New ALPS Results on Hidden-Sector Lightweights

Abstract: The ALPS collaboration runs a "Light Shining through a Wall" (LSW) experiment to search for photon oscillations into "Weakly Interacting Sub-eV Particles" (WISPs) often predicted by extensions of the Standard Model. The experiment is set up around a superconducting HERA dipole magnet at the site of DESY. Due to several upgrades of the experiment we are able to place limits on the probability of photon-WISP-photon conversions of a few 10^{-25}. These limits result in today's most stringent laboratory constraints on the existence of low mass axion-like particles, hidden photons and minicharged particles.

• The paper presents a novel optical resonator setup that achieves conversion probabilities as low as 10^-25 to constrain ALP-photon interactions.
• The paper establishes record sensitivity for detecting hidden photons in the meV range, challenging existing cosmic radiation models.
• The paper refines laboratory techniques for mini-charged particle searches, extending investigations into previously unexplored parameter spaces.

An Overview of the ALPS Experiment on WISPs in Hidden-Sectors

The paper presents findings from the Any-Light-Particle-Search (ALPS) collaboration, focusing on the experimental search for Weakly Interacting Sub-eV Particles (WISPs) such as axion-like particles (ALPs), hidden photons, and mini-charged particles. These entities are often predicted by theoretical extensions of the Standard Model of particle physics and are hypothesized to interact very weakly with known particles.

Experiment Overview

The ALPS experiment operates on the "light shining through a wall" (LSW) concept, designed to detect the transition of photons into WISPs and back into photons. The fundamental setup involves directing a powerful laser through a superconducting dipole magnet, which may facilitate the conversion of photons to WISPs. A physical barrier—the "wall"—blocks light, but any WISPs traversing it can revert to detectable photons on the other side.

This experiment took place at the Deutsches Elektronen-Synchrotron (DESY) accelerator in Hamburg and incorporated a series of advancements over previous setups, notably the introduction of an optical resonator light enhancement system and a new CCD camera with improved efficiency for the detection of regenerated photons. With these upgrades, it was capable of analyzing the smallest conversion probabilities yet—on the order of $\times 10^{-25}$.

Key Results and Analysis

1. ALP Constraints: The ALPS experiment sets stringent laboratory limits on ALP-photon couplings in the sub-eV mass range. These results are particularly relevant since astrophysical constraints can be evaded in specific WISP models. The experiment covers masses corresponding to previously untested regions by introducing specific values of gas (argon), which compensates for coherence loss in oscillations due to massive ALPs.
2. Hidden Photon Searches: The experiment achieved unparalleled sensitivity to hidden photons of meV mass scales. The observed limits extend previous experimental constraints and challenge hypothetical contributions to the cosmic radiation density from hidden photons, which have been suggested to obscure the effective number of relic cosmic neutrinos.
3. Mini-Charged Particles (MCPs): Constraints on MCPs derived from the ALPS experiment represent substantial progress in the laboratory-based investigation of these particles, especially within the meV energy range. Though weaker than astrophysical limits, these lab-based constraints are indispensable for establishing boundaries in scenarios where astrophysical arguments may not apply.

Theoretical and Practical Implications

The findings from the ALPS experiment significantly impact both theoretical and experimental physics. From a theoretical standpoint, the experiment's limits guide modifications to models that propose WISPs beyond the Standard Model, refining our understanding of particle coupling and mass predictions.

Practically, this research underlines the importance of LSW experiments in probing the low-energy frontier of particle physics. The demonstrated experimental strategies—especially in increasing laser power and introducing resonators—set a precedent for future investigations into WISPs. Additionally, the improvement in laboratory-specific techniques such as resonant regeneration enhances the feasibility of detecting weakly interacting new physics particles.

Forward Glance

This work not only benchmarks current technological capabilities in quantum optical experiments but also marks a pathway for future research to detect or constrain fundamental particles in hidden sectors. Advancing technologies like increased magnet strength, power, and improved detector sophistication, alongside pioneering concepts such as resonant regeneration, could catalyze the next generation of precision experiments in unexplored particle physics sectors.