Rare Low-Energy Event Searches with the Majorana Demonstrator

TL;DR

The Majorana Demonstrator searches for neutrinoless double-beta decay and explores additional rare low-energy events like dark matter and axions, demonstrating low-background detection at underground facilities.

Contribution

This paper presents the results and ongoing efforts of rare-event searches using the Majorana Demonstrator, highlighting its low-background and low-threshold capabilities for multiple physics investigations.

Findings

Successful detection of low-energy rare events

Demonstrated low-background and low-threshold performance

Projected sensitivity for future rare-event searches

Abstract

The Majorana Demonstrator is currently searching for neutrinoless double-beta decay in $^{76}$Ge and will demonstrate the feasibility to deploy a tonne-scale experiment in a phased and modular fashion. It consists of two modular arrays of natural and $^{76}$Ge-enriched germanium detectors totaling 44.1 kg, of which 29.7 kg is enriched, located at the 4850' level of the Sanford Underground Research Facility in Lead, South Dakota, USA. The low-backgrounds and low trigger thresholds ($<$1 keV) achieved by the Demonstrator allow for additional rare-event searches at low-energies, e.g. searches for WIMPs, bosonic dark matter, and solar axions. In this work, we will present results and ongoing efforts related to these rare-event searches and discuss the future reach of Majorana.

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Full text

1 The Majorana Demonstrator

The Majorana Demonstrator [1] is a search for neutrinoless double-beta decay ($0\nu\beta\beta$) in 76Ge currently operating at the 4850’ level of the Sanford Underground Research Facility (SURF) in Lead, SD, USA. The goals of the Majorana Demonstrator are to demonstrate backgrounds low enough to justify building a tonne-scale experiment, to demonstrate the feasibility to construct modular arrays of high-purity Ge (HPGe) detectors, and for additional searches for physics beyond the Standard Model (SM). The Majorana Demonstrator employs 44.1 kg P-type point contact (PPC) HPGe detectors, 29.7 kg of which is enriched to 88% 76Ge. The detectors are arranged into two separate cryostats constructed of ultra pure underground electroformed copper (UGEFCu). Starting from the innermost region, two cryostat modules are surrounded by an inner layer of electroformed copper, an outer layer of commercial copper, lead, an active muon veto, borated polyethylene, and polyethylene.

The Majorana Demonstator is currently operating with the best energy resolution of any $0\nu\beta\beta$ experiment with an energy resolution of 2.5 keV FWHM at 2039 keV, the Q-value for $0\nu\beta\beta$ in 76Ge. Recent results released at the Neutrino 2018 conference include a blind analysis of data with 26 kg-yr enriched exposure (11.85 kg-yr blind data). An observed background rate of 15.4 $\pm$ 2.0 cts/FWHM t yr was found near the 76Ge Q-value for a full exposure limit on the half life of $T^{0\nu}_{1/2}>2.7\times 10^{25}$ [2, 3].

2 The Majorana Low-Energy Program

The low backgrounds and detector thresholds achieved by the Majorana Demonstrator allow for additional searches for physics beyond the SM at low-energies ($<$100 keV). These include searches for bosonic dark matter, Pauli exclusion principle violation, electron decay, solar axions [4], and light ($\sim$10 GeV/c2) WIMPs. Careful material selection and handling, as well as extreme care taken to reduce the amount of cosmogenic activation in the enriched detectors results in low-backgrounds down to low-energies. Additionally, the low-capacitance achieved by PPC detector technology coupled with the development of a low-noise, low-mass front-end [5] enable low trigger thresholds ($<$1 keV) and excellent energy resolution (0.4 keV FWHM at 10.4 keV). Excellent pulse-shape analysis abilities enable further background discrimination.

3 Previous Results

The Majorana background spectrum from 5-50 keV is shown in Fig. 1. There is a clear difference visible in the enriched and natural detectors due to the fact that, unlike the enriched detectors, the natural detectors had a much longer surface exposure, resulting in increased cosmogenic activation.

The Majorana collaboration has set limits on bosonic dark matter [4]. This is accomplished by searching for peaks at the mass of the dark matter particle in the spectrum, which is assisted by the excellent energy resolution of the Majorana detectors. The spectrum in solid blue in Fig. 1 was used to set the previous limits on pseudoscalar bosonic dark matter shown as the solid red line in Fig. 2. The dotted black line shows the projected sensitivity using new data with 9497 kg-d of exposure including blind data, assuming a background rate of 0.01 cts/keV/kg/d.

In addition to dark matter, the Majorana collaboration has set limits on lightly-ionizing particles (LIPs). LIPs are theoretical particles whose electromagnetic interactions are suppressed compared to the normal interaction strength. This would manifest experimentally as particles which have a fractional charge compared to the fundamental charge. Limits are typically set based on a reduction factor f. The charge $q$ detected is related to the reduction factor f by $q=e/f$, where $e$ is the fundamental charge. The Majorana Demonstrator completed a background free search for LIPs resulting in the limit shown in Fig. 3 [6].

4 Ongoing Analyses

In addition to the results shown in the previous section, the Majorana collaboration is working on pulse-shape analysis efforts to further reduce backgrounds at low-energies in order to improve the analysis thresholds. Lower analysis thresholds would enable a host of additional searches at low energies, including searches for solar axions and low-mass WIMPs.

The Majorana collaboration is able to search for solar axions through two different coupling mechanisms: the Primakov effect as well as axio-electric effect. Solar axions can be detected via the Coherent Primakoff effect, even without exact knowledge of the directions of the crystal axes, if there are more than about 15 detectors [7]. In this case, the true total axion rate can be approximated by averaging angle-specific rates over all possible angles. This would manifest experimentally as a time-dependent spectrum with signals expected between 3-10 keV. Additionally, solar axions can be detected via the axio-electric effect [8]. This is analogous to the photoelectric effect, in that an axion is absorbed and the energy is released as electrons. This would manifest as characteristic peaks in a spectrum below 15 keV.

The Majorana Demonstrator is also sensitive to low-mass WIMPs ($\sim$10 GeV/c2). Though the Demonstrator is unable to distinguish between events originating from nuclear recoils and those due to electron recoils, the low intrinsic backgrounds will enable a search which can provide an important check for experiments searching for WIMPs using Ge.

Acknowledgments

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, the Particle Astrophysics and Nuclear Physics Programs of the National Science Foundation, and the Sanford Underground Research Facility. This work is also funded by the National Science Foundation Graduate Research Fellowship Program under grant number DGE-1144081.