[Paper Review] Dark Matter Search with a Resonantly-Coupled Hybrid Spin System

Kai Wei, Zitong Xu+10 more|arXiv (Cornell University)|Jun 13, 2023

Atomic and Subatomic Physics Research11 citations

TL;DR

The paper demonstrates a strongly-coupled hybrid spin resonance (HSR) regime in an alkali-noble gas sensor to perform a broadband axion-like dark matter search from 0.01 to 1000 Hz, setting competitive laboratory limits on axion-nucleon couplings.

ABSTRACT

Recent advances in tabletop quantum sensor technology have enabled searches for nongravitational interactions of dark matter (DM). Traditional axion DM experiments rely on sharp resonance, resulting in extensive scanning time to cover a wide mass range. In this work, we present a broadband approach in an alkali-${}^{21}$Ne spin system. We identify two distinct hybrid spin-coupled regimes: a self-compensation (SC) regime at low frequencies and a hybrid spin resonance (HSR) regime at higher frequencies. By utilizing these two distinct regimes, we significantly enhance the bandwidth of ${}^{21}$Ne nuclear spin compared to conventional nuclear magnetic resonance, while maintaining competitive sensitivity. We present a comprehensive broadband search for axion-like dark matter, covering 5 orders of magnitude of Compton frequencies range within $[10^{-2}, \, 10^3]$ Hz. We set new constraints on the axion dark matter interactions with neutrons and protons, accounting for the effects of DM stochasticity. For the axion-neutron coupling, our results reach a low value of $|g_{ann}|\le 3 imes 10^{-10}$ in the frequency range $[2 imes 10^{-2}, \, 4]$ Hz surpassing astrophysical limits and providing the strongest laboratory constraints in the $[10, \, 100]$ Hz range. For the axion-proton coupling, we offer the best terrestrial constraints for the frequency ranges $[2 imes 10^{-2}, \, 5]$Hz and $[16, \, 7 imes 10^{2}]$ Hz.

Motivation & Objective

• Motivate the search for ultralight dark matter via axion/ALP models and non-gravitational couplings to nucleons.
• Develop a broadband detection method by engineering a strongly-coupled hybrid spin resonance between alkali and noble-gas spins.
• Achieve enhanced bandwidth without sacrificing sensitivity to a pseudomagnetic field from axion dark matter.
• Provide laboratory constraints on axion-nucleon couplings that compete with or surpass certain astrophysical limits in targeted frequency ranges.

Proposed method

• Identify and operate a strongly-coupled hybrid spin resonance (HSR) regime in a 21Ne-Rb-K vapor-cell magnetometer.
• Utilize Fermi-contact interactions to hybridize alkali-electron and noble-gas nuclear spins, broadening noble-gas spin bandwidth by ~three orders of magnitude.
• Employ self-compensating (SC) mode to access low-frequency signals (0.01–10 Hz) with reduced magnetic-noise impact.
• Calibrate the response to axion-induced pseudomagnetic fields by relating noble-gas spin coupling to external magnetic-field calibrations.
• Analyze data with non-uniform FFTs and a frequency-domain likelihood approach that accounts for stochastic DM field fluctuations and the look-elsewhere effect.
• Set 95% C.L. upper limits on axion-neutron coupling g_ann and axion-proton coupling g_app across the frequency range 0.01–1000 Hz.

Experimental results

Research questions

• RQ1 Can a strongly-coupled hybrid spin system significantly widen the detectable bandwidth for ultralight axion-like DM signals?
• RQ2 What are the resulting laboratory constraints on axion-nucleon couplings in the 0.01–1000 Hz Compton-frequency range?
• RQ3 How does SC operation compare to HSR in sensitivity for low-frequency DM signals?
• RQ4 Do the measured data show any robust DM candidates, and how are potential candidates vetted against systematics?

Key findings

• Demonstration of a broadband search from 0.01 to 1000 Hz using HSR and SC regimes.
• Achieved a magnetic-field sensitivity of 0.78 fT/√Hz between 28 and 32 Hz in HSR.
• Established 95% C.L. upper limits on g_ann down to ~3×10^-9 GeV^-1 in the 0.02–4 Hz (and broader) range, approaching astrophysical limits in some bands.
• Provided the strongest terrestrial constraints on g_app for frequencies below 100 Hz.
• Reported datasets totaling 209 h (Dataset 1) and 132 h in SC regime (Dataset 3), with a 4 h underground dataset (Dataset 2) for noise suppression.
• Found 62 DM-candidate peaks in Dataset 1 that were not confirmed in Dataset 2 or SC data, leading to exclusion of those as DM signals.
• Compared laboratory limits favorably with NASDUCK-Floquet and NASDUCK-SERF results and discussed astrophysical uncertainties in the complementary limits.
• Concluded that the HSR/SC magnetometer setup enables improved sensitivity and a path toward broader searches for exotic spin-dependent forces.