Any Light Particle Searches with ALPS II: first science results

The ALPS II experiment at DESY conducted its first science campaign from February to May 2024, achieving a 20-fold improvement in sensitivity limits for axion-like particles without finding evidence of their existence, while demonstrating stable operation and preparing for future upgrades to further enhance detection capabilities.

The Gist

Imagine you are trying to find a ghost. But this isn't a spooky ghost; it's a "ghost particle" that is so light and interacts so weakly with the rest of the universe that it can walk right through solid walls without leaving a scratch. Physicists call these particles axions or WISPs (Weakly Interacting Slim Particles). They are a leading candidate for what makes up "dark matter," the invisible stuff holding galaxies together.

The paper you provided is a report from the ALPS II experiment in Hamburg, Germany. Here is what they did, how they did it, and what they found, explained simply.

The Big Idea: "Light Shining Through a Wall"

The experiment is based on a clever trick called "Light Shining Through a Wall." Here is the analogy:

1. The Conversion: Imagine you have a beam of light (photons). You shine it through a very strong magnetic field. In this field, some of the light might magically turn into a ghost particle (an axion).
2. The Wall: You put a thick, light-tight wall in the path of the beam. The light hits the wall and stops. But the ghost particles? They don't care about the wall. They pass right through it, invisible and undetectable.
3. The Re-Conversion: On the other side of the wall, you have another strong magnetic field. If the ghost particles are real, some of them will turn back into light (photons) as they pass through this second field.
4. The Detection: You look behind the wall with a super-sensitive camera. If you see a tiny, tiny spark of light where there should be total darkness, you have caught a ghost!

The Machine: A Giant, Straightened Tunnel

The ALPS II experiment is built inside a former particle accelerator tunnel (HERA) at DESY in Hamburg.

• The Magnets: They used 24 massive superconducting magnets (originally built 40 years ago) to create the magnetic fields.
• The Challenge: These magnets were originally bent to guide particles around a curve. To make them work for this experiment, the team had to perform a "brute-force" straightening procedure, physically adjusting the magnets so they were perfectly straight without taking them apart. It's like trying to straighten a curved garden hose without cutting it.
• The Wall: In the middle of the tunnel, there is a "cleanroom" with a light-tight wall. The magnets are on both sides.
• The Lasers: They used high-powered lasers and a system of mirrors to create an "optical resonator." Think of this like an echo chamber for light. The light bounces back and forth thousands of times, getting stronger and increasing the chances that it will turn into a ghost particle.

The First Science Run (February – May 2024)

The team ran their first official "science campaign" for a few months. Their goal was to see if any light appeared behind the wall.

The Results:

• No Ghosts Found: They did not find any evidence of axions or similar particles.
• The "Noise": They did see a tiny bit of light behind the wall, but it was too broad and "fuzzy" to be a ghost particle. They determined this was just "stray light"—tiny leaks of light sneaking around the wall, like a draft under a door.
• The Achievement: Even though they didn't find a ghost, they proved the machine works. They successfully kept the lasers stable, calibrated the complex equipment, and demonstrated that the "straightened" magnets perform just as well as they did 40 years ago.

How Much Better Is This?

Before this experiment, other labs had tried similar things. ALPS II is a massive leap forward.

• Sensitivity: They improved the sensitivity by a factor of 20 compared to all previous similar experiments.
• The Limit: They set a new "rule" for how heavy or how strongly these particles can interact. If axions exist with a mass below a certain tiny limit, they must interact with light even less than the new limit ALPS II set. It's like saying, "If ghosts exist, they are at least 20 times harder to catch than we thought before."

What's Next?

The paper states that the experiment is not over. The team is currently upgrading the optical system. Their goal is to make the machine 100 times (two orders of magnitude) more sensitive than it is now. They want to reach a level of sensitivity that rivals what we can learn from looking at stars and the universe (astrophysical observations).

In Summary:
ALPS II is a high-tech "ghost hunting" machine. In its first run, it didn't find any ghosts, but it proved the hunting gear works perfectly and set a much stricter rule for where those ghosts might be hiding. The hunt is just getting started.

Technical Summary

Technical Summary: Any Light Particle Searches with ALPS II: First Science Results

Problem and Motivation
Despite the success of the Standard Model, there is no experimental evidence for physics at energy scales between the electroweak and Planck scales. Theoretical frameworks suggest the existence of Weakly Interacting Slim Particles (WISPs), such as axions, axion-like particles (ALPs), hidden photons, and other bosons. These particles are motivated by solutions to the strong CP problem, explanations for dark matter, and mechanisms for generating small cosmological constants. Because their interactions with Standard Model constituents are extremely weak, they cannot be detected by existing or planned accelerator-based experiments. Instead, dedicated laboratory setups are required to search for them indirectly.

