DELPHI Event r0021982e0012186

e+ e- ---> Tau+ Tau- Mu+ Mu-
The Tau+ (left) decays into a Pi+ (3.1 GeV), identified as minimum ionizing in the HPC and showering in the HAB. An electromagnetic shower is seen in the neighbouring module of the HPC. Its invariant mass with the Pi+ is about 0.7 GeV. It is compatible with the decay chain :
Tau+ --> Rho+ Nu , Rho+ --> Pi+ Pi0

The Tau- (right upper) decays into an e- (19.4 GeV) identified by its shower in the HPC.

Finally, there is a pair Mu+ (9.8 GeV, left down), Mu- (2.7 GeV, left middle), both identified as minimum ionizing in the HPC and HAB and the Mu+ with hits in both planes of the MUB. The low energy Mu- ranges out in the HAB. The invariant mass of the pair is 6.7 GeV.

The event is interpreted as a Tau+ Tau- accompanied by a photon which gives the Muon pair by "internal conversion".

Ref : this event was published in the Cern Courrier Vol 31 N0 8 (Oct 1991) on p. 8, after the "new effect" observed by ALEPH as an excess of Tau Tau V events had been excluded at the 1991 Conference in Geneva.