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

The paper introduces WINTER, a novel broadband interferometry experiment utilizing a high-finesse Fabry-Pérot cavity in a magnetic field to detect photon-axion conversion in vacuum, aiming to achieve photon-axion coupling sensitivities down to $3.7\times10^{-14}$ $\text{GeV}^{-1}$ for axion masses up to 380 $\mu$eV independent of the dark-matter hypothesis.

The Gist

Imagine you are trying to find a ghost. But this isn't a spooky ghost; it's a theoretical particle called an axion. Scientists think axions might be the "dark matter" that holds the universe together, but they are incredibly shy. They rarely interact with normal matter, making them nearly impossible to catch.

For decades, scientists have tried to find them by assuming they are floating around like a fog (the "dark matter hypothesis"). But what if that fog doesn't exist, or isn't where we think it is?

This paper introduces a new, clever way to hunt for axions called WINTER (Weak Interacting Slim Particle INterferometer). Instead of waiting for axions to bump into us, WINTER tries to turn light into axions and see if the light disappears.

Here is how it works, explained through simple analogies:

1. The Setup: A Magical Race Track

Imagine a race track with two lanes.

• Lane A (The Reference Lane): This lane is empty and safe. A beam of laser light runs through it.
• Lane B (The Sensing Lane): This lane is the "danger zone." It is a long, empty vacuum tube (no air) sitting inside a super-strong magnet.

The experiment uses a Mach-Zehnder Interferometer. Think of this as a split-track system where a laser beam is split in two, sent down both lanes, and then brought back together at the finish line.

2. The Magic Trick: The Primakoff Effect

In Lane B, the strong magnetic field acts like a magical wand. According to quantum physics, if you shine light through a strong magnetic field, there's a tiny chance the photons (light particles) will turn into axions.

• The Analogy: Imagine the laser beam is a stream of water. As it flows through the magnet, a tiny, invisible amount of that water turns into "ghost water" (axions) and vanishes into a parallel dimension.
• The Result: When the two lanes meet back at the finish line, the beam from Lane B is slightly weaker than the beam from Lane A because some of it turned into axions and left.

3. The Problem: The Signal is Tiny

The amount of light that turns into axions is so small that it's like trying to hear a whisper in a hurricane. If you just look at the light, you won't see the difference because of all the "noise" (vibrations, heat, electronic fuzz).

4. The Solution: The "Noise-Canceling" Headphones

This is where WINTER gets clever. It uses two main tricks to hear that whisper:

A. The Echo Chamber (Fabry-Pérot Cavity)
Inside the magnet, the laser doesn't just go through once. It bounces back and forth between two mirrors thousands of times.

• The Analogy: Imagine shouting in a small bathroom. The sound bounces off the tiles and gets louder. WINTER does this with light. By bouncing the light back and forth 100,000 times, the "interaction" with the magnet is amplified. If 1 in a million photons turns into an axion, bouncing it 100,000 times makes that effect much easier to spot.

B. The "Wiggle" Code (Modulation)
To distinguish the axion signal from background noise, the scientists "wiggle" the light.

• They rapidly switch the polarization (the direction the light waves are vibrating) back and forth.
• The Analogy: Imagine you are trying to find a specific friend in a crowded, noisy stadium. If you just stand there, you can't see them. But if your friend wears a bright, flashing hat that only they have, you can spot them easily.
• In WINTER, the "flashing hat" is the polarization wiggle. The axion signal only appears when the light is wiggled in a specific way. The computer ignores everything else and only listens for that specific "wiggle" pattern.

5. The Goal: Finding the Invisible

The experiment is designed to be broadband.

• Old experiments were like tuning a radio to one specific station, hoping the axion is on that frequency. If the axion is on a different frequency, you miss it.
• WINTER is like a radio that can scan all frequencies at once. It doesn't need to guess the axion's "weight" (mass) beforehand. It can search a huge range of possibilities simultaneously.

Why is this a big deal?

• No Assumptions: Unlike other experiments that assume axions are the dark matter filling our galaxy, WINTER doesn't care. It just looks for the conversion of light to axions. If axions exist, WINTER can find them, even if they aren't dark matter.
• Extreme Sensitivity: By using a 10-meter long magnet (like the ones used in the Large Hadron Collider) and high-tech mirrors, WINTER is projected to be sensitive enough to detect axions that have never been seen before.
• The Prototype: The team is already building a smaller, "table-top" version in Hamburg to prove the concept works before building the massive full-scale version.

Summary

WINTER is a high-tech, ultra-sensitive light detector that uses a super-strong magnet and a bouncing mirror chamber to see if light can magically turn into invisible particles. By "wiggling" the light and listening for a specific signal, it hopes to catch the first glimpse of the elusive axion, potentially solving one of the biggest mysteries in physics: what is the universe made of?

Technical Summary

1. Problem Statement

The search for axions and Axion-Like Particles (ALPs) is a primary frontier in particle physics, driven by the need to solve the strong CP problem in QCD and to identify the nature of dark matter. Existing experimental approaches face significant limitations:

• Haloscopes: Highly sensitive but rely on the "halo model" assumption regarding the local density and velocity distribution of dark matter. Recent simulations suggest these assumptions may be flawed, limiting their effective sensitivity.
• Light-Shining-Through-the-Wall (LSW): Model-independent and broadband but require two conversion steps (photon $\to$ axion $\to$ photon), which drastically reduces signal strength ($P \propto g^4$).
• Birefringence Experiments (e.g., PVLAS): Detect polarization changes but lack the extreme sensitivity of interferometric amplitude detection.

There is a critical need for a model-independent, broadband experiment capable of detecting axions across a wide mass range (specifically the QCD axion band) without relying on dark matter density assumptions, while maximizing sensitivity through single-stage conversion.

