Abstract
Neutrinoless double-beta decay searches seek to determine the nature of neutrinos, the existence of a lepton violating process, and the effective Majorana neutrino mass. The Majorana Collaboration is assembling an array of high purity Ge detectors to search for neutrinoless double-beta decay in 76Ge. The Majorana Demonstrator is composed of 44.8 kg (29.7 kg enriched in 76Ge) of Ge detectors in total, split between two modules contained in a low background shield at the Sanford Underground Research Facility in Lead, South Dakota. The initial goals of the Demonstrator are to establish the required background and scalability of a Ge-based, next-generation, tonne-scale experiment. Following a commissioning run that began in 2015, the first detector module started physics data production in early 2016. We will discuss initial results of the Module 1 commissioning and first physics run, as well as the status and potential physics reach of the full Majorana Demonstrator experiment. The collaboration plans to complete the assembly of the second detector module by mid-2016 to begin full data production with the entire array.
Highlights
Neutrinoless double-beta (ββ(0ν)) decay searches represent the only viable experimental method for testing the Majorana nature of the neutrino [1]
Discussion of Early Results After all cuts in dataset 1 (DS1), there are 5 events within a 400 keV window centered on Qββ. (See Fig. 2.) This results in a background index of (7.5+−43..45 × 10−3 cnts/(keV kg y))
The efficiency for ββ(0ν) is 0.61 ± 0.04 resulting from cuts due to resolution (0.84), the probability that ββ results in a full energy deposit (0.90), the single-site waveform cut (0.90), and the surface-α cut (0.90)
Summary
Neutrinoless double-beta (ββ(0ν)) decay searches represent the only viable experimental method for testing the Majorana nature of the neutrino [1] The observation of this process would immediately imply that lepton number is violated and that neutrinos are Majorana particles [2]. Evidence from the SNO experiment [4] of a clear departure from non-maximal mixing in solar neutrino oscillation implies a minimum effective Majorana neutrino mass of ∼15 meV for the inverted mass ordering scenario. This target is within reach of next-generation ββ(0ν) searches. For recent comprehensive experimental and theoretical reviews, see Refs. [5, 6, 7, 8, 9, 10, 11, 12]
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