DELPHI Event r0009810e0001341

e+ e- ---> Tau+ Tau- gamma
The Tau pair is cleanly identified by a 1 vs 3 topology

The Tau+ decays into a Pi+ (18.1 GeV), identified by a late shower produced in the HPC (starting at layer 7).

The Tau- decays into 3 particles, Pi- (4.8 GeV), Pi- (3.1 GeV), Pi+ (6.6 GeV), all three being minimum ionizing in the HPC and depositing energy in the HAB. The 3-particle system has 14.5 GeV energy and 1.08 GeV mass, hence compatible with a Tau.

A shower from a gamma (23.4 GeV) is seen in the HPC on the same side as the 3-prong with a small amount of leakage in the HAB. The invariant mass formed with the three particle system is 9.5 GeV, incompatible with the decay of a Tau. The photon is thus of radiative origin, most likely from the final state Tau-.