[Paper Review] Introducing DMRadio-GUT, a search for GUT-scale QCD axions

L. Brouwer, Saptarshi Chaudhuri+30 more|arXiv (Cornell University)|Mar 21, 2022

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper introduces DMRadio-GUT, a proposed experiment to search for GUT-scale QCD axions in the 0.4–120 neV mass range via their coupling to photons. By leveraging high-field superconducting magnets, ultra-low-noise SQUID amplifiers, and active feedback stabilization in a resonant cavity, DMRadio-GUT aims to achieve sensitivity to DFSZ-model axions with a projected 6.2-year scan time, covering a critical and previously unexplored region of the axion parameter space.

ABSTRACT

The QCD axion is a leading dark matter candidate that emerges as part of the solution to the strong CP problem in the Standard Model. The coupling of the axion to photons is the most common experimental probe, but much parameter space remains unexplored. The coupling of the QCD axion to the Standard Model scales linearly with the axion mass; therefore, the highly-motivated region 0.4-120 neV, corresponding to a GUT-scale axion, is particularly difficult to reach. This paper presents the design requirements for a definitive search for GUT-scale axions and reviews the technological advances needed to enable this program.

Motivation & Objective

• To close the critical gap in the axion parameter space between 0.4–120 neV, corresponding to GUT-scale QCD axions.
• To develop a definitive experimental program capable of detecting axion dark matter in the DFSZ model with high sensitivity.
• To overcome the challenge of low signal-to-noise ratio in the low-frequency regime through advanced cryogenic and electronic technologies.
• To enable a full-scan search across the GUT-scale axion mass window using lumped-element resonant techniques.
• To establish a roadmap for technological R&D in high-field magnets, ultra-low-noise amplifiers, and active feedback systems.

Proposed method

• Utilizes a high-Q, high-B0 resonant cavity (16–29 T) to enhance axion-to-photon conversion via the axion-photon coupling.
• Employs ultra-low-noise SQUID amplifiers with active feedback stabilization to reduce backaction noise below the standard quantum limit.
• Applies active feedback stabilization (AFS) and flux-lock loop (FLL) techniques to maintain optimal signal detection in the presence of thermal and quantum noise.
• Uses a discrete Fourier transform (DFT) to analyze power spectral density (PSD) and extract weak axion-induced signals from background.
• Employs a figure of merit (FOM) based on signal-to-noise ratio (SNR), quality factor (Q), and system temperature (T) to optimize detector performance.
• Incorporates Monte Carlo (MC) simulations to model detector response and predict sensitivity across different experimental configurations.

Experimental results

Research questions

• RQ1 Can a resonant cavity-based experiment achieve sufficient sensitivity to detect GUT-scale QCD axions with masses between 0.4 and 120 neV?
• RQ2 What are the optimal trade-offs between magnetic field strength, cavity volume, Q-factor, and amplifier noise to minimize scan time?
• RQ3 Can active feedback stabilization reduce amplifier noise below the standard quantum limit to enable detection in the low-frequency regime?
• RQ4 To what extent can technological advances in high-field magnets and low-noise electronics extend the reach of axion searches?
• RQ5 Is it feasible to achieve a full-scan sensitivity across the entire 0.4–120 neV range within a reasonable time frame (e.g., ~6 years)?

Key findings

• The baseline DMRadio-GUT configuration achieves sensitivity to DFSZ-model axions across the 0.4–120 neV range with a projected scan time of 6.2 years.
• A 29 T magnet configuration with higher amplifier noise can still achieve comparable sensitivity by compensating with increased field strength, reducing scan time to 3.2 years.
• Reducing system temperature to 10 mK and lowering amplifier noise to -25 dB enables a scan time of 7.3 years, demonstrating robustness to performance trade-offs.
• The experiment is designed to be resilient to partial R&D failures, as trade-offs between magnetic field, volume, Q-factor, and noise can be compensated to maintain sensitivity.
• The projected sensitivity curve in Figure 4 shows DMRadio-GUT will cover the entire GUT-scale axion window, surpassing existing limits in the low-mass region.
• The experiment is feasible within current technological R&D trajectories, including 29 T high-field magnets and sub-SQL noise amplifiers.