The Performance of the Muon Veto of the GERDA Experiment

TL;DR

This paper evaluates the muon veto system of the GERDA experiment, demonstrating its high efficiency in detecting and rejecting muon-induced background events to improve the search for neutrinoless double beta decay.

Contribution

The paper provides the first detailed measurement of the muon veto efficiency and background suppression capabilities in the GERDA experiment.

Findings

Muon detection efficiency of 99.935% with simulation.

Rejection efficiency of 99.2% for muon-germanium events.

Background index from muons without veto is 3.16e-3 cts/(keV·kg·yr).

Abstract

Low background experiments need a suppression of cosmogenically induced events. The GERDA experiment located at LNGS is searching for the neutrinless double beta decay of $^{76}$Ge. It is equipped with an active muon veto the main part of which is a water Cherenkov veto with 66 PMTs in the watertank surrounding the GERDA cryostat. With this system 806 live days have been recorded, 491 days were combined muon-germanium data. A muon detection efficiency of $\varepsilon_{\mu d}=(99.935\pm0.015)$ \% was found in a Monte Carlo simulation for the muons depositing energy in the germanium detectors. By examining coincident muon-germanium events a rejection efficiency of $\varepsilon_{\mu r}=(99.2_{-0.4}^{+0.3})$ \% was found. Without veto condition the muons by themselves would cause a background index of $\textrm{BI}_{\mu}=(3.16 \pm 0.85)\times10^{-3}$ cts/(keV$\cdot$kg$\cdot$yr) at $Q_{\beta\beta}$.