[Paper Review] First Results from ABRACADABRA-10cm: A Search for Low-Mass Axion Dark Matter

Chiara P. Salemi|arXiv (Cornell University)|May 16, 2019

Dark Matter and Cosmic Phenomena12 references9 citations

TL;DR

ABRACADABRA-10 cm presents the first results from a prototype experiment searching for low-mass axion dark matter using a toroidal magnet and SQUID-based detection of axion-induced effective currents. The experiment sets 95% confidence level limits on axion-photon coupling in the 0.31–8.3 neV mass range, achieving competitive sensitivity with existing experiments like CAST, and finds no evidence for a signal in its month-long run.

ABSTRACT

The presence of dark matter provides some of the most tangible evidence for the existence of physics beyond the Standard Model. One compelling dark matter candidate is the axion, a light boson that was originally postulated as a solution to another outstanding issue, the strong CP problem in QCD. ABRACADABRA is an experimental program to search for sub-$μ$eV axion and axion-like dark matter. It searches for axion-induced modifications to Maxwell's equations with a toroidal magnet and SQUID magnetometer. This contribution will present the first results from the prototype detector, ABRACADABRA-10cm.

Motivation & Objective

• To test the feasibility of detecting low-mass axion dark matter using a broadband, SQUID-based detection method in a compact prototype.
• To set the first experimental limits on axion-photon coupling for axion masses between 0.31 and 8.3 neV.
• To validate the ABRACADABRA detection concept in a real-world environment with minimal background interference.
• To identify and mitigate instrumental and environmental noise sources that could mimic axion signals.
• To lay the groundwork for larger-scale axion searches using resonant and broadband readout in future ABRACADABRA detectors.

Proposed method

• The experiment uses a 10 cm diameter superconducting toroidal magnet to generate a strong, uniform azimuthal magnetic field.
• Axion dark matter induces an effective current via the axion-photon coupling term $ g_{a\gamma\gamma} \mathbf{E} \cdot \mathbf{B} $, modeled as $ \mathbf{J}_{\text{eff}} = g_{a\gamma\gamma} \frac{\partial a}{\partial t} \mathbf{B} $.
• The oscillating axion field drives a time-varying magnetic flux in the toroid's center, detectable via a superconducting pickup loop.
• Signal readout is performed in both broadband and resonant modes using SQUID amplifiers with high sensitivity.
• Data is processed via Fourier transforms to search for narrow peaks in frequency space, with a coherence width $ \Delta f / f \approx 10^{-6} $.
• Statistical analysis uses a log-likelihood ratio test with a 5σ threshold to assess significance, accounting for the Look Elsewhere Effect across ~8.1 million mass points.

Experimental results

Research questions

• RQ1 Can a compact, SQUID-based detector in broadband mode detect axion dark matter in the sub-micro-eV mass range?
• RQ2 What are the dominant sources of noise in a low-noise, cryogenic SQUID system that could mimic an axion signal?
• RQ3 How sensitive is the ABRACADABRA-10 cm prototype to axion-photon coupling in the 0.31–8.3 neV mass window?
• RQ4 Can magnet-off data be used to effectively veto environmental and instrumental background peaks?
• RQ5 Does the observed data show any statistically significant excess above background that could be attributed to axion dark matter?

Key findings

• No 5σ excess was observed in the month-long data run, indicating no evidence for axion dark matter in the 0.31–8.3 neV mass range.
• The experiment sets 95% confidence level upper limits on the axion-photon coupling strength $ g_{a\gamma\gamma} $ that are competitive with CAST limits on solar axions.
• The final data set underwent stringent cuts: 70% efficiency after removing spectra with more than thirty 3σ detections, and magnet-off data was used to veto persistent environmental peaks.
• The analysis accounted for the Look Elsewhere Effect by considering ~8.1 million independent mass points in the statistical test.
• The sensitivity was reduced by a factor of 6.5 compared to expectations, prompting planned hardware upgrades for the next run.
• The results validate the ABRACADABRA detection method and support the path toward larger, higher-field detectors for future axion searches.