[Paper Review] Search for hidden-photon Dark Matter with FUNK

Funk Experiment, :+14 more|arXiv (Cornell University)|Nov 1, 2017

Dark Matter and Cosmic Phenomena6 references3 citations

TL;DR

This paper presents a search for hidden-photon dark matter using the FUNK experiment, which detects visible and near-UV photons produced when hidden photons mix with standard photons at material interfaces. No significant signal was found; a preliminary upper limit on the mixing parameter χ is set, with systematic uncertainties dominating the sensitivity in the eV to sub-eV mass range.

ABSTRACT

It has been proposed that an additional U(1) sector of hidden photons could account for the Dark Matter observed in the Universe. When passing through an interface of materials with different dielectric properties, hidden photons can give rise to photons whose wavelengths are related to the mass of the hidden photons. In this contribution we report on measurements covering the visible and near-UV spectrum that were done with a large, 14 m2 spherical metallic mirror and discuss future dark-matter searches in the eV and sub-eV range by application of different electromagnetic radiation detectors.

Motivation & Objective

• To test the hypothesis that hidden photons—proposed as a candidate for dark matter—can convert into detectable photons at dielectric interfaces.
• To extend the search for dark matter into the visible and near-UV spectrum using a large spherical mirror and a low-noise photomultiplier.
• To establish a preliminary upper limit on the hidden-photon mixing parameter χ, accounting for systematic uncertainties.
• To lay the groundwork for future searches in the MHz, GHz, and THz frequency ranges.

Proposed method

• A 14 m² spherical metallic mirror composed of 36 segments is used to focus photons produced by hidden-photon-to-photon conversion at dielectric interfaces.
• A low-dark-current photomultiplier tube (PMT) ET9107BQ with bialkali photocathode is employed for single-photon detection with high quantum efficiency in the visible and UV range.
• The PMT is mounted on a motorized linear stage to scan positions in and out of the mirror’s focal point, enabling differential measurements to isolate potential dark matter signals.
• A motorized shutter allows background characterization by comparing open and closed shutter states, while a flasher system calibrates single-photoelectron response.
• Digitized signals from the PMT are recorded using a PicoScope 6404D oscilloscope, with pulse charge and timing used to identify single-photon events and reject cosmic-ray showers.
• Statistical cuts are applied to the charge spectrum to retain only single-photoelectron events, achieving an estimated 75% efficiency for single-photon selection.

Experimental results

Research questions

• RQ1 Can hidden-photon dark matter be detected via its conversion into visible and near-UV photons at material interfaces?
• RQ2 What is the sensitivity of the FUNK experiment to the hidden-photon mixing parameter χ in the eV and sub-eV mass range?
• RQ3 How do systematic effects—such as temperature drifts or timing inaccuracies—affect the measurement of potential dark matter signals?
• RQ4 What is the impact of the PMT position (in vs. out of the focal point) on the observed count rate, and how does this influence the signal-to-noise ratio?

Key findings

• No significant excess signal was observed in a 30-day measurement run conducted in February–March 2017, with an average count rate of 0.535 Hz.
• The difference in count rates with the shutter open versus closed when the PMT was at the focal point was ΔR = 0.0032 ± 0.0014 Hz, interpreted as a proxy for a potential dark matter signal.
• A systematic difference of ΔR ≈ 0.025 Hz was observed between the open-shutter count rates at the focal point and 8 cm outside, indicating a potential systematic uncertainty.
• This 0.025 Hz difference was used as an upper bound on systematic uncertainty, leading to a preliminary upper limit on the mixing parameter χ denoted as 'FUNK preliminary (sys)' in Fig. 5.
• The statistical uncertainty dominates the sensitivity in the lower mass range, while systematic effects dominate in the higher mass range, as reflected in the sensitivity curves.
• The experiment sets a conservative preliminary limit on χ, with the sensitivity curve in Fig. 5 showing the current reach in the eV to sub-eV range, with future extensions planned into MHz–THz bands.