William & Mary physicist featured in Fermilab neutrino video
Detector Patricia Vahle, assistant professor of physics at William & Mary, checks the operation of a NOvA detector prototype at Fermilab. Vahle was featured in a Fermilab video on the experiment, under construction and set to begin in 2013. Photo by Reidar Hahn/Fermilab
A William & Mary physicist is featured in a video explaining the latest chapter in the investigation of mysterious, flavor-shifting particles called neutrinos.
“Neutrinos are interesting because they’re one of the most abundant particles in the universe,” Patricia Vahle says in the video. “So if you want to understand how the universe was born, how it is evolving, what it’s eventual fate is, you need to understand the properties of neutrinos.”
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The video, titled NOvA: Exploring Neutrino Mysteries, was produced by Fermilab, the U.S. Department of Energy’s facility in Batavia, Ill., dedicated to high-energy physics. Vahle, assistant professor of physics at William & Mary, leads one of the working groups of physicists involved in the NOvA experiment.
NOvA is the latest experiment established to probe the odd, identity-swapping property of neutrinos called oscillation. Vahle explains that physicists have known for decades that neutrinos exist in three different states—or “flavors”; physicists know them variously as electron, muon and tau neutrinos.
One of the challenging aspects of neutrino study stems from their property of changing flavors in mid-flight. This change in flavor is known as oscillation, the subject of NOvA and a number of other high-energy physics experiments.
Vahle explained that NOvA is particularly interested in aspects of the muon-to-electron neutrino oscillation. This study will tell physicists about a certain parameter known as theta 1-3. The parameter is also the focus of a separate experiment at the Daya Bay nuclear complex in China.
“Daya Bay has already measured the value of theta 1-3,” she explained. “But at NOvA, we’ll do it in a different way than Daya Bay did, in a way that will tell us other things about neutrino oscillation as well.”
Scientists hope NOvA will shed some light on the mass of neutrinos, particles so light that for years they were thought to be utterly massless. In the complex world of neutrinos, mass states are related to flavor states, but there’s not a one-to-one correlation, Vahle explains.
“In the oscillation experiments, we kind of measure the mass indirectly. We don’t know which neutrino is the most massive or the least massive,” she said. “It’s a little bit tricky to explain, but we measure the difference in the masses, not the absolute mass.”
This problem is known as the “mass hierarchy” of neutrinos, one of the problems NOvA was designed to solve. Vahle points out that mass hierarchy could be an important concept that discriminates among versions of the Grand Unified Theory of physics, models that seek to reconcile the fundamental forces of nature.
Vahle says that she and the collaborators at NOvA hope to be able to understand how neutrinos behave under charge parity, or CP, transformations.
Physicists are excited because CP violation among neutrinos could help to explain one of the most fundamental questions of physics: why there is matter in the universe.
“Right after the Big Bang, matter and antimatter should have been made in equal parts. And then matter and antimatter should have annihilated with one another and just left energy,” Vahle explains. “And so the fact that we have a universe made out of matter and that we don’t have much antimatter lying around is a big question.”
"Quarks violate CP, but just a little bit," Vahle explained. "There's room in the neutrino sector for there to be larger violation of CP, and people are excited about that."
She added that the CP violation displayed by the quarks is 100,000 times too small to account for the matter-antimatter asymmetry of the universe.
“There is the possibility that we could find the answer to this question in neutrinos,” she said. “If neutrinos violate CP, and they violate it in a big way, we may kind of have the first hint to the answer to that question.”
Vahle says she was picked for the video just by being in the right place at the right time. She was one of a group of William & Mary physicists participating in another neutrino experiment known as MINOS and appeared in a Fermilab video ten years ago.
“They just remembered my name and contacted me, asked if I was willing to do something for NOvA,” she said. “They just wanted me to come down and talk on the video about why I was excited about NOvA, about what I, as a neutrino physicist, was hoping to learn from this experiment.”
William & Mary’s Department of Physics is well represented in participation in neutrino experiments. The university’s standing in contemporary neutrino physics was highlighted by William & Mary being chosen to host NuFact 2012, the annual international neutrino workshop. The July event brought more than 150 particle physicists from all over the world to Williamsburg.
