First results on angular distributions of thermal dileptons in nuclear collisions

Understand

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.