Axion Search in DarkSide-20k

Project Information

ASiD

Grant agreement ID: 101031397

DOI

10.3030/101031397

Project closed

EC signature date 9 March 2021

Start date 1 January 2022

End date 31 December 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 212 933,76

EU contribution

€ 212 933,76

Coordinated by

ROYAL HOLLOWAY AND BEDFORD NEW COLLEGE
United Kingdom

Periodic Reporting for period 1 - ASiD (Axion Search in DarkSide-20k)

Reporting period: 2022-01-01 to 2023-12-31

Summary of the context and overall objectives of the project

The existence of dark matter was suggested in the early 20th century independently by multiple astronomers. Further evidence was provided by Vera Rubin’s observations of the velocity curves of the spiral galaxy. Nearly 85 % of the Universe is made up of yet-to-be discovered particles called dark matter. Two of the most promising candidates are relic axions and Weakly Interacting Massive Particles (WIMPs). If they exist, the dark matter particles must be non-baryonic, electrically neutral, and rarely interact with ordinary matter.

The Darkside-20k experiment uses a dual phase argon-based time projection chamber filled with underground liquid argon of 20-ton fiducial mass as target volume to detect the WIMP-nucleon scattering. The time projection chamber is placed inside the Laboratori Nazionali del Gran Sasso underneath the Gran Sasso mountains. The experiment uses photodetector modules made up of state-of-the-art silicon photomultipliers. U.K. is producing nearly a total of 6 sq. meter photodetector units for inner veto of DarkSide 20k.

In addition to searching wimps, the silicon inside the silicon photomultipliers itself can be used as a target for searching solar axions via axioelectric effect. This is analogues to photoelectric effect but with a solar axion instead of a photon. The energy required to promote an electron to conduction band in silicon is much lower than in liquid argon and liquid xenon.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The Darkside 20k is a low background search. Therefore, we expect a low occupancy during the experiment. We built a laser test stand at Royal Holloway to mimic the experimental conditions and calibrate the single photoelectron response. We cryogenically tested the tiles inside an ISO7 clean room. We kept the occupancy low by setting the intensity of the laser accordingly. We acquired data at a range of bias voltages above the breakdown and qualified the silicon photomultipliers at 7V above the breakdown. The quantities we qualified were signal to noise ratio, single photo electron response and noise spectra.

During the characterization, we found that the single photo electron response and the signal to noise ratio as a function of bias voltage was linear as expected. The noise spectra showed a characteristic shape which is an artifact of the front-end board electronics of the silicon photomultipliers. During the test, we identified bad tile behavior. These were mainly due to flaky connectors and smushed wire bonds. We used the noise analysis as an early warning system to identify this bad behavior in the test stand.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The proposal went beyond the state-of-the-art and proposed the search for solar axions with the detector itself. Using the assayed silicon-32 beta decay spectrum, a PhD student is currently working on a Monte Carlo simulation to generate a sensitivity plot.

A simple table top setup to cryogenically test the silicon photomultipliers.

A picture of two silicon photomultiplier setup being prepared for cryogenic testing.