Dr Ashlea Kemp will join RAL Particle Physics Department on a Future Leaders Fellowship with funding for a ‘comprehensive and exciting research project dedicated to experimentally searching for dark matter candidates with masses on the order of the proton.
Dark Matter was first hypothesised to exist in the 1930s following observations of anomalies in galaxy rotation: these anomalies could be solved by introducing a new particle that does not interact with light. In light of further evidence such as gravitational lensing and cosmic background radiation, there is a broad consensus for the existence of dark matter, although alternative theories are still studied.
Many theories of dark matter particles have been proposed, spanning a wide range of masses and possible interactions. These include axions and Weakly Interacting Massive Particles (WIMPs). The former is the lightest candidate: very light but still far heavier than an electron, while the latter has 10 - 1000x the mass of a proton. With many dark matter searches looking for WIMPs and axions, there is a desire among physicists to explore beyond these boundaries. Recently, there has been more interest in proton-mass dark matter candidates motivated by ‘asymmetric’ dark matter, in which the prevalence of dark matter is determined by the imbalance between matter and antimatter in the universe.
Dark matter particles have eluded detection for nearly 40 years: any interaction with ordinary matter is believed to be very rare and weak. Dark matter experiments therefore face several challenges, such as finding new technologies capable of detecting the lowest possible energies, and building detectors as ‘background-free’ as possible: any other particles, from atmospheric cosmic rays or even from slightly radioactive material in the detector, can potentially mimic a dark matter signal.
Dark Matter detection: DarkSide-20k
Most dark matter experiments are built underground, using the Earth as a shield from cosmic rays, and are also increasingly large to maximise the chance of an interaction being measured. This is the case for one of the experiments that Dr Kemp works on, DarkSide-20k, a new 20 tonne two-phase Liquid Argon Time Projection Chamber (LArTPC) currently being built in the Laboratori Nazionali del Gran Sasso in Italy. DarkSide-20k is designed to reach a level of sensitivity capable of either discovering WIMP dark matter or observing atmospheric neutrino interactions, a key benchmark in the field.
Display of photomultipliers used within DarkSide 20k’s predecessor; the DarkSide 50 experiment. These conventional photosensors within the time projection chamber achieved world leading sensitivity to spin independent asymmetric dark matter. Image credit : DAR-025-YU_SU-2013 © Yura Suvorov/LNGS-INFN
Whilst originally designed to search for WIMP dark matter, DarkSide-20k has excellent physics potential for asymmetric dark matter. The photon detection system is one of the key technical innovations of DarkSide-20k, which is the first experiment to fully instrument the detector with Silicon Photomultipliers (SiPMs). Compared to its predecessor photomultipliers as depicted below it has a factor of 3x higher photon detection efficiency thanks to the enhanced quantum efficiency of its SiPMs, in a target 1000x greater, DarkSide-20k has strong potential for unprecedented sensitivity to asymmetric dark matter. This potential can only be realised however if background signals induced from radioactive decays, and correlated noise sources such as single electrons, can be kept under control, modelled, and understood. Dr Kemp will use her extensive expertise in data acquisition systems (DAQ), statistical data analysis techniques for dark matter search parameter estimation in liquid noble experiments, and in SiPM technology to drive the search for asymmetric dark matter in DarkSide-20k, focusing on mitigating crucial backgrounds and leveraging the quantum efficiency increase of the novel photon detection system.
Dark matter detection: Quest-DMC
Dr Kemp also works on the QUEST-DMC experiment, one of seven flagship experiments funded by the UKRI Quantum Technologies for Fundamental Physics (QTFP) programme. QUEST-DMC is a unique collaboration of particle physicists and ultra-low temperature, physicists are searching for evidence of asymmetric dark matter using superfluid Helium-3 at extremely cold temperatures (< 300 μK). A dark matter interaction will deposit energy in the form of heat, forming quasiparticles, and through ionisation of nearby helium atoms which will produce scintillation photons (for energies greater than the ionisation energy of Helium).
The energy of an interaction in noble liquid experiments is based on the number of quanta produced, and in He-3, it only takes 10^-7 eV to generate a quantum compared to 10 eV in liquid argon; QUEST-DMC therefore has huge transformative potential to search for extremely low-energy signals induced by asymmetric dark matter, which can only be realised with novel quantum instrumentation that is new to the experimental dark matter community. QUEST-DMC has the potential to achieve world-leading sensitivity to spin-dependent, asymmetric dark matter, if similarly to DarkSide-20k, backgrounds are kept under control.
QUEST-DMC is not currently located underground, and as such, cosmic rays are the limiting background for the dark matter search. It is therefore integral for QUEST-DMC to have a system to mitigate these backgrounds, such as through detecting photons produced in materials surrounding the target from cosmic ray interactions. However, it is technologically very challenging to find photon detectors capable of operating at such low temperatures. Dr Kemp is uniquely positioned with her expertise in photon sensors from DarkSide-20k to meet this challenge, and her research will help in the development of an ultra-low temperature photon detection system for QUEST-DMC; a brand-new instrument imperative for the experiment’s success. This system could also potentially be used to detect the scintillation photons produced in the He-3 target itself; another novel path that Dr Kemp intends to explore during the fellowship on her own ‘quest’ to search for spin-dependent, asymmetric dark matter.
Dr Kemp's statement
Dr Kemp stated "I am honoured to have been awarded this fellowship, and I look forward to joining the prestigious STFC particle physics team at RAL. I hope this project will play an important role in changing the landscape of present and future direct dark matter detection, particularly highlighting the benefits of interdisciplinary research and collaboration”.
