Thermal dileptons in high-energy nuclear collisions

TL;DR

This paper reports on precise measurements of excess dileptons in high-energy nuclear collisions, revealing thermal radiation signatures and a transition from hadronic to partonic sources.

Contribution

The study provides detailed spectral and polarization data supporting the interpretation of excess dileptons as thermal radiation from a hot, dense medium, with new insights into the rho spectral function and source dynamics.

Findings

Broadening of the rho spectral function without mass shift

Prompt excess in the >1 GeV region not due to charm

Evidence of a transition from hadronic to partonic emission sources

Abstract

Clear signs of excess dileptons above the known sources were found at the SPS since long. However, a real clarification of these observations was only recently achieved by NA60, measuring dimuons with unprecedented precision in 158A GeV In-In collisions. The excess mass spectrum in the region M<1 GeV is consistent with a dominant contribution from pi+pi- -> rho -> mu+mu- annihilation. The associated rho spectral function shows a strong broadening, but essentially no shift in mass. In the region M>1 GeV, the excess is found to be prompt, not due to enhanced charm production. The inverse slope parameter Teff associated with the transverse momentum spectra rises with mass up to the rho, followed by a sudden decline above. While the initial rise, coupled to a hierarchy in hadron freeze-out, points to radial flow of a hadronic decay source, the decline above signals a transition to a low-flow source, presumably of partonic origin. The mass spectra show the steep rise towards low masses characteristic for Planck-like radiation. The polarization of the excess referred to the Collins Soper frame is found to be isotropic. All observations are consistent with a global interpretation of the excess as thermal radiation. We conclude with a short discussion of a possible link to direct photons.