First results on angular distributions of thermal dileptons in nuclear collisions

TL;DR

This study presents initial measurements of angular distributions of thermal dileptons in nuclear collisions, showing no polarization and supporting their thermal radiation origin from a randomized system.

Contribution

First measurement of angular distributions of thermal dileptons in nuclear collisions, confirming their unpolarized nature and supporting thermal radiation interpretation.

Findings

Angular distributions are uniform in polar and azimuth angles.

Structure function parameters mbda, ta, mbda are zero.

Results support thermal radiation hypothesis.

Abstract

The NA60 experiment at the CERN SPS has studied dimuon production in 158A GeV In-In collisions. The strong excess of pairs above the known sources found in the complete mass region 0.2<M<2.6 GeV has previously been interpreted as thermal radiation. We now present first results on the associated angular distributions. Using the Collins-Soper reference frame, the structure function parameters \lambda, \mu and \nu are measured to be zero, and the projected distributions in polar and azimuth angles are found to be uniform. The absence of any polarization is consistent with the interpretation of the excess dimuons as thermal radiation from a randomized system.