Dilepton radiation at the CERN super-proton synchrotron

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≲1GeV) dominated by vector mesons (ρ, ω, ϕ) and an intermediate-mass region (IMR, 1GeV⩽M⩽3GeV) 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 transverse-momentum (qT) spectra reveals discrepancies with thermal emission for qT≳1GeV in non-central In–In collisions, which we address by extending our calculations by: (i) a refined treatment of ρ decays at thermal freeze-out, (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 freeze-out 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.