NA60 results on $p_T$ spectra and the $\rho$ spectral function in In-In collisions

TL;DR

The NA60 experiment analyzed low-mass muon pairs in In-In collisions, revealing a broadening of the $ ho$ spectral function without mass shift and complex $p_T$ spectra indicating multiple sources.

Contribution

This study provides detailed measurements of the $ ho$ spectral function and $p_T$ spectra in In-In collisions, highlighting spectral broadening and mass-dependent source contributions.

Findings

Significant broadening of the $ ho$ spectral function.

No observable mass shift in the $ ho$.

Complex $p_T$ spectra indicating different sources.

Abstract

The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158 AGeV In-In collisions. A strong excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size of close to 400K events and the good mass resolution of about 2% have made it possible to isolate the excess by subtraction of the decay sources (keeping the $\rho$). The shape of the resulting mass spectrum exhibits considerable broadening, but essentially no shift in mass. The acceptance-corrected transverse-momentum spectra have a shape atypical for radial flow and show a significant mass dependence, pointing to different sources in different mass regions.