NA60 results on thermal dimuons

TL;DR

The NA60 experiment measured muon pairs in In-In collisions, revealing a thermal radiation signature with a broadened rho spectral function and a transition from hadronic to partonic emission sources across the mass spectrum.

Contribution

This study provides detailed measurements of thermal dimuons, demonstrating rho broadening without mass shift and evidence for partonic emission at higher masses, advancing understanding of quark-gluon plasma signatures.

Findings

Strong excess of muon pairs above known sources

Rho spectral function shows broadening, no mass shift

Evidence for partonic emission at M>1 GeV

Abstract

The NA60 experiment at the CERN SPS has measured muon pairs with unprecedented precision in 158A GeV In-In collisions. A strong excess of pairs above the known sources is observed in the whole mass region 0.2<M<2.6 GeV. The mass spectrum for 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. For M>1 GeV, the excess is found to be prompt, not due to enhanced charm production, with pronounced differences to Drell-Yan pairs. The slope parameter Teff associated with the transverse momentum spectra rises with mass up to the rho, followed by a sudden decline above. The rise for M<1 GeV is consistent with radial flow of a hadronic emission source. The seeming absence of significant flow for M>1 GeV and its relation to parton-hadron duality is discussed in detail, suggesting a dominantly partonic emission source in this region. A comparison of the data to the present status of theoretical modeling is also contained. The accumulated empirical evidence, including also a Planck-like shape of the mass spectra at low pT and the lack of polarization, is consistent with a global interpretation of the excess dimuons as thermal radiation. We conclude with first results on omega in-medium effects.