Thermal Dimuon Emission in In-In at the CERN SPS

TL;DR

This study measures low-mass dimuon production in In-In collisions at CERN SPS, revealing a broadened rho spectral function and a transition from hadronic to partonic emission sources based on spectral and flow analysis.

Contribution

It provides the first precision measurements of the dimuon mass and transverse momentum spectra, highlighting spectral broadening and flow behavior changes indicating a transition in emission sources.

Findings

Significant rho spectral broadening observed

Mass spectra show no significant mass shift

Flow parameter T_eff rises then drops with mass

Abstract

The NA60 experiment at the CERN SPS has studied low-mass dimuon production in 158A GeV In-In collisions. A significant excess of pairs is observed above the yield expected from neutral meson decays, consistent with a dominant contribution from $\pi\pi\to\rho\to\mu\mu$. This paper presents precision results on the mass and transverse momentum spectra of the excess pairs. The space-time averaged rho spectral function associated to the measured mass distribution shows a significant broadening, but essentially no mass shift. The slope parameter $T_{eff}$ 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 indicates a transition to an emission source with much smaller flow, possibly of partonic origin.