The Majorana Collaboration is operating an array of high purity Ge detectors to search for neutrinoless double-β decay in ^{76}Ge. The Majorana Demonstrator comprises 44.1 kg of Ge detectors (29.7 kg enriched in ^{76}Ge) split between two modules contained in a low background shield at the Sanford Underground Research Facility in Lead, South Dakota. Here we present results from data taken during construction, commissioning, and the start of full operations. We achieve unprecedented energy resolution of 2.5 keV FWHM at Q_{ββ} and a very low background with no observed candidate events in 9.95 kg yr of enriched Ge exposure, resulting in a lower limit on the half-life of 1.9×10^{25}  yr (90% C.L.). This result constrains the effective Majorana neutrino mass to below 240-520 meV, depending on the matrix elements used. In our experimental configuration with the lowest background, the background is 4.0_{-2.5}^{+3.1}  counts/(FWHM t yr).

The MajoranaDemonstratorwill search for the neutrinoless double-beta(ββ0ν)decay of the isotopeGe with a mixed array of enriched and natural germanium detectors. The observation of this rare decay would indicate that the neutrino is its own antiparticle, demonstrate that lepton number is not conserved, and provide information on the absolute mass scale of the neutrino. The Demonstratoris being assembled at the 4850-foot level of the Sanford Underground Research Facility in Lead, South Dakota. The array will be situated in a low-background environment and surrounded by passive and active shielding. Here we describe the science goals of the Demonstratorand the details of its design.

We present new limits on exotic keV-scale physics based on 478 kg d of Majorana Demonstrator commissioning data. Constraints at the 90% confidence level are derived on bosonic dark matter (DM) and solar axion couplings, Pauli exclusion principle violating (PEPV) decay, and electron decay using monoenergetic peak signal-limits above our background. Our most stringent DM constraints are set for 11.8 keV mass particles, limiting gAe < 4.5 × 10 −13 for pseudoscalars and α ′ α < 9.7 × 10 −28 for vectors. We also report a 14.4 keV solar axion coupling limit of g eff AN × gAe < 3.8 × 10 −17 , a 1 2 β 2 < 8.5 × 10 −48 limit on the strength of PEPV electron transitions, and a lower limit on the electron lifetime of τe > 1.2 × 10 24 yr for e − → invisible.PACS numbers: 95.35.+d

a b s t r a c tThe MAJORANA collaboration is constructing the MAJORANA DEMONSTRATOR at the Sanford Underground Research Facility at the Homestake gold mine, in Lead, SD. The apparatus will use Ge detectors, enriched in isotope 76 Ge, to demonstrate the feasibility of a large-scale Ge detector experiment to search for neutrinoless double beta decay. The long half-life of this postulated process requires that the apparatus be extremely low in radioactive isotopes whose decays may produce backgrounds to the search. The radioassay program conducted by the collaboration to ensure that the materials comprising the apparatus are sufficiently pure is described. The resulting measurements from gamma-ray counting, neutron activation and mass spectroscopy of the radioactive-isotope contamination for the materials studied for use in the detector are reported. We interpret these numbers in the context of the expected background for the experiment.

Neutron inelastic scattering as a background in the Majorana neutrinoless double-beta decay experiment V E Guiseppe, M Devlin, S R Elliott et al. 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 76 Ge. The Majorana Demonstrator is composed of 44.8 kg (29.7 kg enriched in 76 Ge) 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. IntroductionNeutrinoless 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]. A measurement of the ββ(0ν) decay rate may also yield information on the absolute neutrino mass. Measurements of atmospheric, solar, and reactor neutrino oscillation [3] indicate a large parameter space for the discovery of ββ(0ν) decay just beyond the current experimental bounds below m ββ ∼50 meV. Moreover, 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. An experiment capable of observing this minimum rate would therefore help elucidate the Majorana or Dirac nature of the neutrino for invertedhierarchical neutrino masses. Even for the normal hierarchy, these experiments would improve the existing sensitivity by ≈1 order of magnitude. A nearly background-free tonne-scale 76 Ge experiment would be sensitive to effective Majorana neutrino masses below ∼15 meV. For recent comprehensive experimental and theoretical reviews, see Refs. [5,6,7,8,9,10,11,12].Recent developments in germanium detector technology make a large-scale ββ(0ν) decay search feasible using 76 Ge. In these proceedings we describe the Majorana Demonstrator, an experimental effort completing construction during 2016, whose goal is to demonstrate the techniques required for a next-generation ββ(0ν) decay experiment with enriched Ge detectors. A complementary 76 Ge ef...