Methodology
The ALPS II experiment at DESY in Hamburg employs a "light-shining-through-a-wall" (LSW) approach. This purely laboratory-based method is independent of astrophysical or cosmological assumptions. The experimental setup consists of two strings of 24 modified superconducting dipole magnets (originally from the HERA accelerator) separated by a light-tight wall.

• Production: A high-power laser beam passes through the first magnetic field string, where photons may convert into WISPs (axions or similar bosons).
• Blocking: A light-tight wall blocks the original photons, while any generated WISPs pass through unhindered.
• Regeneration: In the second magnetic field string, WISPs may convert back into photons.
• Detection: The regenerated photons are detected behind the wall.

For the first science campaign (February–May 2024), the experiment utilized a regeneration cavity (RC) behind the wall to resonantly enhance the conversion probability of WISPs back into photons. The production cavity was not yet implemented. The optical system employed a heterodyne sensing scheme involving three lasers (high-power, reference, and local oscillator) to stabilize frequencies and measure the interference beatnote between the local oscillator and any regenerated light. This setup required sub-µHz frequency control and the ability to sense powers below $10^{-22}$ W.

To calibrate the system and determine conversion probabilities, a shutter within the light-tight wall was periodically opened to allow a tiny fraction of the high-power laser light to enter the regeneration cavity. By comparing the power measured with the shutter open versus closed, the experiment determined the probability of photon-WISP-photon conversion ($P_{\gamma \leftrightarrow \text{WISP}}$).

Key Contributions

1. First Science Campaign Results: The paper reports the first physics results from ALPS II, demonstrating the stable operation and robust calibration of a complex, resonantly enhanced LSW experiment.
2. Technical Achievement: The experiment successfully recovered a horizontal aperture of 50 mm in the HERA dipole magnets through a "brute-force" straightening procedure, allowing for the installation of a high-finesse optical resonator.
3. Comprehensive Limits: The analysis provides limits on the coupling strengths for scalar, pseudoscalar, vector, and tensor bosons, covering a broad range of theoretical models.

Experimental Results
During the first science campaign, the experiment collected over 580,000 seconds of data for perpendicular polarization ($\gamma_\perp$) and over 1,060,000 seconds for parallel polarization ($\gamma_\parallel$).

• No Detection: No evidence for the existence of axions or similar lightweight particles was found.
• Pseudoscalar Bosons (Axions): For masses below approximately 0.1 meV, the experiment established a 95% confidence level (CL) upper limit on the di-photon coupling strength of $|g^p_{\phi\gamma\gamma}| < 1.5 \cdot 10^{-9} \text{ GeV}^{-1}$. This represents an improvement of more than a factor of 20 compared to all previous similar LSW experiments (such as OSQAR and ALPS).
• Scalar Bosons: A limit of $|g^s_{\phi\gamma\gamma}| < 1.8 \cdot 10^{-9} \text{ GeV}^{-1}$ was set. The authors note that LSW experiments are less competitive than fifth-force searches for scalar bosons.
• Vector Bosons (Hidden Photons): A limit on the kinetic mixing parameter of $|\epsilon| < 2.0 \cdot 10^{-7}$ was derived. The improvement over previous results is described as "modest" because the unique magnetic field configuration of ALPS II is less relevant for vector boson searches, which do not require a magnetic field for mixing.
• Tensor Bosons: Limits on the coupling strength $G'$ relative to the standard graviton coupling $G$ were established, with a combined limit of $G'/G < 5.8 \cdot 10^{18}$.

Significance and Future Outlook
The primary significance of this work is the successful demonstration of the ALPS II concept, proving that a resonantly enhanced LSW experiment can operate stably and achieve sensitivities far exceeding previous non-resonant attempts. The results are purely laboratory-based and do not rely on astrophysical assumptions.

The paper modestly states that while the first campaign did not detect new particles, it has significantly improved the sensitivity limits for axion-photon coupling. The authors note that the optical system is currently being upgraded to include a production cavity and to mitigate stray light and technical noise. The goal of these upgrades is to achieve a sensitivity increase of another two orders of magnitude, aiming to reach the sensitivity levels of astrophysical analyses and to probe more recent QCD axion models.