Abstract
The Majorana Collaboration has assembled an array of high purity Ge detectors to search for neutrinoless double-beta decay in 76Ge with the goal of establishing the required background and scalability of a Ge-based next-generation ton-scale experiment. The Majorana Demonstrator consists of 44 kg of high-purity Ge (HPGe) detectors (30 kg enriched in 76Ge) with a low-noise p-type point contact (PPC) geometry. The detectors are split between two modules which are contained in a single lead and high-purity copper shield at the Sanford Underground Research Facility in Lead, South Dakota. Following a commissioning run that started in June 2015, the full detector array has been acquiring data since August 2016. We will discuss the status of the Majorana Demonstrator and initial results from the first physics run; including current background estimates, exotic low-energy physics searches, projections on the physics reach of the Demonstrator, and implications for a ton-scale Ge-based neutrinoless double-beta decay search.
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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