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Social distancing at the far detector: The NOvA beam may be off, but the physics is very much on

  • A technician works on the installation of the far detector
    Socially distant:  A technician works on the installation of the far detector in Ash River, Minnesota. The NOvA neutrino beam is dark as part of social-distancing measures, so the experiment’s far detector is registering transit of neutrinos and other subatomic particles that are an unseen, unfelt and (mostly) undetected part of our existence.  Fermilab file photo by Reidar Hahn
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The beam is off, but high energy physics research is very much on at one of the world’s premier particle physics labs.

The requirements of social distancing have prompted scientists at the U.S. Department of Energy’s Fermilab to turn off the accelerator that makes a beam of neutrinos for the NOvA experiment, but William & Mary physicists are among those monitoring the still-active NOvA neutrino detectors.

Without a neutrino beam, data collection for the experiment’s main physics program is paused. Instead, the scientists are recording the interactions of naturally made particles, with an eye on the long-shot possibility that they’ll be able to witness a critical moment in the evolution of a nearby star.

“We have decided to leave our neutrino detectors on,” said Patricia Vahle. “There are risks to power cycling these complex machines, so it’s better to keep them running. By staying up, we might catch Betelgeuse if it decides to go supernova.”

William & Mary’s physicists have a control room in Small Hall from which they can monitor the NOvA experiment. But now that the beam is dark, Vahle added that the physicists will be working at home, as they are now able to monitor activity at the far detector using web applications.

“People will be able to do everything at home and will not need to go to the remote control room on campus,” she said. “We’re also busy analyzing the data we’ve collected since last summer. Our big summer conference where everyone presents new results is still being held, just online.”

Vahle is a professor in the Department of Physics at William & Mary and is the co-spokesperson for the NOvA experiment. NOvA is dedicated to investigating a puzzling quality of superabundant subatomic particles known as neutrinos.

The very idea of neutrinos may seem preposterous to the layperson. Neutrinos are teeny subatomic bits that zip around just short of the speed of light. Even though neutrinos are unbelievably abundant, they are challenging to detect. For one thing, they zip through matter, rarely colliding or interacting. Then there’s oscillation. Neutrinos come in three known “flavors” — tau, muon and electron. And they change — oscillate — from one flavor to another mid-flight.

The NOvA experiment was designed to study oscillation and other properties of neutrinos. The experiment sends a beam of neutrinos from a source through two detectors. The beam of particles shoots through the near detector, close to the beam source at Fermilab in Batavia, Illinois.

The beam then goes through 500 miles of earth before going through the far detector at Ash River, Minnesota. Even now, with the beam turned off, the detector still lights up.

“We have plenty of atmospheric neutrinos zipping through, but they are hard to identify in our detector because it’s on the surface,” Vahle said. “Atmospheric neutrinos are created when high-energy particles from outer space bombard our atmosphere.”

She said there is plenty to be learned from those atmospheric neutrinos. Vahle pointed out that researchers at Super-Kamiokande — a Japanese neutrino observatory also known as Super-K — won the 2015 Nobel Prize in Physics for work on the oscillation of atmospheric neutrinos.

“We have a whole physics working group devoted to looking at non-beam related physics,” Vahle said. “Some of it is recording the rate and multiplicity of cosmic rays as a function of time. Seasonal variations can tell us about production of different types of particles in our atmosphere.”

Vahle notes that neutrinos are not the only things lighting up the detector; a whole menagerie of high-energy particles rains down continuously. There are nuclei of hydrogen, helium and other elements that zoom along until they smack into earthly atoms, a collision that creates other particles. These particles, largely pions, degrade into muons.

The NOvA far detector can register particles coming up from the center of the Earth, or rather from the other side of the Earth, she said.

“An atmospheric neutrino that was created on the other side of the Earth interacts with a nucleus in the earth nearby our detector, sending a muon off through our detector,” Vahle explained. “We can tell that the muon is going up by looking at the timing of the track of hits it leaves in our detector. There are very few things other than an atmospheric neutrino that can send an energetic muon going up from underground.”

Vahle added that the physicists also are keeping an eye out for evidence of “exotic things like magnetic monopoles or dark matter.” She consulted her collaborator, Matt Strait, a postdoctoral researcher at the University of Minnesota, as the expert on how a supernova might be witnessed by the watchers of the far detector.

There has been some buzz in scientific circles about the apparent dimming of Betelgeuse for months, inspiring speculation that it may be close to going supernova. (Or might be just dusty). If this star, part of the constellation Orion, were to enter supernova phase, it would be the first Milky Way star to do so since before Isaac Newton was born. It would be visible, even during the day. Watchers of neutrino detectors would get advance notice of a day or two.

“Neutrinos come before the light,” Vahle said, “so we could send advance notice to optical astronomers.”

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Check it out: Patricia Vahle explains what's going on in the NOvA far detector

“This is 500 microseconds of activity in our detector. The top panel shows what our detector looks like if you were looking down on it; the bottom panel shows the detector from the side. The horizontal axis is the depth in the detector, while the vertical axis is the transverse position in the detector. Each colored box corresponds to an individual detector channel that saw some light. The hits are colored by their arrival time.

“Most of what you see are cosmic ray muons. Muons charged particles and are the heavy cousins of electrons. High-energy muons can travel through a lot of material, just leaving behind a little bit of energy at each step, so they show up as long straight tracks in our detector. These muons were created high in the atmosphere and are constantly raining down all over the earth.”