The Large-Angle Photon Veto System for the NA62 Experiment at CERN

TL;DR

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

Contribution

It introduces a novel veto system using recycled lead glass blocks, with innovative electronics for precise photon detection and timing, validated by test-beam data and simulations.

Findings

Achieved photon detection inefficiency below 10^-4 for 200 MeV photons.

Developed electronics enabling simultaneous time and energy measurements.

Validated system performance with test-beam data and Monte Carlo simulations.

Abstract

The branching ratio (BR) for the decay K^+ \rightarrow \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 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.