Pre-equilibrium dilepton production from an anisotropic quark-gluon plasma

TL;DR

This paper models dilepton production from an early-stage anisotropic quark-gluon plasma, showing that pre-equilibrium effects can significantly enhance high-energy dilepton yields, which could help determine plasma isotropization times experimentally.

Contribution

It introduces two phenomenological models for the time evolution of anisotropic quark-gluon plasma and quantifies the impact of pre-equilibrium emission on dilepton yields.

Findings

Pre-equilibrium emission enhances high-energy dilepton production.

Dependence on isotropization time is reduced when fixing final pion multiplicity.

Enhancement of up to 50% at LHC energies for 2 fm/c isotropization time.

Abstract

We calculate leading-order dilepton yields from a quark-gluon plasma which has a time-dependent anisotropy in momentum space. Such anisotropies can arise during the earliest stages of quark-gluon plasma evolution due to the rapid longitudinal expansion of the created matter. Two phenomenological models for the proper time dependence of the parton hard momentum scale, p_hard, and the plasma anisotropy parameter, xi, are constructed which describe the transition of the plasma from its initial non-equilibrium state to an isotropic thermalized state. The first model constructed interpolates between 1+1 dimensional free streaming at early times and 1+1 dimensional ideal hydrodynamical expansion at late times. In the second model we include the effect of collisional broadening of the parton distribution functions in the early-time pre-equilibrium stage of plasma evolution. We find for both cases that for fixed initial conditions high-energy dilepton production is enhanced by pre-equilibrium emission. When the models are constrained to fixed final pion multiplicity the dependence of the resulting spectra on the assumed plasma isotropization time is reduced. Using our most realistic collisionally-broadened model we find that high-transverse momentum dilepton production would be enhanced by at most 40% at the Relativistic Heavy Ion Collider and 50% at CERN Large Hadron Collider if one assumes an isotropization/thermalization time of 2 fm/c. Given sufficiently precise experimental data this enhancement could be used to determine the plasma isotropization time experimentally.