The large-angle photon veto system for the NA62 experiment at CERN

The branching ratio (BR) for the decay K⁺→π⁺νν̅ is a sensitive probe for new physics. The NA62 experiment at the CERN SPS will measure this BR to within about 10%. To reject the dominant background from channels with final state photons, the large-angle vetoes (LAVs) must detect photons of energy as low as 200 MeV with an inefficiency of less than 10^-4, as well as provide energy and time measurements with resolutions of 10% and 1 ns for 1 GeV photons. The LAV detectors make creative reuse of lead glass blocks recycled from the OPAL electromagnetic calorimeter barrel. We describe the mechanical design and challenges faced during construction, the characterization of the lead glass blocks and solutions adopted for monitoring their performance, and the development of front-end electronics to allow simultaneous time and energy measurements over an extended dynamic range using the time-over-threshold technique. Our results are based on test-beam data and are reproduced by a detailed Monte Carlo simulation that includes the readout chain.