The Cosmic Axion Spin Precession Experiment (CASPEr): a dark-matter search with nuclear magnetic resonance

TL;DR

CASPEr is an NMR-based experiment designed to detect axion-like particles as dark matter by employing resonant and non-resonant measurement schemes across a wide frequency range, improving sensitivity especially at low frequencies.

Contribution

The paper introduces new detection modalities for CASPEr, extending the search bandwidth for axion-like particles from Hz to mHz frequencies.

Findings

Resonant NMR method sensitive from Hz to hundreds of MHz.

Non-resonant frequency-modulation scheme extends detection to mHz frequencies.

Proposes new data processing techniques for low-frequency axion detection.

Abstract

The Cosmic Axion Spin Precession Experiment (CASPEr) is a nuclear magnetic resonance experiment (NMR) seeking to detect axion and axion-like particles which could make up the dark matter present in the universe. We review the predicted couplings of axions and axion-like particles with baryonic matter that enable their detection via NMR. We then describe two measurement schemes being implemented in CASPEr. The first method, presented in the original CASPEr proposal, consists of a resonant search via continuous-wave NMR spectroscopy. This method offers the highest sensitivity for frequencies ranging from a few Hz to hundreds of MHz, corresponding to masses $ m_{\rm a} \sim 10^{-14}$--$10^{-6}$ eV. Sub-Hz frequencies are typically difficult to probe with NMR due to the diminishing sensitivity of magnetometers in this region. To circumvent this limitation, we suggest new detection and data processing modalities. We describe a non-resonant frequency-modulation detection scheme, enabling searches from mHz to Hz frequencies ($m_{\rm a} \sim 10^{-17}$--$10^{-14} $ eV), extending the detection bandwidth by three decades.