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Faculty Profile

Ziqing Hong

Assistant Professor
Experimental Dark Matter and Neutrino Physics

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Welcome to the Department of Physics Professor Hong!
You received your PhD from Texas A & M University. Can you tell our readers about the research you were involved in?

I did collider physics for my PhD thesis. I joined the CDF experiment at the Fermilab Tevatron and did a data analysis related to the top quark-antiquark differential production cross-section. I joined CDF a bit late, as the experiment ceased operation in 2011 and I joined one year later. At the time, there were a lot of unresolved data analyses to be finished; one outstanding one was the anomaly in the top quark “forward-backward production asymmetry”. The experimental results departed from the theoretical predictions with the “Standard Model of elementary physics”, in some cases by as much as 3 standard deviations. I cleaned up the last bit of the CDF data and performed a statistical analysis with the then best knowledge about top quarks. In the end, with more data and with theorists refining the predictions, we brought the differences down to about 1 standard deviation, at which point we no longer consider it an anomaly. We hoped we had signs of new physics at the beginning and put up a good chase for it. In the end, we didn’t find new physics, but a better understanding of the Standard Model.

Why did you pursue Dark Matter after completing your PhD? What made you interested in it?

Chasing for physics beyond the Standard Model has always been a long-time interest for me. The Standard Model is great in explaining elementary particle physics. But it has its own challenges. Particle physicists are pushing in all directions where a potential deviation could be identified and hoping that’ll lead to a series of realizations of where physics beyond the Standard Model would go. The evidence of the existence of dark matter, something not predicted by the Standard Model, has been around for decades now, although the direct interaction of dark matter in a terrestrial detector is yet to be observed. It is a direction that could tell us what nature is hiding beyond the current model we have. I thus joined the work force to directly search for hints of dark matter.

Can you tell our readers a little bit about the “SuperCDMS” experiment and your role it that?
The SuperCDMS experiment is a dark matter direct search experiment. “CDMS” is short for Cryogenic Dark Matter Search, while the prefix Super speaks about the quality of our experiment. The kernel of SuperCDMS is its ultra-pure silicon and germanium detectors equipped with cutting-edge technology called Transition Edge Sensors. These detectors work at temperatures below 50 milli-Kelvin, thus they require a world class cryogenic facility to cool them down to proper working temperature. On top of the cryogenic requirement, the experiment also needs to be well shielded from the outside world, including the cosmic ray and radiogenic contaminations. We are in the process of constructing such a facility at the SNOLAB, Canada's world-leading astroparticle physics laboratory located 2 km below the surface in the Vale Creighton Mine near Sudbury. My business line in the SuperCDMS experiment lies in detector testing and calibration. Before putting detectors in the SuperCDMS SNOLAB experiment, they need to be thoroughly tested. The detector testing happens at three places, at SLAC where they are fabricated, and at two underground facilities, NEXUS@FNAL and CUTE@SNOLAB. I joined forces with other SuperCDMS collaborators in operating the two underground facilities, NEXUS and CUTE. In addition, I am leading the detector nuclear-recoil calibration program, Ionization Measurement with Phonons At Cryogenic Temperatures (IMPACT), aiming at calibrating the detector response with neutron beams.

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The new-style SuperCDMS SNOLAB detector.
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Schematics for the SuperCDMS and CUTE at SNOLAB.

Your other research interest is in neutrino coherent scattering detection, why?
Neutrino coherent scattering is interesting to me both because it will soon be an intrinsic background for any dark matter search experiment, and because it has it can provide possible hints to deviations from the Standard Model. As dark matter detectors are becoming more and more sensitive, the underground dark matter search experiments will start seeing, or have already seen, neutrinos from the sun. This is through the Coherent Elastic neutrino-nucleus scattering (CEvNS) process. To push the dark matter search further we need a good understanding of the CEvNS process, in order to model this intrinsic background. One good way to gain this knowledge is to deploy a dark matter search type detector next to a nuclear reactor and measure the CEvNS signal from it. At the same time, there are anomalies seen with other ongoing reactor-based neutrino experiments. Measuring CEvNS signals from reactors would serve as a complementary measurement to independently probe what is going on, and hopefully, jointly we can understand the neutrinos from reactors better, or confirm hints beyond the Standard Model.

Can you tell our readers what the Ricochet experiment is?
The Ricochet experiment is a reactor-based CEvNS experiment. It employs cutting-edge cryogenic detector technology to measure the neutrinos from a nuclear reactor. The current experimental site is at Institut Laue-Langevin in Grenoble, France. We are just starting the experiment construction phase and he first data are expected in 2023.

What is your role in this experiment?
I’m involved in the effort of designing and optimizing part of the Ricochet detector payload. The cryogenic detectors to be deployed in Ricochet requires a slightly different optimization than the SuperCDMS detectors. They need to be smaller in size individually, to not be too sensitive to the muon flux in an above-ground laboratory environment. To accumulate enough mass for a sensible measurement, we need tens to hundreds of them forming a detector array. We are in the process of taking what we have learned in optimizing dark matter detectors and coming up with a novel idea of a thermal TES sensor chip. We can then couple these sensor chips to our choice of target easily. This new detector scheme reduces the requirement on the detector fabrication process, and makes it easier to build a large payload.

What are some real world implications of neutrino scattering?
Comparing to the other neutrino detector technologies, CEvNS process require a very sensitive cryogenic detector to make observations, but not a ton-scale detector. Detecting CEvNS signal with cryogenic detectors, once realized, would have a big impact on nuclear reactor monitoring and nuclear non-proliferation.

What are your research plans at the University of Toronto?
I plan to keep collaborating with the SuperCDMS and Ricochet members, both locally at UofT and internationally, to pursue the two scientific goals: dark matter search and CEvNS detection. I am in the process of building up a hardware-orientated research group to complement the existing SuperCDMS effort at UofT led by Prof. Miriam Diamond and Prof. Pekka Sinervo, who focus on simulation and analysis efforts. We will establish a local cryogenic lab, with detectors running at below 50 milliKelvin. The local lab would be focusing on fast-turnaround detector R&D, testing and calibration efforts. Once the new detectors are produced, we would calibrate them in neutron beams and deploy them either at SNOLAB to detect dark matter, or at ILL in Grenoble to detector neutrinos.

What are you most excited about being in Toronto?
Toronto is fairly close to Sudbury, where SNOLAB resides. I am excited to the possibility of traveling to SNOLAB frequently to contribute to the SuperCDMS SNOLAB and CUTE@SNOLAB experiments. The department here is also very large, with experts on all topics of physics. I am excited to learn more from my colleagues and collaborate on various fronts to find physics beyond the Standard Model. I have heard good words about the students at UofT and am looking forward to working with the talented younger generation. Beyond research, the restaurant density around the university also makes me excited...

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Noah Kurinsky (left), Ziqing Hong (right) and Brian Nebolsky (in the back) installing a SuperCDMS-type detector at a SuperCDMS test facility.