Broadband interferometry-based searches for photon-axion conversion in vacuum

TL;DR

WINTER presents a broadband, model-independent search for photon-axion conversion in vacuum using a free-space Mach-Zehnder interferometer with one arm in a high-field vacuum region and a long, high-finesse Fabry-Pérot cavity. The method leverages polarization and amplitude modulation, coupled with lock-in demodulation, to extract a small axion-induced signal from a dark-port output, achieving a projected sensitivity of $g_{aγγ} \gtrsim 5.5×10^{-15}$ GeV$^{-1}$ for axion masses up to $\sim 84.8$ μeV. A table-top prototype and a full-scale WINTER instrument are analyzed, including detailed cavity-locking, interferometer-stabilization, and noise-budget estimates, showing competitive reach in a broad mass range and independence from local dark matter assumptions. The work highlights the potential to explore the QCD axion parameter space with a broadband laboratory approach, offering a complementary path to existing helioscope and light-shining-through-wall experiments.

Abstract

A novel experiment is introduced to detect photon-axion conversion independent of the dark-matter hypothesis in a broad mass-range called WISP Interferometer (WINTER). The setup consists of a free-space Mach-Zehnder-type interferometer incorporating an external magnetic field and vacuum in one of the arms, where photon-axion mixing occurs via the Primakoff effect and is detected through changes in amplitude. The expected axion-induced signal is then modulated by polarization changes. The experiment is designed to integrate a Fabry-Pérot cavity with a finesse of $10^{5}$ that will be operated in a vacuum environment, significantly enhancing the sensitivity. The setup is sensitive to photon-axion coupling strengths values $g_{aγγ}\gtrsim 5.5\times10^{-15}$ $\text{GeV}^{-1}$ for axion masses up to 84.8 $μ$eV

Figures (4)

• Figure 1: Schematic view of the experimental setup of WINTER using a free space MZI for broadband detection. In red, the laser beam in free space is shown. The sensitive arm of the interferometer is placed inside a vacuum chamber with a FPC of 10m length integrated in a dipole magnet with 9T of the same length. The acronyms correspond to: electro-optical modulator (EOM) with amplitude (EOM-AM), phase (EOM-PM), and polarization (EOM-PC) modulation, Faraday isolator (FI), linear polarizer (LP), signal generator (SG), beam-splitter (BS), photo-detector (PD), mirror (M), and low-pass filter (LPF). More details about the setup are given in Sect. \ref{['sec:setup_overview']}.
• Figure 2: Projected sensitivity for WINTER experiment and a prototype under construction. As a comparison, the CAST CAST_Micromegas_2024 limit is also shown in gray. The specific parameters used for the calculation of both setups are shown in Tab. \ref{['tab:we_comparison']}. Note that the differences in the cutoff axion masses between the prototype and the full WINTER setup are due to the different wavelengths and magnet length along which the FPC is integrated.
• Figure 3: Representation of the total losses in ppm (in blue) for WINTER experiment as a function of the beam waist at the plane mirror (M2). The selected parameters are shown in Tab. \ref{['tab:we_fpc_comparison']}.
• Figure 4: Representation of the finesse variation (in blue) for WINTER experiment as a function of the beam waist at the plane mirror (M2), accounting for diffraction and clipping losses. At the secondary y-axis, the beam waist at the concave mirror (M3) is also shown (in orange). The selected FPC details are shown in Tab. \ref{['tab:we_fpc_comparison']}.