[Paper Review] Axion-like particle searches with sub-THz photons

L.M. Capparelli, G. Cavoto+4 more|arXiv (Cornell University)|Oct 23, 2015

Dark Matter and Cosmic Phenomena4 references3 citations

TL;DR

This paper proposes a novel Light-Shining-through-Wall (LSW) experiment using intense sub-THz photons from gyrotrons at ~30 GHz to search for axion-like particles (ALPs). By leveraging ultra-high photon flux and single-photon detectors in the microwave domain, the method could improve current exclusion limits on ALP couplings by up to four orders of magnitude for masses ≤0.01 meV.

ABSTRACT

We propose a variation, based on very low energy and extremely intense photon sources, on the well established technique of Light-Shining-through-Wall (LSW) experiments for axion-like particle searches. With radiation sources at 30 GHz, we compute that present laboratory exclusion limits on axion-like particles might be improved by at least four orders of magnitude, for masses m_a

Motivation & Objective

• To overcome current limitations in axion-like particle (ALP) searches by exploiting low-energy, high-intensity sub-THz photon sources.
• To address the gap in sensitivity for ALP couplings below ∼10−10 GeV−1, particularly for very light ALPs with masses ≤0.01 meV.
• To motivate the development of single-photon detectors in the 30 GHz range, which are essential for achieving the proposed sensitivity.
• To explore the regime near the photon energy resonance with ALP mass (Eγ ≈ ma), where standard LSW approximations break down.
• To demonstrate that Fabry-Perot cavities and high-quality gyrotron sources can significantly enhance event rates and detection sensitivity.

Proposed method

• Utilizes gyrotron sources operating at ~30 GHz to generate extremely intense, continuous sub-THz photon beams with photon fluxes exceeding 10^28 photons/second.
• Applies a strong, uniform magnetic field (H ≈ 15 T) over a path length L to enable photon-to-axion conversion via the axion-photon interaction Lagrangian L_I = (G/4) a Fμν F̃μν.
• Employs Fabry-Perot resonant cavities with high quality factors (Q > 10^4) to enhance the effective interaction length and boost the probability of photon-axion conversion.
• Derives a modified photon-to-axion conversion probability formula that accounts for the energy regime near Eγ ≈ ma, where the standard approximation F ≈ 1 breaks down.
• Incorporates a modified denominator in the conversion factor F = Eγ / √(Eγ² − ma²) that includes the path length L, leading to F ≈ 1/(L + √(Eγ² − ma²)) near resonance.
• Relies on single-photon detection in the microwave domain to achieve the required sensitivity for detecting rare conversion events.

Experimental results

Research questions

• RQ1 Can sub-THz photon sources at ~30 GHz significantly improve the sensitivity of Light-Shining-through-Wall experiments for axion-like particles?
• RQ2 How does the photon-axion conversion probability change when the photon energy approaches the axion mass (Eγ ≈ ma), and what modifications are needed in the standard LSW formalism?
• RQ3 What is the expected improvement in exclusion limits on the axion-photon coupling G for masses ≤0.01 meV using this sub-THz approach?
• RQ4 What are the technical requirements for realizing such an experiment, particularly in terms of detector sensitivity and source intensity?
• RQ5 Can the use of high-Q Fabry-Perot cavities in the microwave domain enhance the effective interaction length and event rate in low-energy LSW experiments?

Key findings

• Using 30 GHz gyrotron sources with ~10^28 photons/second, the experiment could achieve event rates of ~10 per year at G ≈ 10−10 GeV−1, significantly improving on current limits.
• The proposed method could improve present laboratory exclusion limits on axion-like particle couplings by at least four orders of magnitude for masses m_a ≤ 0.01 meV.
• The photon-axion conversion probability formula must be modified near Eγ ≈ ma, where the standard factor F ≈ 1 breaks down, and the path length L appears explicitly in the denominator.
• The modified conversion probability includes a term F ≈ 1 / (L + √(Eγ² − ma²)), which enhances sensitivity when Eγ ≈ ma, enabling resonant production of axions at rest.
• Theoretical analysis confirms that the imaginary part of the self-energy Σ(k) governs the decay rate of photons into axions, and this rate scales as G²H²Eγ / √(Eγ² − ma²) near resonance.
• The feasibility of detecting single photons at 30 GHz is identified as a critical R&D frontier, and the paper argues that such detectors are essential for pushing sensitivity beyond current limits.