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

TL;DR

The paper details the design, construction, and testing of the large-angle photon veto system for the NA62 experiment at CERN, crucial for background suppression in rare kaon decay measurements.

Contribution

It introduces a novel veto detector system using recycled lead glass and advanced electronics, with comprehensive testing and simulation validation.

Findings

Achieved photon detection inefficiency below 10^{-4} for energies as low as 200 MeV.

Developed electronics enabling simultaneous time and energy measurements over a wide dynamic range.

Validated detector performance through test-beam data and detailed Monte Carlo simulations.

Abstract

The branching ratio (BR) for the decay K^+ \to \pi^+\nu\bar{\nu} 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 background from dominant kaon decays with final state photons, the large-angle photon vetoes (LAVs) must detect photons of energy as low as 200 MeV with an inefficiency of less than 10^{-4}. The LAV detectors make use 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.