Fermilab’s Robert Bernstein had been conducting neutrino research for 25 years when he began searching for a new project.
“One of my friends came up to me and said, ‘You should be Mu2e spokesperson.’ And I said, ‘What’s Mu2e?’ I went off and did some homework and I said, ‘OK, it’s terrifically interesting, really hard, and I don’t know anything about it. Sure. Sign me up!” he said.
Although the muon was discovered in 1936, physicists still seek to understand why the particle exists, thus the need for the Mu2e experiment (pronounced “mew 2 E,” which stands for muon-to-electron conversion). It’s part of a larger question: Why do multiple generations of subatomic particles exist? The quark family is made of three generations of particles — up and down, strange and charm, bottom and top — each heavier than the next. Similarly, the lepton family has three generations — electrons, muons and taus — and again, the members have different masses.
“Why is that? Why are there these generations of particles. That’s a fundamental question,” Bernstein said.
Bernstein first became co-spokesperson with Boston University’s James Miller in 2007, the year Mu2e received formal approval. He continued in that role until 2014, when Fermilab scientist Doug Glenzinski succeeded him. In March, Bernstein returned to the co-spokesperson role after Glenzinski became Fermilab’s chief project officer.
“Bob and I were co-spokespersons at the beginning of Mu2e, and it’s great to work with him again,” Miller said. “He brings a deep knowledge of the experiment and a proven record of leadership.”
Bernstein returned the sentiments.
“He has a long and distinguished career in muon physics and precision experiments,” Bernstein said of Miller. “He’s been doing this even longer than I have.”
The Mu2e collaboration consists of nearly 250 scientists at 40 institutions. The MBA that Bernstein earned from the University of Chicago in 2006 has helped him grapple with the organizational challenges involved in working with such a large group.
“I majored in organizational behavior. In terms of thinking about how to get people to work together on a task or a big project or a big problem, I use the ideas I learned there every day,” he said.
Bernstein noted that the experiment is changing from construction to installation.
“The building, along with most components for the infrastructure, experiment and beam have been built, and over the next couple of years, we’ll turn Mu2e into a running experiment,” he said. “It’s an exciting time, and a great one for young people who will put the parts together. When someone is analyzing data, they’ll know they we’re the ones who installed the piece that gave them that data, and they’ll know all its quirks. That’s a lot of fun and incredibly satisfying.”
The collaboration has needed to invent technology to get to this point. No one, for example, has ever built anything like Mu2e’s solenoid system, which serves as the heart of the experiment. The tracker, the instrument that detects the electron, boasts novel technology that includes straw-like tubes whose walls are about half as thin as a typical human hair.
Mu2e meshes nicely with Fermilab’s NOvA neutrino experiment, which Bernstein also is involved in. NOvA’s goals include observing the oscillation of muon neutrinos into electron neutrinos. Mu2e similarly seeks evidence for the conversion of muons to electrons. Such a discovery could put physicists on the trail of new particles or new forces of nature. To get there, the collaboration continues to solve new problems as they arise.
“For me on a daily basis the most fun part of the experiment is working on really hard problems with really smart people,” Bernstein said. “That’s motivating for me.”
Muon research at Fermilab is supported by the Department of Energy Office of Science.
Fermilab is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.
In late 2019, Betelgeuse, the star that forms the left shoulder of the constellation Orion, began to noticeably dim, prompting speculation of an imminent supernova. If it exploded, this cosmic neighbor a mere 700 light-years from Earth would be visible in the daytime for weeks. Yet 99% of the energy of the explosion would be carried not by light, but by neutrinos, ghost-like particles that rarely interact with other matter.
If Betelgeuse does go supernova soon, detecting the emitted neutrinos would “dramatically enhance our understanding of what’s going on deep inside the core of a supernova,” said Fermilab theorist Sam McDermott. And it would present a unique opportunity to investigate the properties of neutrinos themselves. The Deep Underground Neutrino Experiment, hosted by Fermilab and planned to begin operation in the late 2020s, is being developed with these goals in mind.
