Two showers are detected, one with 9.0 GeV in the HPC, the other with
6.4 GeV in the FEMC. The effective mass with the closest track is:
M(Mu+,Pi0(6.4)) = 8.4 GeV
M(Mu-,Pi0(9.0)) = 6.0 GeV
hence excluding that Taus are at the origin of the muons, (mis-interpreted charged pions).
The event can thus be interpreted as a Mu+ Mu- with two photons radiated in the initial or final state. The latter is the most likely explanation, since the angles of the photons with the outgoing tracks are smaller than with the beam.