e+e- Pairs: a clock and a thermometer of heavy ion collisions

TL;DR

This paper discusses how e+e- pairs serve as probes for the properties and evolution of the quark-gluon plasma created in heavy ion collisions, revealing excess dilepton production linked to thermal photon emission.

Contribution

It presents new measurements of dilepton spectra in Au+Au collisions at RHIC, highlighting an excess that correlates with collision centrality and transverse momentum, indicating thermal photon production.

Findings

Enhanced dilepton yield in 150-750 MeV/c^2 mass range

Excess increases with collision centrality

Low p_T excess linked to thermal photon production

Abstract

Recently, there is growing evidence that a new state of matter is formed in sqrt(s_NN)= 200 GeV Au+Au collisions at RHIC: a strongly coupled Quark Gluon Plasma of partonic degrees of freedom which develops a collective motion. Dilepton spectra are not affected by strong interaction and can therefore probe the whole time evolution of the collision. Thus they may be sensitive to onset of deconfinement, chiral symmetry restoration, as well as the production of thermal photons. The PHENIX experiment measured the production of e+e- pairs in p+p and Au+Au collisions at sqrt(s_NN)= 200 GeV. An enhanced dilepton yield in the mass range 150<m_ee<750 MeV/c^2 is measured. The excess increases faster with centrality than the number of participating nucleons and is concentrated at p_T<1GeV/c. At higher p_T the excess below 300 MeV/c^2 has been related to an enhanced production of direct photons possibly of thermal origin.