Resonant laser power build-up in ALPS -- a "light-shining-through-walls" experiment

TL;DR

This work demonstrates the first successful integration of a large-scale optical cavity into a light-shining-through-walls experiment (ALPS) to boost photon production for WISP searches. By embedding a production cavity inside a 5.30 T HERA dipole and stabilizing the laser via Pound-Drever-Hall locking, the team achieved a power build-up of about $PB\approx43$, enhancing the potential WISP production by more than an order of magnitude relative to a bare laser. Using a 1064 nm master-oscillator MOPA with SHG to 532 nm and a low-background CCD detector, the exemplary run yields no WISP signal, but sets competitive 95% C.L. limits on ALP couplings $g$ (both $0^-$ and $0^+$), hidden-photon kinetic mixing $\chi$, and MCP+HP scenarios, e.g. $g<4.1\times10^{-7}\ \text{GeV}^{-1}$ for $m_-\lesssim0.5$ meV and $\chi<2.1\times10^{-6}$ for $m_{\gamma'}\approx0.7$ meV. The results establish cavity-enhanced LSW as a viable approach and highlight main loss channels (notably AR-window losses) as actionable targets for near-term sensitivity improvements.

Abstract

The ALPS collaboration runs a light-shining-through-walls (LSW) experiment to search for photon oscillations into "weakly interacting sub-eV particles" (WISPs) inside of a superconducting HERA dipole magnet at the site of DESY. In this paper we report on the first successful integration of a large-scale optical cavity to boost the available power for WISP production in this type of experiments. The key elements are a frequency tunable narrow line-width continuous wave laser acting as the primary light source and an electronic feed-back control loop to stabilize the power build-up. We describe and characterize our apparatus and demonstrate the data analysis procedures on the basis of a brief exemplary run.

Figures (15)

• Figure 1: Schematic overview of a light-shining-through-walls experiment. The gray blob indicates the mixing term between photons and the "weakly interacting sub-eV particle" (WISP).
• Figure 2: Feynman diagrams responsible for the mixing term between photons and different hypothetical "weakly interacting sub-eV particles" (WISPs). Photon oscillations into Axion-like particles (ALPs) and massless hidden photons (HPs) via mini-charged particle (MCP) loops require the presence of a background magnetic field, denoted by a crossed circle. Oscillations into massive Hidden Photons occur regardless of such a background. See Table \ref{['tab:WISPstuff']} for details on the photon-WISP couplings and references.
• Figure 3: Schematic overview of the whole experimental setup of the ALPS experiment comprising laser, second harmonic generation in the PPKTP crystal, production cavity, magnet and detector. Magnified is the schematic representation of the ALPS master-oscillator power amplifier laser system.
• Figure 4: Selection of a typical one hour exposure recorded with the SBIG CCD. Tracks from radioactivity and cosmics are visible as well as warm pixels, i.e. pixels showing large pedestals or high noise rates. The signal (left) and reference (right) regions are shown enlarged. No evidence for photons from WISPs is seen while the reference beam shows up clearly and well focused.
• Figure 5: Beam radius of the fundamental eigenmode of the ALPS optical resonator together with the radius at which $\unit[0.2]{\%}$ of the mode's power would be clipped. The position of the end mirror EM is shown as a vertical line. Clearly, the beam size is always well below the minimum aperture of our production vacuum tube (14 mm).