Dilepton Production in Transport Calculations and Coarse-Grained Dynamics

Stephan Endres,Janus Weil,Hendrik Van Hees,Marcus Bleicher

doi:10.1088/1742-6596/503/1/012039

Abstract

We present transport calculations with the Ultra-relativistic Quantum Molecular Dynamics approach (UrQMD) for dilepton spectra at SIS energies and simulations for NA60 within a coarse-graining approach. While the transport model obtains a good agreement with experiment for elementary reactions, in heavy-ion collisions an excess in the invariant mass spectra is observed which cannot be described by the model. As the pure transport calculations do not include explicit in-medium effects and are limited to hadronic degrees of freedom, we present an alternative approach that uses coarse-grained output from transport calculations to determine thermal dilepton emission rates. For this we apply the medium-modified ρ spectral function by Eletsky et al. In a first exemplary comparison to data from the NA60 experiment we find that the coarse-graining approach gives reasonable results.

Highlights

Dileptons are a unique tool to study the properties of hot and dense matter created in nuclear collisions
For this we apply the mediummodified ρ spectral function by Eletsky et al In a first exemplary comparison to data from the NA60 experiment we find that the coarse-graining approach gives reasonable results
For a hot and dense evironment as created in ultra-relativistic heavy-ion collisions it is supposed that medium effects play a crucial role for dilepton production, but it is highly difficult to implement them in a transport model

Summary

Introduction

Dileptons are a unique tool to study the properties of hot and dense matter created in nuclear collisions. We gain insight into all the different stages of a nuclear collision, from the first nucleon-nucleon interactions to the final freeze-out This means that in experimental measurements we obtain time-integrated spectra only, stemming from a broad variety of sources. At lower bombarding energies, where the dominant dilepton contribution originates from hadronic decays, transport models have been successful in describing the experimentally measured dilepton spectra [4,5,6,7]. In these proceedings we present recent results from our calculations with the UrQMD model at SIS energies. We argue here that a coarse-graining approach is a good way to apply in-medium spectal functions within an underlying microscopic description of the reaction dynamics

HADES Acceptance θee

Vacuum ρ