DELPHI Event r0035606e0003559

e+ e- ---> Tau+ Tau-
Excellent illustration of particle identification by means of the calorimetry in the DELPHI detector.

The Tau- (down) decays to (Mu Nu). The muon is measured with the VD, ID, TPC, OD and has 13.0 GeV/c momentum. It appears as a MIP in the HPC and in the four layers of the HAB. It has hits associated in both planes of the muon chambers.

The upper jet has three fast collimated tracks. The one on the right is positive with 7.9 GeV/c momentum measured by the VD, ID, TPC, OD. It leaves no detectable energy in the HPC and produces an energetic shower in the HAB. This identifies the particle as a hadron (most likely a Pi).

The other two tracks (in red) are probably due to a photon conversion. The one on the right is positive with 13.6 GeV/c momentum and is measured in the VD, ID, TPC, OD. It has an associated shower in the HPC. The one on the left is only measured in the VD, ID and TPC. It is negative and has 13.1 GeV/c momentum. It made an interaction at the end of the RICH, which is seen as a shower in the OD (represented by the green box). It is accompanied by two energetic showers in the HPC. As seen from a blown-up view with the VD, the two tracks cross close to the position of the beam pipe.

There are, in addition, two slow tracks (in green) seen in the central detectors. One is a positive with 118 MeV/c, the other a negative with 135 MeV/c, both producing a spiral. They are probably electrons from the conversion of a photon in the beam pipe (see their extrapolation in the blown-up view with the VD).

The Tau+ therefore decayed to a (Pi+ Nu) and one or more Pi0s.

To be completely fair, there is a small uncertainty about the hard conversion pair. The energy of the showers in the HPC is of 5 and 7 GeV, only half the energy of the tracks. However, the positive track hits the HPC very close to a module boundary in z and energy can be lost between the modules. The negative track has interacted in the RICH and part of the energy is probably absorbed before reaching the HPC. So, it can be understood that both have an underestimated energy in the HPC. In addition, both showers start in the first layer of the HPC and are completely contained in the detector. They appear to be of electromagnetic origin and are associated to the tracks. An interpretation as hadrons from a K0 decay is therefore very unlikely.