[Paper Review] Exotic dark matter search with the Majorana Demonstrator

I. J. Arnquist, F. T. Avignone+57 more|arXiv (Cornell University)|Jun 21, 2022

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper presents new experimental limits on exotic dark matter using low-energy data from the Majorana Demonstrator, a high-purity germanium detector array. By analyzing rare energy peaks in the 1–100 keV range with a profile-likelihood method, it sets the most sensitive constraints to date for pseudoscalar and vector bosonic dark matter in germanium, with upper limits of $ g_{ae} < 1.425 \times 10^{-13} $ at 17.28 keV and $ \alpha' / \alpha < 2.414 \times 10^{-30} $ at 2.29 keV, surpassing previous Ge-based limits despite smaller active mass than xenon experiments.

ABSTRACT

With excellent energy resolution and ultra-low 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, $ν_s ightarrow ν_a$. We report new limits on fermionic dark matter absorption ($χ+ A ightarrow ν+ A$) and sub-GeV DM-nucleus 3$ ightarrow$2 scattering ($χ+ χ+ A ightarrow 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.

Motivation & Objective

• To search for sub-GeV dark matter via rare inelastic scattering events in high-purity germanium detectors.
• To set new experimental limits on pseudoscalar (axion-like) and vector (dark photon) bosonic dark matter using low-energy data from the Majorana Demonstrator.
• To improve sensitivity in the keV-scale energy region by reducing the energy threshold and controlling surface exposure time.
• To test whether keV-scale sterile neutrino dark matter could explain the XENON1T excess, finding the required transition magnetic moment too low.

Proposed method

• A profile-likelihood method at 90% confidence level was used to set upper limits on counts from rare energy peaks in the 1–100 keV range.
• The analysis focused on inelastic fermionic dark matter absorption (χ + A → ν + A) and 3 → 2 scattering processes with sub-GeV dark matter.
• For pseudoscalar dark matter, the coupling constant $ g_{ae} $ was constrained using the observed event rate and detector exposure.
• For vector dark matter (dark photons), the kinetic mixing parameter $ \kappa^2 = \alpha' / \alpha $ was derived from the flux and cross-section product, using the formula $ \Phi_{DM} \sigma_{ve} = \frac{4 \times 10^{23}}{m_\chi} \left( \frac{\alpha'}{\alpha} \right) \frac{\sigma_{pe}(m_\chi)}{A} $.
• The energy threshold was reduced by a factor of five compared to prior analyses, enhancing sensitivity in the low-energy region.
• Surface exposure time was carefully controlled to minimize background contributions and improve statistical sensitivity.

Experimental results

Research questions

• RQ1 Can the Majorana Demonstrator detect sub-GeV pseudoscalar dark matter via axion-electron coupling in the keV energy range?
• RQ2 What are the tightest experimental limits on vector bosonic dark matter (dark photons) in germanium detectors?
• RQ3 Is the transition magnetic moment between active and sterile neutrinos sufficient to explain the XENON1T excess if keV-scale sterile neutrinos are the dark matter?
• RQ4 How do the limits from this germanium-based experiment compare to those from large-scale xenon experiments?
• RQ5 Can inelastic fermionic dark matter absorption (χ + A → ν + A) be ruled out or constrained at the sub-GeV scale?

Key findings

• The Majorana Demonstrator sets a new upper limit on the pseudoscalar axion-electron coupling constant of $ g_{ae} < 1.425 \times 10^{-13} $ at 17.28 keV, representing the tightest constraint from any germanium experiment.
• For the lowest energy point, a limit of $ g_{ae} < 1.542 \times 10^{-13} $ was obtained at 2.64 keV, demonstrating improved sensitivity in the low-energy regime.
• The experiment establishes a best upper limit of $ \alpha' / \alpha < 2.414 \times 10^{-30} $ for vector bosonic dark matter at 2.29 keV, the most sensitive result to date among germanium-based searches.
• The analysis rules out the possibility that keV-scale sterile neutrino dark matter with a transition magnetic moment sufficient to explain the XENON1T excess is responsible for the observed signal.
• The study confirms that xenon-based experiments still achieve tighter limits due to their larger active mass, despite the improved energy threshold and exposure control in the Majorana Demonstrator.
• The results demonstrate the potential of future ton-scale $ \beta\beta(0\nu) $ experiments like LEGEND to significantly improve on these limits with 10 ton-year exposure.