The photon veto system for the NA62 rare kaon decay experiment

The NA62 experiment at CERN SPS, aimed to measure ~80 events of very rare decay K⁺¿ ¿+ ¿ anti-v (BR~8.5×10-10), poses stringent requirements on PID capabilities to reject the overwhelming K_¿2(63%) and K_¿2(21%) backgrounds. The photon veto system must provide a rejection factor of 10^-8 on ¿⁰ decays. As main ¿ veto detector, the NA48 liquid Kr calorimeter will be used. To have full geometrical acceptance, a set of 12 veto stations will be placed along the vacuum decay tank, with an inefficiency <10^-4 in a wide energy range (50 MeV-35 GeV). Good energy resolution (~10% at 1 GeV) for threshold definition, good time resolution (~1 ns) to be used at the trigger level, sensitivity to mip, for calibration with muons of the beam halo are needed. We performed an intense R&D program on 3 solutions. Studies have been performed at the Frascati BTF beam and all 3 meet the efficiency requirements. The final choice uses a peculiar radial arrangement of lead glasses in rings. Studies on the front-end electronics are under way. The first full veto station has been constructed installed and tested at CERN.