The photon veto system for the P326 experiment at CERN

The P326 experiment at CERN aims at measuring the very rare decay K + → π + ν ν ¯ , which is immersed in an overwhelming background from K + → μ + ν , K + → π + π 0 and other decays. This calls for an impressive rejection capability ( 10 12 ) in order to keep a significant signal/background ratio while keeping a good overall efficiency. One of the key components of the detector is the photon veto system which should have a global π 0 inefficiency of 10 - 8 or better and should almost hermetically cover the decay volume down to zero degrees. The detectable energies range from about 100 MeV to almost 70 GeV. The photon veto system will be composed of a large angle veto system, the NA48 Liquid Krypton Calorimeter for intermediate angles and Intermediate Ring Calorimeters as well as a Small Angle Calorimeter to close the acceptance at small angles. An intense R&D program for these detectors is in progress.