Exotic Dark Matter Search with the Majorana Demonstrator.

With excellent energy resolution and ultralow-level radiogenic backgrounds, the high-purity germanium detectors in the Majorana Demonstrator enable searches for several classes of exotic dark matter (DM) models. In this work, we report new experimental limits on keV-scale sterile neutrino DM via the transition magnetic moment from conversion to active neutrinos ν_{s}→ν_{a}. We report new limits on fermionic dark matter absorption (χ+A→ν+A) and sub-GeV DM-nucleus 3→2 scattering (χ+χ+A→ϕ+A), and new exclusion limits for bosonic dark matter (axionlike particles and dark photons). These searches utilize the (1-100)-keV low-energy region of a 37.5-kg y exposure collected by the Demonstrator between May 2016 and November 2019 using a set of ^{76}Ge-enriched detectors whose surface exposure time was carefully controlled, resulting in extremely low levels of cosmogenic activation.

Final Result of the Majorana Demonstrator's Search for Neutrinoless Double-ß Decay in

The Majorana Demonstrator searched for neutrinoless double-ß decay (0νßß) of ^{76}Ge using modular arrays of high-purity Ge detectors operated in vacuum cryostats in a low-background shield. The arrays operated with up to 40.4 kg of detectors (27.2 kg enriched to ∼88% in ^{76}Ge). From these measurements, the Demonstrator has accumulated 64.5 kg yr of enriched active exposure. With a world-leading energy resolution of 2.52 keV FWHM at the 2039 keV Q_{ßß} (0.12%), we set a half-life limit of 0νßß in ^{76}Ge at T_{1/2}>8.3×10^{25} yr (90% C.L.). This provides a range of upper limits on m_{ßß} of (113-269) meV (90% C.L.), depending on the choice of nuclear matrix elements.

Constraints on the Decay of

^{180m}Ta is a rare nuclear isomer whose decay has never been observed. Its remarkably long lifetime surpasses the half-lives of all other known ß and electron capture decays due to the large K-spin differences and small energy differences between the isomeric and lower-energy states. Detecting its decay presents a significant experimental challenge but could shed light on neutrino-induced nucleosynthesis mechanisms, the nature of dark matter, and K-spin violation. For this study, we repurposed the Majorana Demonstrator, an experimental search for the neutrinoless double-beta decay of ^{76}Ge using an array of high-purity germanium detectors, to search for the decay of ^{180m}Ta. More than 17 kg, the largest amount of tantalum metal ever used for such a search, was installed within the ultralow-background detector array. In this Letter, we present results from the first year of Ta data taking and provide an updated limit for the ^{180m}Ta half-life on the different decay channels. With new limits up to 1.5×10^{19} yr, we improved existing limits by 1-2 orders of magnitude which are the most sensitive searches for a single ß and electron capture decay ever achieved. Over all channels, the decay can be excluded for T_{1/2}<0.29×10^{18} yr.

Measurement of the 2νßß Decay Rate and Spectral Shape of

Neutrinoless double beta decay (0νßß) is a yet unobserved nuclear process that would demonstrate Lepton number violation, a clear evidence of beyond standard model physics. The process two neutrino double beta decay (2νßß) is allowed by the standard model and has been measured in numerous experiments. In this Letter, we report a measurement of 2νßß decay half-life of ^{100}Mo to the ground state of ^{100}Ru of [7.07±0.02(stat)±0.11(syst)]×10^{18} yr by the CUPID-Mo experiment. With a relative precision of ±1.6% this is the most precise measurement to date of a 2νßß decay rate in ^{100}Mo. In addition, we constrain higher-order corrections to the spectral shape, which provides complementary nuclear structure information. We report a novel measurement of the shape factor ξ_{3,1}=0.45±0.03(stat)±0.05(syst) based on a constraint on the ratio of higher-order terms from theory, which can be reliably calculated. This is compared to theoretical predictions for different nuclear models. We also extract the first value for the effective axial vector coupling constant obtained from a spectral shape study of 2νßß decay.

Erratum: Search for Spontaneous Radiation from Wave Function Collapse in the Majorana Demonstrator [Phys. Rev. Lett. 129, 080401 (2022)].

This corrects the article DOI: 10.1103/PhysRevLett.129.080401.

Search for Spontaneous Radiation from Wave Function Collapse in the Majorana Demonstrator.

