P326 Photon Vetoes Simulation

The P326 experiment aims to a very rare decay, K⁺ rarr pi⁺nunu^-, immersed in a overwhelming K⁺ rarr pi⁺nu, K⁺ rarr pi⁺pi⁰ (and other decays) background. This calls for an impressive rejection capability (10¹²) in order to keep a significant signal/background ratio while keeping a good overall efficiency. The detector should then be hermetic to photons in order to have a pi⁰ inefficiency at the level of 10^-8, that translates in an unprecedented single photon inefficiency of 10 ^-5 (above 1 GeV). The design of such a detector is a challenge both from the instrumental and from the simulation point of view. Three detector elements will cover the P326 experiment acceptance: the liquid krypton calorimeter of NA48, a set of large angle veto rings and small angle counters. In order to control the inefficiency at the level of 10^-5, all the uncertainties in the detector response simulation/parametrization should be kept at the same level of accuracy. This calls for very detailed simulation description both at the level of accurate geometry description and of realistic digitization and signal generation. At the same time, in order to carefully check the uniformity of the detector response over the whole acceptance, the mechanical and construction details should be precisely included. Finally, all the physics processes in the materials should be reproduced at the same level of accuracy. A very detailed study of the large angle veto structure exploiting the full potential of the GEANT4 technology has been developed in order to design the detector and to validate the required photon detection inefficiency on real-scale prototypes.