2. Methodology: The WINTER Experiment

The authors propose WINTER (Weak Interacting Slim Particle INTERferometer), a novel setup utilizing a free-space Mach-Zehnder Interferometer (MZI) to detect photon-axion conversion via the Primakoff effect in a vacuum.

Core Architecture

• Interferometer: A free-space MZI where one arm (the sensing arm) is placed inside a strong transverse magnetic field ($B_{ext}$) and a high-vacuum environment. The other arm serves as a reference.
• Photon-Axion Mixing: In the presence of $B_{ext}$, photons can convert into axions. This conversion reduces the photon flux in the sensing arm.
• Fabry-Pérot Cavity (FPC): The sensing arm incorporates an FPC with high finesse ($\mathcal{F} \approx 10^5$). This increases the effective path length and circulating power, enhancing the conversion probability quadratically ($P_{\gamma \to a} \propto L^2$).
• Signal Modulation & Detection:
  • Polarization Modulation: An Electro-Optical Modulator (EOM-PC) switches the laser polarization between two orthogonal states at a frequency $\omega_{sig}$. Only the polarization component parallel to the magnetic field couples to axions. This modulates the conversion signal, allowing it to be distinguished from background noise.
  • Amplitude Modulation: A second EOM (EOM-AM) modulates the amplitude at frequency $\omega_m$ to lock the interferometer near a "dark fringe" (destructive interference).
  • Readout: The system operates near the dark fringe to minimize shot noise. The signal is extracted by demodulating the power at the dark port first at $\omega_m$ (to stabilize the lock) and then at $\omega_{sig}$ (to isolate the axion-induced amplitude change).

Key Technical Parameters

• Laser: 1064 nm continuous wave (CW) laser.
• Magnet: Proposed configurations include a 10 m, 9 T dipole magnet (similar to LHC test magnets) or a 200 m, 5.3 T string of magnets (similar to ALPS-II).
• Cavity: Finesse $\mathcal{F} \approx 10^5$, length $L \approx 10$ m (for the primary proposal).
• Locking: Uses Pound-Drever-Hall (PDH) technique for cavity locking and homodyne detection for interferometer stabilization.

3. Key Contributions

1. Single-Stage Conversion: Unlike LSW experiments, WINTER relies on a single photon-to-axion conversion step. The signal is detected as a reduction in photon flux (amplitude loss) rather than a regenerated photon, avoiding the $g^4$ suppression.
2. Model Independence: The search does not assume the local dark matter density or velocity distribution, making it robust against uncertainties in halo models.
3. Broadband Sensitivity: The experiment is sensitive to axion masses up to $\approx 380$ $\mu$eV (for the 10 m setup) and $\approx 85$ $\mu$eV (for the 200 m setup), covering a significant portion of the theoretically motivated QCD axion band (50–1500 $\mu$eV).
4. Noise Suppression: By operating near the dark fringe and using polarization modulation, the experiment effectively suppresses laser shot noise and environmental instrumental noise.

4. Results and Projected Sensitivity

The authors perform a sensitivity analysis based on the signal-to-noise ratio (SNR) considering dark current noise and shot noise from residual light at the dark fringe.

• Primary Configuration (LHC-type Magnet):

  • Parameters: $B_{ext} = 9$ T, $L = 10$ m, $\mathcal{F} = 10^5$, $P_{tot} = 130$ W, integration time = 1 year.
  • Projected Sensitivity: $g_{a\gamma\gamma} \simeq 3.7 \times 10^{-14}$ GeV$^{-1}$.
  • Mass Range: Up to $m_a \approx 380$ $\mu$eV.
  • Significance: This reaches the theoretical Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) axion band.

• Alternative Configuration (ALPS-II Magnet String):

  • Parameters: $B_{ext} = 5.3$ T, $L = 200$ m, $\mathcal{F} = 1.2 \times 10^4$.
  • Projected Sensitivity: $g_{a\gamma\gamma} \simeq 9.2 \times 10^{-15}$ GeV$^{-1}$.
  • Mass Range: Cutoff at $m_a \approx 85$ $\mu$eV due to the longer interaction length.
  • Trade-off: Higher sensitivity at the cost of a reduced mass range.

• Prototype Status:

  • A table-top prototype is under construction at the University of Hamburg using a 1 m magnet (1.2 T) and a 1 m FPC ($\mathcal{F}=3\times10^4$).
  • Projected Limit: $g_{a\gamma\gamma} \gtrsim 7.1 \times 10^{-11}$ GeV$^{-1}$ for masses up to 996 $\mu$eV, which already improves upon existing limits for certain mass ranges.

5. Significance

The WINTER proposal represents a paradigm shift in axion detection strategies:

• Exploration of Uncharted Territory: It targets the "QCD axion band" in a mass range (tens to hundreds of $\mu$eV) that is difficult for haloscopes (due to frequency tuning limits) and LSW experiments (due to sensitivity limits).
• Robustness: By decoupling the search from dark matter density assumptions, WINTER provides a definitive test of the axion-photon coupling regardless of whether axions constitute the entirety of dark matter.
• Technological Feasibility: The design leverages established technologies (high-finesse cavities, LIGO-grade mirrors, and standard interferometric locking techniques) and demonstrates that a prototype can be built with current resources.
• Complementarity: WINTER complements existing efforts by offering a unique detection channel (amplitude reduction via single conversion) that is orthogonal to birefringence and regeneration-based searches.

In conclusion, WINTER offers a highly sensitive, broadband, and model-independent pathway to discovering axions, potentially reaching the theoretical DFSZ limit and significantly advancing the search for physics beyond the Standard Model.