DELPHI Event r0015236e0003792

e+ e- ---> q qbar gamma , very hard gamma
This event is characterized by a very hard (about 40 GeV) gamma completely isolated and opposite to 4 charged tracks. It is an extreme case of a radiative event.

The single gamma has 37 GeV measured in the HPC. Behind it is a 1.2 GeV shower in the HAB, probably due to leakage as the shower is close to an HPC module boundary.

The 4 charged particles have a (charge x momentum) of +4.6, +1.2, -5.1 and -3.3 GeV/c respectively, when ordered clockwise. They are well measured with the VD, ID, TPC and OD (except the 1.2 GeV which is without OD). They are accompanied by neutral electromagnetic activity in the HPC, which was resolved into 3 showers totalling 23 GeV energy. The shower in the HAB has 9+-7 GeV and is probably produced by the 5.1 GeV/c particle.

This event can be interpreted as a very hard initial state radiation, which would leave the e+ e- interacting at about 30 GeV in their cms. The final state is most probably hadronic, although several combinations of the charged and neutral particles would also be compatible with Tau+ Tau- in a 3 vs 1 topology. A "back of the envelope" calculation leads to a rate of a few 10**-4 for such events with respect to all Z0 hadronics.