DELPHI Z0 PEAK EVENT

DELPHI Event R0021584E0007221

e+ e- ---> Tau+ Tau- gamma gamma

The Tau+ gives a Pi+ (down right) of 3.6 GeV, identified as minimum ionizing in the HPC and depositing its energy in the first layer of the HAB. A 1.4 GeV shower is seen in the HPC, close to its flight direction. The effective mass of the Pi with this Pi0 is 620 MeV, hence the Tau probably decayed via a Rho.

The Tau- decays into a likely e- of 14.5 GeV, passing through a crack in the HPC and depositing all its energy in the first layer of the HAB.

In addition, two hard showers of 18.3 and 13.5 GeV are detected in the HPC. Assuming that they are Pi0s, their invariant mass with the closest track is:
M(Pi+,Pi0(18.3)) = 5.6 GeV
M(Pi-,Pi0(13.5)) = 15.5 GeV
which excludes that they originate from the decay of a Tau. They are thus probably due to radiation from the final state Taus (both gammas are emitted at about 90 degree with the beam direction, very unlikely for initial state radiation).