Dilepton radiation at the CERN super-proton synchrotron

Abstract

A quantitative evaluation of dilepton sources in heavy-ion reactions is performed taking into account both thermal and non-thermal production mechanisms. The hadronic thermal emission rate is based on an electromagnetic current-correlation function with a low-mass region (LMR, M 1 GeV) dominated by vector mesons (ρ, ω, φ) and an intermediate-mass region (IMR, 1 GeV ≤ M ≤ 3 GeV) characterized by (the onset of) a multi-meson continuum. A convolution of the emission rates over a thermal fireball expansion results in good agreement with experiment in the low-mass spectra, confirming the predicted broadening of the ρ meson in hadronic matter in connection with the prevalence of baryon-induced medium effects. The absolute magnitude of the LMR excess is mostly controlled by the fireball lifetime, which in turn leads to a consistent explanation of the dilepton excess in the IMR in terms of thermal radiation. The analysis of experimental transversemomentum (qT ) spectra reveals discrepancies with thermal emission for qT 1 GeV in noncentral In-In collisions, which we address by extending our calculations by: (i) a refined treatment of ρ decays at thermal freezeout, (ii) primordially produced ρ's subject to energy-loss, (iii) Drell-Yan annihilation, and (iv) thermal radiation from t-channel meson exchange processes. We investigate the sensitivity of dilepton spectra to the critical temperature and hadro-chemical freezeout of the fireball. The ρ broadening in the LMR turns out to be robust, while in the IMR Quark-Gluon Plasma radiation is moderate unless the critical temperature is rather low.

Dileptons and Chiral Symmetry Restoration

“…Of course, one should keep in mind that the chiral condensate at T pc is still at about half of its vacuum value, which, again is in the vicinity of recent estimates in cold nuclear matter at twice nuclear saturation density [13]. Figure 1: Dilepton spectra in heavy-ion collisions from HADES at SIS [3] (left), CERES [5] and NA60 [6] at SPS (middle panels) and STAR [7] at RHIC (right panel), compared to theoretical calculations with QGP and hadronic emission using the same model for the in-medium ρ spectral function [9,10,11,12].…”

Resonance studies with the UrQMD model

“…Hadronic resonances decay on time scales of several fm / c, which is well within the expected time scales of the developing medium in heavy ion collisions, even after taking into account resonance kinematics [29]. Within a hot and dense medium resonances, most notably vector mesons like the ρ meson are supposed to change their spectral function [30][31][32], however in order to disentangle interesting effects like e.g. the restoration of chiral symmetry from trivial effects it is crucial to understand the kinematics of the resonances and the surrounding medium.…”

Electromagnetic probes in heavy-ion collisions

“…In addition, in the intermediate-mass region also contributions from multi-pion processes (like a 1 , ω+π reactions) are implemented via low-density approximations, including vector-axial-vector mixing [20], using the empirical vacuum spectra of the vector-and axial-vector current-correlation function from τ-decay data [21,22]. Figure 1.…”