Dilepton spectra in p+p and Au+Au collisions at RHIC

TL;DR

This paper measures electron-positron pair production in p+p and Au+Au collisions at RHIC, revealing significant low-mass dielectron enhancement in heavy-ion collisions that challenges current theoretical models and suggests thermal photon radiation.

Contribution

It provides new measurements of dielectron spectra in p+p and Au+Au collisions, highlighting unexplained low-mass enhancements and thermal photon signals in heavy-ion collisions.

Findings

Low-mass dielectron yield in Au+Au is enhanced by a factor of 4.

The excess dielectrons have lower average p_T than expected.

Thermal photon radiation models can qualitatively explain the observed excess.

Abstract

This work presents measurements of electron-positron pairs from p+p collisions at sqrt(s) = 200 GeV collected during the 2005 RHIC run and compares them to results from Au+Au collisions at sqrt(s_NN) = 200 GeV taken in 2004 with the PHENIX detector. The invariant mass distribution of e+e- pairs in p+p is consistent with the expected contributions from Dalitz decays of light hadrons, dielectron decays of vector mesons and correlated charm production, which have been measured in the same experiment. The charm and bottom cross section extracted from the measured dielectron yield are sigma_cc = 544 +/- 39(stat.) +/- 142(syst.) +/- 200(model) microbarns and sigma_bb = 3.9 +/- 2.4(stat.) +3/-2(syst.) microbarns, respectively. In min. bias Au+Au collisions the yield of dielectrons in the low mass region is enhanced by a factor of 4.0 +/- 0.3(stat.) +/- 1.5(syst.) +/- 0.8(model) compared to the known hadronic sources. The excess dominates the yield in the transverse momentum region below 1 GeV/c and shows significantly lower average p_T than the expected sources. The low p_T enhancement is currently not understood by any theoretical model of heavy ion collisions. The enhancement extends to larger transverse momenta where it is also observed in p+p and explained by virtual direct photons. The p+p measurement serves as an important test to pQCD calculations of direct photon production from hard scattering processes in this momentum range. An excess with an inverse slope of T_eff = 221 +/- 23(stat.) +/- 18(syst.) MeV is observed in central Au+Au collisions above the binary scaled direct photon yield in p+p. This can be qualitatively explained by hydrodynamical models including thermal photon radiation with initial temperatures of 300 - 600 MeV and formation times of 0.12 - 0.6 fm/c.