[Paper Review] ALPS II technical overview and status report

Aaron Spector|arXiv (Cornell University)|Nov 17, 2016

Dark Matter and Cosmic Phenomena4 references9 citations

TL;DR

ALPS II is a next-generation light-shining-through-a-wall experiment using dual resonant optical cavities—production and regeneration—to enhance sensitivity to weakly interacting sub-eV particles (WISPs), particularly axion-like particles and hidden photons. By employing 100 m cavities with 40,000 power buildup factors and dual-frequency stabilization, ALPS IIc achieves sensitivity to couplings as low as gaγ ≳ 2 × 10⁻¹¹ GeV⁻¹ for masses below 10⁻⁴ eV.

ABSTRACT

The Any Light Particle Search II (ALPS II) is an experiment that utilizes the concept of resonant enhancement to improve on the sensitivity of traditional light shining through a wall style experiments. These experiments attempt to detect photons passing through an opaque wall by converting to relativistic weakly interacting sub-eV particles and then reconverting back to photons. ALPS II at DESY in Hamburg, Germany will use dually resonant optical cavities before and after the wall to increase the probability of this interaction occurring. This paper gives a technical overview and status report of the experiment.

Motivation & Objective

• To develop and test a dual-resonant optical cavity system for enhanced WISP detection in light-shining-through-a-wall experiments.
• To achieve sub-picometer mirror position control for resonant frequency matching between production and regeneration cavities.
• To demonstrate single-photon detection with transition edge sensors (TES) and heterodyne interferometry for low-background signal measurement.
• To validate the technical feasibility of ALPS IIa before full-scale deployment of ALPS IIc with superconducting magnets.
• To enable the most sensitive search to date for hidden photons with mγ′ > 5 × 10⁻⁵ eV and axion-like particles below 1 meV.

Proposed method

• Utilizes a 1064 nm laser system with 35 W input power and 5000 power buildup in the production cavity (PC).
• Employs a second harmonic generation (SHG) crystal to frequency-double light from the PC, enabling isolation from the regeneration cavity (RC).
• Uses feedback control systems to stabilize PC and RC resonance by actuating on laser frequency and piezoelectric mirror actuators.
• Maintains dual cavity resonance within 2 Hz and spatial mode overlap via central breadboard (CBB) with mirrors mounted directly to it.
• Employs two independent detection schemes: transition edge sensors (TES) for single-photon energy measurement and heterodyne detection with phase tracking.
• Uses 5.3 T superconducting dipole magnets from HERA, arranged in 10-magnet strings to provide 466 T·m magnetic field length.

Experimental results

Research questions

• RQ1 Can dual-resonant optical cavities with sub-picometer stability achieve resonant enhancement of WISP-to-photon conversion?
• RQ2 Can TES and heterodyne detection systems achieve sufficient energy resolution and background suppression for single-photon detection?
• RQ3 Can the PC and RC be stabilized in frequency and spatial mode while preventing light leakage from PC to RC?
• RQ4 What is the achievable sensitivity of ALPS IIc to axion-like particle couplings (gaγ) in the sub-eV mass range?
• RQ5 Can the technical systems of ALPS IIa successfully validate the optical and control subsystems before full-scale ALPS IIc deployment?

Key findings

• The 20 m test cavity achieved a power buildup factor of 1,010, demonstrating feasibility of long-baseline cavity stabilization.
• Frequency noise was suppressed to a unity gain frequency of 55 kHz, enabling stable resonance for up to 48 hours.
• Mirror alignment on the CBB was achieved with better than 5 µrad accuracy, meeting ALPS IIa requirements.
• TES detection efficiency is projected to exceed 30% with energy resolution better than 0.1 eV, with NIST benchmarks showing 95% efficiency at 0.29 eV resolution.
• Two HERA magnets were successfully unbent and operated, validating magnet preparation for ALPS IIc.
• Theoretical sensitivity analysis shows ALPS IIc can probe gaγ ≳ 2 × 10⁻¹¹ GeV⁻¹ for masses below 10⁻⁴ eV.