Chelsea Bartram and Brian Lenardo have been named the 2022 Panofsky Fellows at the Department of Energy's SLAC National Accelerator Laboratory. Bartram will help develop an experiment to search for dark matter particles called axions, while Lenardo will aid the search for a phenomenon called neutrinoless double beta decay that would hint at a world beyond the Standard Model of particle physics.
The Panofsky Fellowship, named after SLAC's founder and first director Wolfgang K. H. "Pief" Panofsky, recognizes exceptional early-career scientists who would most benefit from the opportunity to do their research at the lab. It provides generous funding for five years of research and an opportunity for continuing appointment at SLAC.
From ham radio to dark matter radio
Growing up in Ohio, Chelsea Bartram was enchanted by her dad’s amateur radio setup in the basement of their house, with antennas hidden around the yard so as not to attract attention. “I thought it was so magical that you could talk to someone on the other side of the world with this,” she said.
She thought she’d major in engineering at Boston University, where one of the first courses she took was on electromagnetism – the phenomenon behind ham radio. Then, seeking an even deeper level of understanding about how things work, she switched to physics.
“That set me on this trajectory where I eventually started working in particle physics,” Bartram said. She was one of the first students at her university to attend a six-month study abroad program at the University of Geneva in Switzerland, where they got to explore CERN, home of the Large Hadron Collider, before the collider’s massive particle detectors closed up and began operation. That trip, she said, cemented her interest in the field.
In grad school at the University of North Carolina at Chapel Hill, she joined a group focused on nuclear physics and built a tabletop experiment called CALIOPE to see if the decay of positroniums – which are bound states of electrons and their antimatter cousins, positrons –violated any fundamental symmetries.
She was still in grad school when she started working on the Axion Dark Matter Experiment (ADMX) at the University of Washington, and continued there after earning her PhD in 2019.
“ADMX is essentially like a giant AM radio that’s searching for dark matter particles called axions,” she said. The experiment consists of a cylindrical cavity, immersed in a very strong magnetic field at a very low temperature, in which passing axions could covert to photons, creating an electromagnetic signal in the cavity. The trick is to tune the cavity to just the right frequency – like turning the dial on a radio – to detect this signal.
When she arrives at to SLAC in October, she’ll be working with SLAC and Stanford Professor Kent Irwin and his team on the Dark Matter Radio (DMR) experiment, which will also search for axions by picking up radio signals.
“This is very exciting,” she said. “I can pull from my experience at ADMX to inform the design work that needs to be done on DMR. More broadly, I’d like to test a technique I developed with a collaborator for searching for axions at a faster rate. I started to do that at UW, and that’s what I’d like to do at SLAC.”
Bartram said the most important benefit of the Panofsky Fellowship is that it will enable her to develop her own research group. “I know there are other potential collaborators at SLAC and Stanford who would be very exciting to work with in developing these ideas,” she said, “plus I look forward to using facilities like the new Detector Microfabrication Facility to make quantum devices for this research.”
From dark matter to neutrinos
From early on, Lenardo's parents encouraged him to study math, figuring that it would be very important whatever career he ended up pursuing, Lenardo said. That career began to take shape in high school, when he took his first physics class. "I thought, wow, I can use this math I've learned to predict things in the real world," he said.
Lenardo's path toward neutrino physics began as an undergraduate at the University of Maryland. At first, he said, like Bartram he didn't entirely know whether to pursue physics or engineering. Then, he took a winter-break seminar that introduced him to the IceCube Neutrino Observatory, which uses sensitive detectors buried in Antarctic ice to detect cosmic neutrinos.
"That really sold it," Lenardo said, and he chose physics.
As an undergrad at Maryland, he worked on the High-Altitude Water Cherenkov Gamma-Ray Observatory, a cosmic-ray observatory in Mexico.
For graduate school, he moved to the University of California, Davis, where he started working on the LUX project, a dark matter experiment using a giant tank of liquid xenon. The basic idea is that if dark matter strikes xenon atoms, it should create a flash of light and kick out an electron, both of which are easy to detect. By carefully analyzing the light and electrons and comparing what the experiment sees against other potential sources of light and electrons, LUX and other similar experiments, such as the successor LUX-ZEPLIN project, aim to directly detect dark matter.
But Lenardo's PhD research, which focused on understanding xenon's interactions with a variety of particles, also positioned him for another research area: experiments aimed at better understanding the strange properties of neutrinos, including why they have mass when the Standard Model predicts they have none.
"The fact that neutrinos don't really fit into the Standard Model tells us this is a good place to look for new physics," Lenardo said. "It's a clue left on the ground that points in a certain direction, and I think it makes studying neutrinos extremely compelling.”
Lenardo is now pursuing research on neutrinos as a postdoctoral fellow at Stanford, where he works on the nEXO experiment, which like LUX relies on liquid xenon-based detection. That experiment aims to find signs of a hypothetical process whereby xenon atoms transform into barium, kicking off photons and electrons but no neutrinos. That process, an example of something called neutrinoless double beta decay, is impossible according to the Standard Model; but if it does exist, it could yield clues to why neutrinos have mass, why there is more matter than antimatter in the universe, and more.
As a Panofsky Fellow, Lenardo will continue his work on nEXO, focusing in particular on sensor development for nEXO and on ways to calibrate the system. He’ll also work on possible extensions of the technology to explore other physics, such as the study of solar neutrinos.
Lenardo said he is particularly excited to work with SLAC scientists and engineers. "SLAC is a world-leading institution in particle physics and has all that institutional expertise on the specifics of how to build these kinds of experiments," he said. "I'm excited to leverage that expertise to build the next generation of experiments."
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