Only once before have scientists detected the neutrinos emitted by a supernova: During SN 1987A (bright star at center), detectors spotted only about two dozen neutrino interactions. The exploding star was in the Large Magellanic Cloud, 240 times more distant from Earth than Betelguese. Photo: ESO
DUNE’s far detector — an enormous tank of liquid argon at the Sanford Underground Research Facility in South Dakota — will pick up signals left by neutrinos beamed from Fermilab as well as those arriving from space. Since a supernova emits neutrinos evenly in all directions, the number of neutrinos that DUNE could detect falls off as the square of the distance between the supernova and Earth. That is, the number of neutrinos that could be spotted 10,000 light-years away from a supernova is 100 times smaller than the number that could be detected from an equally powerful supernova 1,000 light-years away.
For this reason, if a supernova occurs in the middle of our galaxy, tens of thousands of light-years away, DUNE will likely detect a few thousand neutrinos. Because of Betelgeuse’s relative proximity, however, scientists expect DUNE to detect around a million neutrinos if the red supergiant explodes in the coming decades, offering a bonanza of data.
Although the light from the Betelgeuse supernova would linger for weeks, the burst of neutrinos would last only minutes.
“Imagine you’re in the forest, and there’s a meadow and there’s fireflies, and it’s the time of night where thousands of them come out,” said Georgia Karagiorgi, a physicist at Columbia University who leads the data selection team at DUNE. “If we could see neutrino interactions with our bare eyes, that’s kind of what it would look like in the DUNE detector.”
The detector will not directly photograph incoming neutrinos. Rather, it will track the paths of charged particles generated when the neutrinos interact with argon atoms. In most experiments, neutrino interactions will be rare enough to avoid confusion about which neutrino caused which interaction and at what time. But during the Betelgeuse supernova, so many neutrinos arriving so quickly could present a challenge in the data analysis — similar to tracking a single firefly in a meadow teeming with the insects.
“To remove ambiguities, we rely on light information that we get promptly as soon as the interaction takes place,” Karagiorgi said. Combining the light signature and the charge signature would allow researchers to distinguish when and where each neutrino interaction occurs.
From there, the researchers would reconstruct how the types, or flavors, and energies of incoming neutrinos varied with time. The resulting pattern could then be compared against theoretical models of the dynamics of supernovae. And it could shed light on the still-unknown masses of neutrinos or reveal new ways that neutrinos interact with each other.
Of course, astronomers who hope for Betelgeuse to go supernova are also interested in the light generated by the star explosion. When complete, DUNE will join the Supernova Early Warning System, or SNEWS, a network of neutrino detectors around the world designed to automatically send an alert when a supernova is in progress in our galaxy. Since neutrinos pass through a supernova unimpeded, while particles of light are continually absorbed and reemitted until reaching the surface, the burst of neutrinos arrives at Earth hours before the light does — hence the early warning.
SNEWS has never sent out an alert. Although hundreds of supernovae are observed each year, the most recent one close enough to Earth for its neutrinos to be detected occurred in 1987, more than a decade before SNEWS came online. Based on other observations, astronomers expect a supernova to occur in our galaxy several times per century on average.
“If we run DUNE a few decades, we have pretty good odds of seeing one, and we could extract a lot of science out of it,” said Alec Habig, a physicist at the University of Minnesota, Duluth, who coordinates SNEWS and is involved with data acquisition on DUNE. “So let’s make sure we can do it.”
Given the enormous radius of the red supergiant, Habig said, DUNE would detect neutrinos from Betelgeuse up to 12 hours before light from the explosion reaches Earth, giving astronomers plenty of time to point their telescopes at Orion’s shoulder.
Continuing observations of Betelgeuse suggest that its recent dimming was a sign of its natural variability, not an impending supernova. Current estimates give the star up to 100,000 years to live.
But if scientists get lucky, “an explosion at Betelgeuse would be an amazing opportunity,” McDermott said, “and DUNE would be an incredible machine for the job.”
Fermilab astrophysics research and the Deep Underground Neutrino Experiment are supported by the Department of Energy Office of Science.
Fermilab is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.