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Search for light Dark Matter with a Spherical Proportional Counter

Periodic Reporting for period 1 - DarkSphere (Search for light Dark Matter with a Spherical Proportional Counter)

Reporting period: 2019-05-01 to 2021-04-30

Dark Matter (DM) constitutes 85% of the matter in the Universe, but what is it made of? This is the most compelling question in physics for over a century. Astrophysical observations and cosmological constraints point towards particle dark matter, which is "cold", long-lived, and electrically neutral. Weakly Interacting Massive Particles (WIMPs) in the mass range between 10 - 1000 GeV have been the main focus of experimental scrutiny. However, lack of conclusive evidence calls for a broadening of our approach to the DM question: Modern theories may explain the observed DM abundance with light dark matter candidates. The current generation of state-of-the-art dark matter detectors, optimised for WIMPs, are not sensitive to such light DM candidates.

DarkSphere aims to shed new light on DM through a novel direct search of unprecedented sensitivity for light DM candidates in the 0.05-10 GeV mass region. The primary tool to achieve this goal is the Spherical Proportional Counter (SPC), a novel gaseous detector. The detector combines large volume with a low energy threshold down to a single electron; a unique feature among DM detectors. It will be filled with light (helium, neon) and hydrogen-rich gases (alkanes) to optimise projectile-target momentum transfer. The NEWS-G collaboration develops and operates SPCs across the globe for light DM searches, installed at the deep-underground laboratories of SNOLAB (Canada), LSM (France) and Boulby (UK).

The goals of DarkSphere are achieved through its mains objectives:
-The study of detector properties, such as the gain, ionisation statistics, and drift time of ions, as well as the production of a detailed simulation framework to reproduce the measured detector properties in order to support measurements and physics analysis.
-The estimation of the ionisation quenching factor - the parameter used to measure the relative ionisation yield of nuclear recoils and electrons - and to study the Migdal effect, both being critical parameters for recoil ionisation modelling.
-The development of a method for fast neutron spectroscopy to study neutron-induced background in rare-event search experiments.
-The discovery or placing stringent constraints on light DM through physics analysis of NEWS-G data, including signal/background discrimination, background estimation, statistical analysis, and phenomenological result interpretation.
The objectives of DarkSphere were neatly divided into six work packages to facilitate the completion of the project goals.

The properties of several gas mixtures interesting for the NEWS-G experiment were studied. The gas is the active component of the SPC and the target in the case of DM searches. Particular attention was given to Penning effects, which lead to larger ionisation and higher detector gain for the same applied voltages, allowing for operation at higher pressure.

A simulation framework was developed to realistically simulate the detector response to ionising radiation. Geant4 is used to transport simulated particles through the detector geometry. GARFIELD++ was integrated into GEANT4 to allow physical processes in the SPC to be simulated, providing a wealth of information including simulated detector pulses that are directly comparable to measurements.

The ionisation quenching factor for protons, α-particles and heavy ions in several gases was estimated using different methods. Measurements were performed at CoMIMAC, a low-energy ion accelerator at LPSC Grenoble (France), and using a focused monoenergetic neutron beam to produce low energy recoils at TUNL (USA). Moreover, a new method for estimating the ionisation quenching factor using W-values for ions and electrons was developed.

The Migdal effect -the increased probability for electron ionisation when a recoil nucleus is displaced with respect to its electron cloud after nuclear scattering - is being investigated through
the MIGDAL experiment.

An excellent way to study neutron backgrounds is to measure the energy spectra of environmental fast neutrons. For this reason, SPCs installed in the University of Birmingham and Boulby Underground Laboratory were adapted into nitrogen-based neutron detectors, which take advantage of the 14N(n, p)14C and 14N(n, α)11B reactions.

Data from the operation of the NEWS-G detector at the Laboratoire Souterrain de Modane (LSM) were used to perform physics analysis towards estimating the sensitivity of NEWS-G in DM-nucleon spin-independent and spin-dependent interactions. The NEWS-G detector was successfully commissioned and operated for the first tests in LSM in 2019, and in 2021 was installed in SNOLAB.
DarkSphere addresses head-on the most pressing question in physics: the nature of Dark Matter (DM). For four decades, experiments focused on searches for Weakly Interacting Massive Particles, with masses in the 10–1000 GeV range. However, lack of conclusive evidence demands broadening of DM searches. Modern theories, e.g. hidden sectors, asymmetric DM, and effective field theory descriptions, motivate light DM searches. Current DM detectors are not sensitive to these candidates. DarkSphere proposed a research programme that can lead to the world's best sensitivity to light DM in the 0.05 – 10 GeV mass range through the NEWS-G experiment and the use of Spherical Proportional Counters, covering new ground in DM searches, hitherto inaccessible.

The results and methods developed to estimate the ionisation quenching factor and study the Migdal effect immediately affect direct DM searches. At the same time, the neutron background measurements provide a lasting contribution to experiments in two of the deepest underground laboratories. The fast neutron spectroscopy method can also be used in industrial and medical applications as a safe and cost-effective alternative to 3He detectors for fast neutron measurements.

DarkSphere advanced the light DM physics programme and can potentially deliver the next-generation detectors for rare-event searches. DarkSPHERE, a future SPC, 3 m in diameter, has been proposed to be installed at the Boulby, with a sensitivity reaching the neutrino floor. DarkSPHERE can also act as a multi-physics platform for supernova neutrino detection and neutrinoless double beta decay searches.

A new laboratory founded within BILPA (Birmingham Instrumentation Laboratory for Particle physics and Applications), the Birmingham Gaseous Detector Laboratory to support DarkSphere, strengthen Birmingham's research capabilities and promoted gaseous detector applications in physics and industry in the UK.

DarkSphere produced a series of publications in peer-reviewed physics journals. Results were presented in international and national conferences, topical workshops, and invited seminars.
Schematic of the spherical proportional counter, showing the operation principle.