Initial Results from the Majorana Demonstrator

Abstract

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...

Neutrinoless double beta decay in chiral effective field theory: lepton number violation at dimension seven

“…The observation of 0νββ would have far reaching implications: it would demonstrate that neutrinos are Majorana fermions [2], shed light on the mechanism of neutrino mass generation, and give insight on leptogenesis scenarios for the generation of the matterantimatter asymmetry in the universe [3]. The current experimental limits on the halflives are already impressive [4][5][6][7][8][9][10][11][12][13], at the level of T 0ν 1/2 > 5.3 × 10 25 y for 76 Ge [12] and T 0ν 1/2 > 1.07 × 10 26 y for 136 Xe [13], with next generation ton-scale experiments aiming at a sensitivity of T 0ν 1/2 ∼ 10 27−28 y. By itself, the observation of 0νββ would not immediately point to the underlying physical origin of LNV.…”

Renormalized approach to neutrinoless double- β decay

“…0νββ [7] is by far the most sensitive laboratory probe of lepton number violation (LNV). Current experimental limits are very stringent [8][9][10][11][12][13][14][15][16][17][18][19][20], e.g. T 0ν 1/2 > 1.07 × 10 26 yr for 136 Xe [12], with the next-generation ton-scale experiments aiming for improvements by one or two orders of magnitude.…”

Long-distance contributions to neutrinoless double beta decay π−→π+ee

“…Due to its importance, the detection of 0ν2β decays is being pursued by many experiments around the world [3][4][5][6][7][8][9][10][11][12][13][14]. Current experimental measurements of the decay's half-lives T 0ν 1/2 have reached the level of T 0ν 1/2 > 1.07×10 26 yr for 126 Xe [7], with a new generation of ton-scale experiments aiming for the level of sensitivity improved by 1 or 2 orders of magnitude.…”