Moritz Greif

Electric conductivity of the quark-gluon plasma investigated using a perturbative QCD based parton cascade

Electric conductivity is sensitive to effective cross sections among the particles of the partonic medium. We investigate the electric conductivity of a hot plasma of quarks and gluons, solving the relativistic Boltzmann equation. In order to extract this transport coefficient, we employ the Green-Kubo formalism and, independently, a method motivated by the classical definition of electric conductivity. To this end we evaluate the static electric diffusion current upon the influence of an electric field. Both methods give identical results. For the first time, we obtain numerically the Drude electric conductivity formula for an ultrarelativistic gas of quarks and gluons employing constant isotropic binary cross sections. Furthermore, we extract the electric conductivity for a system of massless quarks and gluons including screened binary and inelastic, radiative 2

Heat conductivity in relativistic systems investigated using a partonic cascade

3 perturbative QCD scattering. Comparing with recent lattice results, we find an agreement in the temperature dependence of the conductivity.

Importance of initial and final state effects for azimuthal correlations in p+Pb collisions

Motivated by the classical picture of heat flow we construct a stationary temperature gradient in a relativistic microscopic transport model. Employing the relativistic Navier-Stokes ansatz we extract the heat conductivity κ for a massless Boltzmann gas using only binary collisions with isotropic cross sections. We compare the numerical results to analytical expressions from different theories and discuss the final results. The directly extracted value for the heat conductivity can be referred to as a literature reference within the numerical uncertainties.

Erratum: Electric conductivity of a hot hadron gas from a kinetic approach [Phys. Rev. D 93 , 096012 (2016)]

Abstract We study the influence and interplay of initial state and final state effects in the dynamics of small systems, focusing on azimuthal correlations at different multiplicities. To this end we introduce a new model, matching the classical Yang-Mills dynamics of pre-equilibrium gluon fields (IP-GLASMA) to a perturbative QCD based parton cascade for the final state evolution (BAMPS) on an event-by-event basis. Depending on multiplicity of the event, we see transverse momentum dependent signatures of the initial, but also the final state in azimuthal correlation observables, such as v 2 {2 PC }( p T ). In low-multiplicity events, initial state correlations dominate for transverse momenta p T > 2 GeV, whereas in high-multiplicity events and at low momenta final state interactions dominate and initial state correlations strongly affect v 2 {2 PC }( p T ) for p T > 2 GeV as well as the p T integrated v 2 {2 PC }. Nearly half of the final pT integrated v 2 {2 PC } is contributed by the initial state in low-multiplicity events, whereas in high-multiplicity the share is much less. Based on Ref. [M. Greif, C. Greiner, B. Schenke, S. Schlichting, Z. Xu, Phys. Rev. D96 (9) (2017) 091504], we are now able to carry out a systematic multiplicity scan, probing the dynamics on the border of initial state dominated to final state dominated – but not yet hydrodynamic regime.

Nonequilibrium photon production in partonic transport simulations

We calculate the electric conductivity of a gas of relativistic particles with isotropic cross sections using the Boltzmann equation as the starting point. Our analyses is restricted to elastic collisions. We show the perfect agreement with previously published numerical results for a massless quarkgluon plasma, and give results for the electric conductivity of an interacting hadron gas, employing realistic resonance cross sections. These results for the electric conductivity of a hot hadron gas, as created in (ultra-)relativistic heavy-ion collisions, are of rich phenomenological as well as theoretical interest and can be compared to, e.g., lattice quantum field theory calculations.

Magnetic field influence on the early time dynamics of heavy-ion collisions

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 using the partonic transport approach 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.

Collectivity in small systems - Initial state vs. final state effects

In high-energy heavy-ion collisions, the magnetic field is very strong right after the nuclei penetrate each other and a nonequilibrium system of quarks and gluons builds up. Even though quarks might not be very abundant initially, their dynamics must necessarily be influenced by the Lorentz force. Employing the (3+1)-d partonic cascade Boltzmann approach to multiparton scatterings (BAMPS), we show that the circular Larmor movement of the quarks leads to a strong positive anisotropic flow of quarks at very soft transverse momenta. We explore the regions where the effect is visible and explicitly show how collisions damp the effect. As a possible application, we look at photon production from the flowing nonequilibrium medium.

Quark gluon plasma studies within a partonic transport approach

Observations of long rang azimuthal correlations in small collision systems (p+p/A) have triggered an enormous excitement in the heavy-ion community. However, it is presently unclear to what extent the experimentally observed correlations should be attributed to initial state momentum correlations and/or the final state response to the initial state geometry. We discuss how a consistent theoretical description of the nonequilibrium dynamics is important to address both effects within a unified framework and present first results from weakly coupled non-equilibrium simulations in [1] to quantify the relative importance of initial state and final state effects based on theoretical calculations.

Electric Conductivity of a hot hadron gas from a kinetic approach

Aiming for the simultaneous description of the hard and the soft regime of ultra-relativistic heavy-ion collisions, we present our recent findings within the partonic transport model BAMPS (Boltzmann Approach to Multi-Parton Scatterings). While using both elastic and radiative 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. BAMPS facilitates investigations of jet quenching, heavy flavor and elliptic flow within the partonic phase of heavy-ion collisions as well as studies of QGP medium properties in terms of e.g. transport coecients like /s and the electric conductivity.