DELPHI Z0 PEAK EVENT

DELPHI Event r0010600e0004505


e+ e- ---> e+ e- q qbar
The e+ and e- are both identified by their showers in the HPC.

The e+ (19 GeV, down) produces a clean shower and gives consistent energy measurements in the TPC(19.3), the track fit(17.3) and the HPC(20.7).

The e- (29? GeV) is badly measured in the TPC (close to a sector boundary), has 10.5 GeV in the track fit and 28.8 GeV in the HPC. In addition, there is a sign of pre-showering in the OD and a close shower of 1.1 GeV in the HPC. Hence its energy is not accurately determined.

Using the above energies, the invariant mass of the e+e- pair is 44.1 GeV. The recoiling system has an energy of 43.2 and a mass of 38.8 GeV.

The visible energy, excluding the pair, is about 20 GeV. Hence, the event is not fully balanced. The polar angle of the missing momentum is around 13 degree, and the particles could have escaped detection in the forward region. One of the tracks (down left in x,y projection), with 3.6 GeV, is minimum ionizing in the HPC and the HAB, but leaves no energy in the HAF. It misses the Muon chambers. If this were a Muon an additional neutrino could have been produced.