[Paper Review] Search for low Energy solar Axions with CAST

G. Cantatore, E. Arık+78 more|arXiv (Cornell University)|Sep 26, 2008

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper presents the first direct search for low-energy solar axion-like particles (ALPs) via their conversion to 2–4 eV photons in the CAST axion helioscope, using a cooled photomultiplier tube coupled to the magnet bore via a 40 m optical fiber and Galilean telescope. No significant excess over background was observed, with a measured background rate of 0.35 ± 0.02 Hz over 75,000 s of solar tracking data, establishing a baseline for future low-energy axion detection efforts.

ABSTRACT

We have started the development of a detector system, sensitive to single photons in the eV energy range, to be suitably coupled to one of the CAST magnet ports. This system should open to CAST a window on possible detection of low energy Axion Like Particles emitted by the sun. Preliminary tests have involved a cooled photomultiplier tube coupled to the CAST magnet via a Galileian telescope and a switched 40 m long optical fiber. This system has reached the limit background level of the detector alone in ideal conditions, and two solar tracking runs have been performed with it at CAST. Such a measurement has never been done before with an axion helioscope. We will present results from these runs and briefly discuss future detector developments.

Motivation & Objective

• To develop and test a detector system sensitive to single photons in the eV energy range for coupling to the CAST magnet bore.
• To search for axion-like particles (ALPs) emitted by the Sun with energies below 100 eV, focusing on the 2–4 eV range via Primakoff conversion to photons.
• To evaluate the background level of the detector system under normal operating conditions at CAST.
• To establish a foundation for future detection of low-energy ALPs using advanced low-background sensors.

Proposed method

• A cooled photomultiplier tube (PMT) and avalanche photodiode (APD) were used as photon detectors, both cooled to -20 °C to reduce dark count rates.
• A Galileian telescope and a 40 m multimode optical fiber were used to couple the detector to the CAST magnet bore, enabling remote detection with moderate light collection efficiency.
• An optical switch enabled alternating between the magnet bore (signal) and a background reference position (dark count measurement).
• The system was operated in two solar tracking campaigns: one with both PMT and APD, and another with only the PMT, including off-center pointing for background validation.
• Dark count rates were measured and subtracted using Poissonian fitting to the count histogram, ensuring single-photon sensitivity.
• Data were collected over 75,000 s of solar tracking, with background and signal periods carefully balanced to enable statistical comparison.

Experimental results

Research questions

• RQ1 Can a detector system sensitive to eV-range photons be effectively coupled to the CAST magnet bore without introducing significant noise?
• RQ2 What is the background count rate for 3–4 eV photons in the CAST environment during solar tracking?
• RQ3 Is there a statistically significant excess of photons during solar tracking that could indicate axion-to-photon conversion?
• RQ4 Can the BaRBE detector system achieve the sensitivity required for future low-energy axion searches?

Key findings

• The BaRBE detector system successfully coupled a cooled PMT to the CAST magnet bore via a 40 m optical fiber, achieving 50% light collection efficiency for the PMT and 10% overall system efficiency when accounting for spectral response.
• The measured background count rate for 3–4 eV photons during solar tracking was 0.35 ± 0.02 Hz over 75,000 seconds of data.
• No statistically significant excess of counts was observed between the 'light' (magnet bore) and 'dark' (background) states, with 1σ error bars confirming consistency with background-only hypothesis.
• The system achieved background levels comparable to the detector's intrinsic noise, demonstrating feasibility for low-background eV-range photon detection in a real helioscope environment.
• The first solar axion search in the 2–4 eV range using an axion helioscope was successfully performed, setting a benchmark for future experiments.
• Three future detector technologies—Transition Edge Sensors (TES), DEPFET sensors, and cryogenically cooled APDs—were identified as promising candidates for improved sensitivity and lower background.