The Majorana Demonstrator neutrinoless double-beta decay experiment comprises a 44 kg (30 kg enriched in ^{76}Ge) array of p-type, point-contact germanium detectors. With its unprecedented energy resolution and ultralow backgrounds, Majorana also searches for rare event signatures from beyond standard model physics in the low energy region below 100 keV. In this Letter, we test the continuous spontaneous localization (CSL) model, one of the mathematically well-motivated wave function collapse models aimed at solving the long-standing unresolved quantum mechanical measurement problem. While the CSL predicts the existence of a detectable radiation signature in the x-ray domain, we find no evidence of such radiation in the 19-100 keV range in a 37.5 kg-y enriched germanium exposure collected between December 31, 2015, and November 27, 2019, with the Demonstrator. We explored both the non-mass-proportional (n-m-p) and the mass-proportional (m-p) versions of the CSL with two different assumptions: that only the quasifree electrons can emit the x-ray radiation and that the nucleus can coherently emit an amplified radiation. In all cases, we set the most stringent upper limit to date for the white CSL model on the collapse rate, λ, providing a factor of 40-100 improvement in sensitivity over comparable searches. Our limit is the most stringent for large parts of the allowed parameter space. If the result is interpreted in terms of the Diòsi-Penrose gravitational wave function collapse model, the lower bound with a 95% confidence level is almost an order of magnitude improvement over the previous best limit.

Search for Solar Axions via Axion-Photon Coupling with the Majorana Demonstrator.

Axions were originally proposed to explain the strong-CP problem in QCD. Through axion-photon coupling, the Sun could be a major source of axions, which could be measured in solid state detection experiments with enhancements due to coherent Primakoff-Bragg scattering. The Majorana Demonstrator experiment has searched for solar axions with a set of ^{76}Ge-enriched high purity germanium detectors using a 33 kg-yr exposure collected between January, 2017 and November, 2019. A temporal-energy analysis gives a new limit on the axion-photon coupling as g_{aγ}<1.45×10^{-9} GeV^{-1} (95% confidence level) for axions with mass up to 100 eV/c^{2}. This improves laboratory-based limits between about 1 eV/c^{2} and 100 eV/c^{2}.

New Limit for Neutrinoless Double-Beta Decay of

The CUPID-Mo experiment at the Laboratoire Souterrain de Modane (France) is a demonstrator for CUPID, the next-generation ton-scale bolometric 0νßß experiment. It consists of a 4.2 kg array of 20 enriched Li_{2}^{100}MoO_{4} scintillating bolometers to search for the lepton-number-violating process of 0νßß decay in ^{100}Mo. With more than one year of operation (^{100}Mo exposure of 1.17 kg×yr for physics data), no event in the region of interest and, hence, no evidence for 0νßß is observed. We report a new limit on the half-life of 0νßß decay in ^{100}Mo of T_{1/2}>1.5×10^{24} yr at 90% C.I. The limit corresponds to an effective Majorana neutrino mass ⟨m_{ßß}⟩<(0.31-0.54) eV, dependent on the nuclear matrix element in the light Majorana neutrino exchange interpretation.

First Limit on the Direct Detection of Lightly Ionizing Particles for Electric Charge as Low as e/1000 with the Majorana Demonstrator.

The Majorana Demonstrator is an ultralow-background experiment searching for neutrinoless double-beta decay in ^{76}Ge. The heavily shielded array of germanium detectors, placed nearly a mile underground at the Sanford Underground Research Facility in Lead, South Dakota, also allows searches for new exotic physics. Free, relativistic, lightly ionizing particles with an electrical charge less than e are forbidden by the standard model but predicted by some of its extensions. If such particles exist, they might be detected in the Majorana Demonstrator by searching for multiple-detector events with individual-detector energy depositions down to 1 keV. This search is background-free, and no candidate events have been found in 285 days of data taking. New direct-detection limits are set for the flux of lightly ionizing particles for charges as low as e/1000.

Search for Neutrinoless Double-ß Decay in

The Majorana Collaboration is operating an array of high purity Ge detectors to search for neutrinoless double-ß 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_{ßß} 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).

Search for Neutrinoless Quadruple-ß Decay of

We report the results of a first experimental search for lepton number violation by four units in the neutrinoless quadruple-ß decay of ^{150}Nd using a total exposure of 0.19 kg yr recorded with the NEMO-3 detector at the Modane Underground Laboratory. We find no evidence of this decay and set lower limits on the half-life in the range T_{1/2}>(1.1-3.2)×10^{21} yr at the 90% C.L., depending on the model used for the kinematic distributions of the emitted electrons.

