3 collisions for gluons as well as quarks. For the inelastic collisions, we employ the improved Gunion–Bertsch matrix element, which cures problems of the original Gunion–Bertsch result in characteristic regions of the phase space. Based on extensive numerical calculations, the improved matrix element agrees well with the exact pQCD calculation. By employing the improved GB matrix element together with a running coupling evaluated at the microscopic scale, we present our recent results for the suppression of high p t particles as well as the collectivity of the bulk medium and compare them to LHC data of the nuclear modification factor R AA and the elliptic flow v 2 . Furthermore, we present studies of R AA for different hadron species and the momentum imbalance A J of reconstructed jets.

BAMPS with the improved Gunion–Bertsch matrix element

Abstract Focusing on the simultaneous investigation of the hard and the soft regime of ultra-relativistic heavy-ion collisions at the Relativistic Heavy-Ion Collider (RHIC) at the Brookhaven National Laboratory (BNL) and the Large Hadron Collider (LHC) at CERN, we present our recent results on the nuclear modification factor R AA and the elliptic flow v 2 within the partonic transport model BAMPS (Boltzmann Approach to Multi-Parton Scatterings). While using interactions provided by perturbative QCD, BAMPS allows the full 3+1D simulation of the quark–gluon plasma (QGP) at the microscopic level by solving the relativistic Boltzmann equation for quarks and gluons. The microscopic interactions include both elastic 2 → 2 collisions, calculated by screened leading order pQCD matrix elements, and inelastic 2 ↔ 3 processes, calculated within an improved Gunion–Bertsch approximation. Furthermore, for all partonic processes a microscopic, running coupling is explicitly taken into account. We show that when fixing the parameter X LPM resulting from the effective modeling of the Landau–Pomeranchuk–Migdal effect, we find not only a good description of R AA at both RHIC and LHC, but also a sizable elliptic flow is built up within the partonic phase.

Momentum imbalance ofDmesons in ultrarelativistic heavy-ion collisions at the CERN Large Hadron Collider

As a new observable for heavy flavor correlations the momentum imbalance $A_D$ of D mesons is proposed. It is defined analogously to the jet momentum imbalance $A_J$ of fully reconstructed jets. However, since D mesons are flavor tagged particles, no jet reconstruction is necessary. $A_D$ quantifies the influence of the medium created in heavy-ion collisions on correlated charm pairs. We present results with the partonic transport model Boltzmann Approach to MultiParton Scatterings (BAMPS), which describes well the nuclear modification factor and elliptic flow of all heavy flavor particles at RHIC and LHC. The $A_D$ distribution in heavy-ion collisions at LHC is shifted to larger values of $A_D$ compared to proton-proton collisions. We argue that this shift is due to medium effects and can be explained partially by a path length imbalance of charm pairs and partially by momentum fluctuations in the initial charm pair distribution.

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.

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/ψ 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/ψ. Where no data is available yet, we make predictions for those observables. Furthermore, results on prompt J/ψ elliptic flow are reported for RHIC energy within the same framework, taking the dissociation as well as regeneration of J/ψ in the quark-gluon plasma into account.

Open heavy flavor and J/ψ 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 / ψ , 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 / ψ mesons, which are treated in the same framework.

Open heavy flavor in Pb + Pb collisions at [formula omitted] within a transport model

The space–time evolution of open heavy flavor is studied in Pb + Pb collisions at s=2.76TeV 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 v2 and nuclear modification factor RAA at RHIC. Within this framework we calculate in a comprehensive study the v2 and RAA of all interesting open heavy flavor particles at the LHC: electrons, muons, D mesons, and non-prompt J/ψ 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 s=7TeV.

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

The description of jet quenching and elliptic flow at the RHIC and the LHC within a common dynamical framework has been notoriously difficult. In this paper, an approach using the perturbative QCD-based partonic transport model BAMPS (Boltzmann approach to multi-parton scatterings) is presented that has recently been extended to include light quarks. The investigations are complemented by a study on the suppression of D-mesons at the LHC based on elastic interactions with the medium.

Energy loss in a partonic transport model including bremsstrahlung processes

A detailed investigation of the energy loss of gluons that traverse a thermal gluonic medium simulated within the perturbative QCD-based transport model BAMPS (a Boltzmann approach to multiparton scatterings) is presented in the first part of this work. For simplicity the medium response is neglected in these calculations. The energy loss from purely elastic interactions is compared with the case where radiative processes are consistently included based on the matrix element by Gunion and Bertsch. From this comparison, gluon multiplication processes $\mathit{gg}\ensuremath{\rightarrow}\mathit{ggg}$ are found to be the dominant source of energy loss within the approach employed here. The consequences for the quenching of gluons with high transverse momentum in fully dynamic simulations of $\mathrm{Au}+\mathrm{Au}$ collisions at the BNL Relativistic Heavy Ion Collider (RHIC) energy of $\sqrt{s}=200A \mathrm{GeV}$ are discussed in the second major part of this work. The results for central collisions as discussed in a previous publication are revisited, and first results on the nuclear modification factor ${R}_{\mathit{AA}}$ for noncentral $\mathrm{Au}+\mathrm{Au}$ collisions are presented. They show a decreased quenching compared to central collisions while retaining the same shape. The investigation of the elliptic flow ${v}_{2}$ is extended up to nonthermal transverse momenta of $10 \mathrm{GeV}$, exhibiting a maximum ${v}_{2}$ at roughly $4$ to $5 \mathrm{GeV}$ and a subsequent decrease. Finally the sensitivity of the aforementioned results on the specific implementation of the effective modeling of the Landau-Pomeranchuk-Migdal (LPM) effect via a formation-time-based cutoff is explored.

Investigation of jet quenching and elliptic flow within a pQCD-based partonic transport model

The partonic transport model BAMPS (a Boltzmann approach to multiparton scatterings) is employed to investigate different aspects of heavy ion collisions within a common framework based on perturbative QCD. This report focuses on the joint investigation of the collective behavior of the created medium and the energy loss of high–pT gluons traversing this medium. To this end the elliptic flow and the nuclear modification factor of gluons in heavy ion collisions at 200 AGeV are simulated with BAMPS.The mechanism for the energy loss of high energy gluons within BAMPS is studied in detail. For this, purely elastic interactions are compared to radiative processes, gg → ggg, that are implemented based on the matrix element by Gunion and Bertsch. The latter are found to be the dominant source of energy loss within the framework employed in this work.