Results on angular distributions of thermal dileptons in nuclear collisions

TL;DR

This study analyzes angular distributions of thermal dileptons produced in nuclear collisions, finding no polarization, which supports their thermal radiation origin from a randomized system.

Contribution

It provides the first measurement of angular distributions of thermal dileptons in nuclear collisions, confirming the absence of polarization and supporting the thermal radiation hypothesis.

Findings

Structure function parameters are consistent with zero.

Angular distributions are uniform in the Collins-Soper frame.

Results support thermal radiation interpretation.

Abstract

The NA60 experiment at the CERN SPS has studied dimuon production in 158 AGeV In-In collisions. The strong pair excess above the known sources found in the mass region $0.2<M<2.5$ GeV has been previously interpreted as thermal radiation. In this paper results on the associated angular distributions for $M<1$ GeV, as measured in the Collins-Soper reference frame, are presented. The structure function parameters $\lambda$, $\mu$, $\nu$ are consistent with zero and the projected polar and azimuth angle distributions are uniform. The absence of any polarization is consistent with the interpretation of the excess dimuons as thermal radiation from a randomized system.