New Limits on Bosonic Dark Matter, Solar Axions, Pauli Exclusion Principle Violation, and Electron Decay from the Majorana Demonstrator.

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 g_{Ae}<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_{AN}^{eff}×g_{Ae}<3.8×10^{-17}, a 1/2ß^{2}<8.5×10^{-48} limit on the strength of PEPV electron transitions, and a lower limit on the electron lifetime of τ_{e}>1.2×10^{24} yr for e^{-}→ invisible.

α -event characterization and rejection in point-contact HPGe detectors.

P-type point contact (PPC) HPGe detectors are a leading technology for rare event searches due to their excellent energy resolution, low thresholds, and multi-site event rejection capabilities. We have characterized a PPC detector's response to α particles incident on the sensitive passivated and p + surfaces, a previously poorly-understood source of background. The detector studied is identical to those in the Majorana Demonstrator experiment, a search for neutrinoless double-beta decay ( 0 ν ß ß ) in 76 Ge. α decays on most of the passivated surface exhibit significant energy loss due to charge trapping, with waveforms exhibiting a delayed charge recovery (DCR) signature caused by the slow collection of a fraction of the trapped charge. The DCR is found to be complementary to existing methods of α identification, reliably identifying α background events on the passivated surface of the detector. We demonstrate effective rejection of all surface α events (to within statistical uncertainty) with a loss of only 0.2% of bulk events by combining the DCR discriminator with previously-used methods. The DCR discriminator has been used to reduce the background rate in the 0 ν ß ß region of interest window by an order of magnitude in the Majorana Demonstrator  and will be used in the upcoming LEGEND-200 experiment.

Measurement of the ßß decay half-life of 130Te with the NEMO-3 detector.

We report results from the NEMO-3 experiment based on an exposure of 1275 days with 661 g of (130)Te in the form of enriched and natural tellurium foils. The ßß decay rate of (130)Te is found to be greater than zero with a significance of 7.7 standard deviations and the half-life is measured to be T(½)(2ν) = [7.0 ± 0.9(stat) ± 1.1(syst)] × 10(20) yr. This represents the most precise measurement of this half-life yet published and the first real-time observation of this decay.

The BiPo-3 detector.

The BiPo-3 detector is a low radioactive detector dedicated to measuring ultra-low natural contaminations of 208Tl and 214Bi in thin materials, initially developed to measure the radiopurity of the double ß decay source foils of the SuperNEMO experiment at the µBq/kg level. The BiPo-3 technique consists in installing the foil of interest between two thin ultra-radiopure scintillators coupled to low radioactive photomultipliers. The design and performances of the detector are presented. In this paper, the final results of the 208Tl and 214Bi activity measurements of the first enriched 82Se foils are reported for the first time, showing the capability of the detector to reach sensitivities in the range of some µBq/kg.

Development of 100 Mo -containing scintillating bolometers for a high-sensitivity neutrinoless double-beta decay search.

This paper reports on the development of a technology involving 100 Mo -enriched scintillating bolometers, compatible with the goals of CUPID, a proposed next-generation bolometric experiment to search for neutrinoless double-beta decay. Large mass ( ∼ 1 kg ), high optical quality, radiopure 100 Mo -containing zinc and lithium molybdate crystals have been produced and used to develop high performance single detector modules based on 0.2-0.4 kg scintillating bolometers. In particular, the energy resolution of the lithium molybdate detectors near the Q-value of the double-beta transition of 100 Mo (3034 keV) is 4-6 keV FWHM. The rejection of the α -induced dominant background above 2.6 MeV is better than 8 σ . Less than 10 µ Bq/kg activity of 232 Th ( 228 Th ) and 226 Ra in the crystals is ensured by boule recrystallization. The potential of 100 Mo -enriched scintillating bolometers to perform high sensitivity double-beta decay searches has been demonstrated with only 10 kg × d exposure: the two neutrino double-beta decay half-life of 100 Mo has been measured with the up-to-date highest accuracy as T 1 / 2 = [6.90 ± 0.15(stat.) ± 0.37(syst.)] × 10 18 years . Both crystallization and detector technologies favor lithium molybdate, which has been selected for the ongoing construction of the CUPID-0/Mo demonstrator, containing several kg of 100 Mo .