Evidence for radial flow of thermal dileptons in high-energy nuclear collisions

TL;DR

This paper presents evidence of radial flow in thermal dileptons from high-energy nuclear collisions, indicating a transition from hadronic to partonic emission sources based on transverse momentum spectra analysis.

Contribution

It provides the first direct evidence of thermal radiation of partonic origin in nuclear collisions through detailed spectral analysis.

Findings

Transverse momentum spectra show a rise and fall in slope parameter Teff with dimuon mass.

The initial rise in Teff is consistent with radial flow in a hadronic source.

The decline in Teff suggests a transition to a source with smaller flow, indicating partonic emission.

Abstract

The NA60 experiment at the CERN SPS has studied low-mass dimuon production in 158 AGeV In-In collisions. An excess of pairs above the known meson decays has been reported before. We now present precision results on the associated transverse momentum spectra. The slope parameter Teff extracted from the spectra rises with dimuon mass up to the rho, followed by a sudden decline above. While the initial rise is consistent with the expectations for radial flow of a hadronic decay source, the decline signals a transition to an emission source with much smaller flow. This may well represent the first direct evidence for thermal radiation of partonic origin in nuclear collisions.