Fermilab honors UC student for 'new physics' quest
UC researchers search for subatomic particles called 'sterile neutrinos,' which could redefine the Standard Model
Physicists at the University of Cincinnati are trying to understand why matter and antimatter weren’t destroyed in equal parts during the Big Bang.
The dominance of matter over antimatter is the reason we have atoms and molecules and hippopotamuses today. But it’s still a mystery that physicists such as UC postdoctoral fellow Jacob Todd are trying to unravel.
Todd was honored this month by two prestigious scientific organizations for his doctoral thesis on the search for new subatomic particles. The Universities Research Association, Inc., and U.S. national physics laboratory Fermilab selected Todd for its annual outstanding graduate thesis award. He was recognized at Fermilab's annual users conference.
Todd and his advisor, UC physics professor Alexandre Sousa, are part of an international team that published its experimental results in March in the prestigious Physical Review Letters, a journal of the American Physical Society.
The researchers are exploring neutrinos, the most abundant subatomic particle with mass in the known universe. Physicists have identified three kinds: electron neutrinos, muon neutrinos and tau neutrinos.
Physicists suspect there is a fourth, called a sterile neutrino. But experiments by Todd, Sousa and their research collaborators at Fermilab did not find them.
The collaboration demonstrates UC's commitment to research as part of its strategic direction called Next Lives Here.
UC physics professor Alexandre Sousa tosses a toy ball in the air that changes color in mid-air to demonstrate the transition of muon neutrinos to electron neutrinos. Photo/Joseph Fuqua II/UC Creative Services
It gives us the key to a completely hidden sector of physics.
Alexandre Sousa UC physics professor talking about Fermilab's DUNE project
At Fermilab, physicists fire a high-intensity neutrino beam at two detectors situated 1 kilometer and 735 kilometers away from the accelerator.
“You can think of it like the flashing of a laser — brief periods or events separated by a fraction of a second. On, off. On, off,” Todd said. “Then you time your detectors to look for those events based on the speed of travel. And they’re traveling at very close to the speed of light.”
The particle accelerator fires a beam of neutrinos in pulses every second, several thousand times per hour, every hour of every day for the past decade.
“We have people on the clock at 3 a.m. on Christmas morning,” Todd said. “That’s the dedication it takes.”
Physicists look for sterile neutrinos by observing the “disappearance” of muon neutrinos between the two detectors. Not all of the muon neutrinos observed in the near detector are observed in the far detector. Where did they go?
Well, some neutrinos change “flavor,” oscillating from muon neutrino to electron neutrino or back again during their transit. And physicists have theorized that some of the neutrinos they don’t find at the far detector might have oscillated into a yet-undiscovered type of neutrino: a sterile neutrino.
But UC’s physicists and their international colleagues did not find evidence of sterile neutrinos. The results are in conflict with a 2018 experiment called MiniBooNE that found significant evidence of sterile neutrinos.
Fermilab's MicroBooNE experiment used more than 8,000 gilded wires submerged in liquid argon to record the tracks of subatomic particles. Scientists found evidence of a fourth type of neutrino called the sterile neutrino. Photo/Cindy Arnold/Fermilab
The discrepancy in results generated a lot of debate in particle physics circles, Sousa said.
“It’s an exact science surrounded by subjective opinion,” he said.
A new Fermilab project called the Deep Underground Neutrino Experiment will feature a detector in South Dakota 800 miles away from Fermilab’s Illinois facility. Sousa said if undiscovered particles indeed do exist, this detector will give researchers the best opportunity yet to find them.
“It’s going to be the largest high-energy physics project in the United States for the next 20 years. It’s a very significant experiment,” Sousa said. “It gives us the key to a completely hidden sector of physics.”
Physicists are curious about sterile neutrinos in part because they might help explain a fundamental question surrounding the origin of the universe — why it has so much matter and so little antimatter. Both should have been annihilated in equal measure during the Big Bang – the presumed origin of the universe – but for some unexplained reason, matter prevailed.
Robots help store data at Fermilab's tape libraries. Photo/Reidar Hahn/Fermilab
There isn’t very much antimatter in the known universe, although physicists have created small amounts in particle accelerators like the one at Fermilab. Antimatter is produced by the sun when particles collide during solar flares and when cosmic rays strike the Earth’s upper atmosphere.
Oddly, bananas create antimatter as well. The decay of trace amounts of radioactive potassium releases subatomic particles called positrons — one about every 75 minutes.
“Bananas are slightly radioactive,” Todd said.
Neutrinos are particles you can actually observe yourself — sort of. YouTube is full of videos about making a cloud chamber. All it takes is a jar, a sponge soaked in rubbing alcohol, dry ice and a flashlight. The dry ice condenses the alcohol into a vapor or cloud. The subatomic particles zipping through space are betrayed by lines of condensation that look like contrails made by the world’s tiniest jets. But Sousa said it would take a stroke of luck to observe a neutrino in your cloud chamber since they so rarely interact with matter.
When matter and antimatter meet, they annihilate into pure energy.
“Imagine if two baseballs made of matter and antimatter collide,” Todd said. “You’d see a massive burst of heat and light. There wouldn’t be a single particle left. It would be the most efficient explosion you’ve ever seen.”
In this field we are always after discoveries, but even when we do not find new particles, we are furthering our understanding of how the universe works.
Alexandre Sousa, UC physics professor
UC graduate Jacob Todd, left, and UC physics professor Alexandre Sousa attend Todd's Ph.D. hooding. Photo/Provided
Hollywood depicted just this kind of spectacle in the Tom Hanks movie “Angels & Demons” in an explosion created when a gram of antimatter annihilates over Vatican City. But researchers have created far less antimatter to date — far less even than it would take to boil water for a cup of tea, CERN physicist Rolf Landua told the news outlet Live Science.
“Thanks to the inefficiency of the transformation process of energy into antimatter we are safe,” Landua said.
Sterile neutrinos are tricky to study because they are virtually impossible to observe, Sousa said.
“They’re particles that only interact with gravity. So they feel gravity but none of the other forces,” Sousa said. “They don’t have a charge. They don’t feel electromagnetic force. They don’t feel the strong force that holds a nucleus together. Unlike neutrinos and other particles, they cannot feel the weak force responsible for radioactive decay.”
The experiment UC contributed to at Fermilab didn’t make a new discovery. But it helped narrow where to look for them, Sousa said.
“In this field we are always after discoveries, but even when we do not find new particles, we are furthering our understanding of how the universe works,” Sousa said.
Sousa said he is optimistic that future experiments at Fermilab such as the upcoming Short-Baseline Neutrino program will bring physicists closer to explaining the origins of the universe.
"I think this result will help us give a definitive answer to the question of whether sterile neutrinos explain the results of the MiniBooNE and LSND experiments,” Sousa said. “Those are the main anomalies we know that indicate sterile neutrinos exist. If we can’t reproduce them, it’s not true.”
Featured image at top: UC postdoctoral fellow Jacob Todd. Photo/Joseph Fuqua II/UC Creative Services
UC postdoctoral fellow Jacob Todd will be honored for his graduate thesis at Fermilab's annual users meeting. Photo/Joseph Fuqua II/UC Creative Services
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