DELPHI Z0 PEAK EVENT

DELPHI Event r0030718e0001588


e+ e- ---> e+ e- gamma gamma
The electrons are identified by their associated showers in the HPC. The e- has a momentum of 3.9 GeV/c and a shower of 6.7 GeV in the HPC. The e+ has a momentum of 14.1 GeV/c and a shower of 15.0 GeV in the HPC. This shower is on the edge of an HPC coffin and produces some leakage into the HAB. In addition, two neutral showers are detected in the HPC, one with 46.4 GeV, the other with 20.9 GeV.

The main kinematical quantities for the particles and their combinations are (values from DELANA):

                   Px       Py      Pz       E       Mass  Theta   Phi
 e+               12.550   1.712   6.224  14.112    0.106   63.8   7.8
 e-                3.070  -2.024   1.353   3.918    0.106   69.8 326.6
 gamma1           13.154 -12.246  10.760  20.947    0.000   59.1 317.0
 gamma2          -37.187  13.053 -24.444  46.377    0.000  121.8 160.7
 e+e-             15.619  -0.312   7.577  18.030    4.861   64.1 358.9
 gamma1gamma2    -24.033   0.807 -13.684  67.323   61.375  119.6 178.1
 e-gamma1         16.223 -14.270  12.113  24.865    2.165   60.7 318.7
 e+gamma2        -24.638  14.765 -18.220  60.489   50.019  122.4 149.1
 e+e-gamma1gamma2 -8.414   0.495  -6.107  85.354   84.717  125.9 176.6
The energy missing in the event is of about 6 GeV and seems to be due to an underestimate of the e+ momentum. Indeed, the missing momentum points in the same direction as the e+ and the Energy-momentum balance requires 22 GeV for this track, the others remaining almost unchanged. It can thus be assumed that all particles are observed. After fitting the momenta from the angles, assuming E-p conservation, the mass of the two gammas is 58.2 +- 0.8 GeV and of the e+e- is 6.7 +- 0.5 GeV. The effective masses of the (e,gamma) pairs exclude that Tau's are at their origin. The event could be interpreted as a e+ e- with two photons radiated in the initial or final state, but the large mass (about 60 GeV) of the 2-gamma system is not very likely for this explanation. On the other hand, the smallest angle between a photon and lepton is only 15.5 degree. This event shows the same characteristics as the L3 events with two hard gammas and the same mass of the two-gamma system.

Ref : Presented by U.Amaldi after the seminar of S.C.C.Ting (CERN, 26 Nov 1992) on high mass photon pairs.