Thermal dileptons at SPS energies

TL;DR

This paper discusses the detailed analysis of excess dileptons observed at SPS energies, providing evidence that they originate from thermal radiation with a broadened rho spectral function and a transition from hadronic to partonic sources.

Contribution

The study offers a comprehensive experimental clarification of dilepton excesses at SPS energies, highlighting the thermal origin and spectral modifications of the rho meson in heavy-ion collisions.

Findings

Excess dileptons are consistent with thermal radiation.

The rho spectral function shows strong broadening without mass shift.

A transition from hadronic to partonic sources is indicated by the inverse slope parameter behavior.

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 at low transverse momenta 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 the interpretation of the excess as thermal radiation.