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.Comment: Submitted to Physical Review Letter