I work in experimental particle physics studying the fundamental building blocks of the universe, the elementary particles, and the forces that bind or repel them. The best theory we currently have is the “Standard Model of Particle Physics” which contains dozens of different particles and describes how they interact. Despite being highly successful in describing parts of our universe, it falls short in other aspects, including when predicting the masses of the neutrinos (proved by the work of the SNO experiment, which led to the Nobel prize shared by Dr. Art McDonald in 2015), and the lack of including particles that could be attributed as the “Dark Matter”. As an experimentalist, together with the experimental collaborations, I am dedicated to chasing down these shortfalls of the Standard Model of Particle Physics, with the goal of gaining more understanding of the universe. This includes searching for “Dark Matter” with the SuperCDMS@SNOLAB experiment and probing the properties of the neutrino particle with the Ricochet@ILL experiment.
Dark matter search with SuperCDMS@SNOLAB
Evidence obtained from astronomy and cosmology calls for a mysterious type of matter, “Dark Matter” (DM), which has only been observed via gravity: measurement shows that DM actually accounts for 85% of all matter in the universe. Due to its gravitational dominance over regular matter, it is a key driver for the evolution of the universe, affecting the formation of galaxies like the Milky Way. Although it is fundamental to our very own existence, we don’t know what it is. Due to the feeble interacting nature of DM, extremely sensitive detectors are needed. Thus, we have built the SuperCDMS experiment at the 2km underground SNOLAB in Lively, ON (40 minutes outside of Sudbury) -- the Nobel-prize-winning lab currently hosting a suite of cutting-edge experiments.
Profs. Miriam Diamond, Pekka Sinervo, and I co-lead the largest team on the experiment, collaborating with research scientists and adjunct Profs. Matt Stukel and Andy Kubik, amongst others. We have recently achieved a milestone of finishing assembling and turning on the experiment. Our students and postdocs are playing leadership roles in almost all aspects of the experiment, from constructing and operating the giant machine to collecting and analyzing data to search for hints of dark matter inside, and everything in between. We are currently going through a period of a few months of “tweaking the machine”, followed by a three-year stable data-taking to give us an unparalleled sensitivity to dark matter candidates that are comparable to the mass of a few protons.
Neutrino elastic scattering sensing with Ricochet@ILL
Aside from chasing dark matter, Prof. Andy Kubik and I are also probing the most enigmatic particles within the Standard Model of Particle Physics: the neutrinos. Although these particles were predicted by the model to have zero masses like the photons, Dr. McDonald and the SNO experiment proved it’s not the case. These elusive particles have been put under the spotlight, going through thorough scrutiny, hoping one day they might tell us some deeper secrets about the universe.
To discover neutrinos’ secrets, we are deploying the sensitive detectors next to their hometown, a nuclear reactor where many neutrinos are born. We are collaborating with the Institut Laue–Langevin (ILL), which operates a research reactor in Grenoble, France, with a detector array placed eight meters from the core of the reactor.
Recent news from the French National Institute of Nuclear and Particle Physics (IN2P3) highlighted the start of the scientific phase of Ricochet, featuring our student Elspeth Cudmore finalizing the detector installation at the reactor site. The experiment will continue for at least another year. By the end of this data collection, we expect a definitive identification of the neutrino scattering signal via the “coherent scattering mode”, in these state-of-the-art cryogenic crystal detectors. A future upgrade with more and better detectors is under planning, hopefully with a new technology that is naturalized from our own lab.
Core technology: Cryogenic crystal detectors
The common feature of both the dark matter search SuperCDMS experiment and the neutrino measurement Ricochet experiment lies in the cryogenic crystal detector. These semiconductor crystals are cooled down to about 0.01 degrees above absolute zero to best suppress their intrinsic noise, then are fitted with either the “Transition-Edge Sensors” utilizing superconductors or the “Neutron-Transmutation-Doped” germanium thermistors, which are the most precisely engineered semiconductor sensors. Their extreme sensitivities to temperature make them the best thermometers existing, measuring the heat induced by the slightest particle interactions, either from the neutrinos or from the potential dark matter particles.
To pursue increasingly fainter signals, we continue to develop this technology in our basement laboratory in the McLennan Physics building. We recently acquired a dilution refrigerator capable of achieving single-digit milliKelvin temperatures and fitted it with an array of Superconducting Quantum Interference Devices (SQUIDs) as the electronics read out system. Benefiting from the Toronto NanoFabrication Center (TNFC) on campus, we can rapidly prototype improved detector designs and test them in our basement lab with a short turnaround time. We also heavily rely on our friendly relations with our basement neighbours, especially groups sharing similar cryogenic facilities and groups who have extensive experience dealing with all sorts of crystals, both in understanding theories about these solid-state systems, and in handling these fragile objects, especially when we face our common challenges: calming down the temperamental fridges.
