Systematic Effects in Pulse Shape Analysis of HPGe Detector Signals for 0νββ V.M. Gehman a,b , S.R. Elliott a , D.-M. Mei c a Los Alamos National Laboratory, Los Alamos, NM 87545 b Center for Experimental Nuclear Physics and Astrophysics, and Department of Physics, University of Washington, Seattle, WA 98195 c Department of Physics, The University of South Dakota, Vermillion, SD 57069 Abstract Pulse shape analysis is an important background reduction and signal identification technique for next generation of 76 Ge 0νββ experiments. We present a study of the systematic uncertainties in one such parametric pulse-shape analysis technique for separating multi-site background from single-site signal events. We examined systematic uncertainties for events in full-energy gamma peaks (predominantly multi-site), double-escape peaks (predominantly single-site) and the Compton continuum near Q ββ (which will be the dominant background for most 0νββ searches). In short, we find total (statistical plus systematic) fractional uncertainties in the pulse shape cut survival probabilities of: 6.6%, 1.5% and 3.8% for double-escape, continuum and γ-ray events respectively. Key words: Neutrinoless double-beta decay, Pulse Shape Analysis, Germanium Detectors 1. 0νββ in Germanium The search for physics beyond the Standard Model has one of its most promising leads in neutrinoless double-beta decay (0νββ) in particular, and neutrino physics more generally. Interest in 0νββ is extremely well-motivated in the literature [1, 2, 3, 4, 5, 6]. There are approximately 10 isotopes known to undergo two-neutrino double beta decay (2νββ, i.e. 76 Ge → 76 Se + 2e - + 2 ν), the slowest nuclear decay allowed in the Standard Model. These are also the isotopes of interest in the search for 0νββ (i.e. 76 Ge → 76 Se + 2e - + 0 ν), a ββ mode forbidden by the Standard Model. If observed, 0νββ would imply the existence of massive Majorana neutrinos[7] and could also lead to the discovery of other physics beyond the Standard Model. If T 0ν 1/2 could be measured in several isotopes, it would do much to elucidate this physics [8]. To make a meaningful comparison between T 0ν 1/2 measurements in several isotopes, the total experimental uncertainty must be sufficiently low. This places significant demands on the size of systematic uncertainties of the global experimental program. The importance of systematic uncertainties in potential results of 0νββ searches motivates the investigation of systematic uncertainties in pulse shape analysis (PSA) presented here. 76 Ge is one of the ββ isotopes under investigation in the current generation of 0νββ searches (the most stringent T 0ν 1/2 limits already come from 76 Ge), and there are two next-generation 76 Ge experiments currently under develop- ment: Majorana[9, 10, 11, 12, 13] and GERDA [14]. This article will present work performed in support of the Majorana experiment. Both Majorana and GERDA, as well as all 0νββ searches (and indeed all searches for rare events), rely heavily on reducing backgrounds while retaining signals as effectively as possible. One of the ways that both experiments plan to reduce background and identify signal events is through PSA on the array of high-purity germanium (HPGe) detectors that will comprise the experiments. Because of the inherent complexity of making pulse shape cuts, systematic uncertainties in PSA-cut efficacy could be one of the leading contributors to the total systematic uncertainty budget of these experiments. An examination of these systematic uncertainties is the primary focus of this article. We will begin with a description of the detector used in this work, then move on to an overview of the use of PSA in HPGe detectors for 0νββ searches up to and including this article. We will then detail our survey of systematic effects on PSA, and close with some discussion of the context of this work in the Majorana experiment. Email address: vmg@lanl.gov (V.M. Gehman) Preprint submitted to Elsevier October 29, 2018 arXiv:0907.2940v3 [nucl-ex] 5 Mar 2010