Not your Standard Model physicist
In the summer of 2005, at a banquet dinner laid out in a Swedish castle for some of the world’s leading physicists, Janet Conrad made a bet.
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Over multiple courses and much wine, Conrad, then a physics professor at Columbia University, playfully sparred with Nobel laureate and MIT physicist Frank Wilczek over the Higgs boson, a fundamental particle that was predicted to exist by the Standard Model of physics but had evaded all attempts at detection. Finding the Higgs would be key to resolving the mystery of how particles acquire mass.
Physicists hoping to observe the elusive particle were pinning their hopes on the Large Hadron Collider, the world’s most powerful particle accelerator, then under construction in cavernous tunnels near the Swiss Alps. The LHC was designed to smash together beams of protons and heavy ions at close to the speed of light. In the aftermath, physicists expected to see signs of the Higgs boson, along with other new physical phenomena.
Conrad, a self-acknowledged contrarian, posed a contrarian question: What if the LHC didn’t detect the Higgs? She had reason to believe that while the particle should exist, it would exist in an energy range that the detector would be unable to pick up. After a spirited debate, a wager, scribbled on a notepad, was struck: a Higgs discovery would favor Wilczek, while no detection would vindicate Conrad. The stakes: chocolate replicas of the Nobel medal, available only at the Nobel Museum in Sweden.
On July 4, 2012, Conrad lost the bet when physicists reported that the LHC had detected a new particle resembling the long-sought Higgs boson—a discovery that resonated across the physics world. When the news broke, Conrad was at the Fermi National Accelerator Laboratory outside Chicago, in the midst of an experiment. She enlisted a friend from Stockholm University to purchase the chocolates, which were ferried to New York by a visiting physicist from Columbia University and then to Chicago by a postdoc heading to Fermilab. Conrad then flew them back to Cambridge en route to a conference, leaving 10 pristine, un-melted chocolate Nobels with her sister, who delivered them to Wilczek’s MIT office.
The New York Times featured the wager in a small write-up, along with a cartoon depicting the byzantine trade-off—a framed copy of which hangs on Conrad’s MIT office wall. She bet that the Higgs wouldn’t exist in the energies that the LHC could probe, she explains, “because that was so much better than the Higgs existing [there], in my opinion.”
In other words, not finding the Higgs boson where so many physicists expected it to be would have exposed a seismic crack in the Standard Model, a theory that the physics community has relied on for decades to describe the fundamental forces and particles in the universe. Such a theoretical shake-up would have revealed a world of new physical unknowns.
“I don’t think our Standard Model makes a lot of sense,” Conrad says. “It fits together really well: you can take pieces of it and predict other pieces of it, which is a really impressive thing. And yet … [it includes] values we cannot explain. I’m convinced this is not the whole story. And I’m really interested in poking around to find out what the whole story is.”
Conrad, who joined MIT’s physics faculty in 2008, has spent her career poking at anomalous experimental results that others have either discounted or accepted unquestioningly as fact. “The thing I find most compelling is when an experiment has seen something interesting, and I want to figure out: did they make a mistake, or is nature actually telling us something new?” she says. “That to me is the most fun.”
That fascination with anomalies has led her on a quest for a particle far more elusive than the Higgs boson. And if she finds it, she will indeed turn the Standard Model of physics on its head.
In the mid-1990s, physicists at Los Alamos National Laboratory in New Mexico reported an unexpected and still controversial signal in the Liquid Scintillator Neutrino Detector (LSND). The detector is essentially designed to count neutrinos—infinitesimally small, nearly undetectable particles that are thought to outnumber all the universe’s ordinary matter particles, such as electrons and neutrons, by a billion to one. Despite their pervasiveness, neutrinos are often called “ghostly” because they are devilishly hard to measure: besides being extremely tiny, they have no charge, so they very rarely interact with ordinary matter and can stream through every cell in our body—even through thousands of tons of granite and steel—without ruffling a single molecule.
The LSND consists of a tank about the size of a city bus, filled with mineral oil, and is designed to receive a beam of neutrinos from a nearby accelerator. Light detectors lining the tank record tiny flashes produced by incoming neutrinos smashing into carbon nuclei in the oil, which is a natural scintillator—that is, it becomes luminescent when excited by ionizing radiation. The pattern and timing of interactions can tell scientists something of the type of neutrinos that streamed through the tank.
According to the Standard Model, neutrinos should exist in three varieties, or “flavors”—an electron neutrino, a muon neutrino, and a tau neutrino. However, in 1994 physicists reported that the LSND had counted more electron neutrinos than predicted. The physicists put forth a shocking theory: that this might be explained by the existence of an entirely new particle, a “sterile neutrino,” which interacts only with gravity and not at all with ordinary matter.
The results were met with intense skepticism. If the sterile neutrino did exist, it would indicate a phenomenon that the Standard Model cannot explain. It could also help explain dark matter, which makes up about a third of the universe’s matter but does not emit or reflect light. While others brushed the results off as a glitch that must have some rational explanation in line with the established rules of physics, Conrad took the anomaly as an opportunity.
Shortly before, she had wrapped up her graduate work in high-energy physics at Harvard University. Most everyone she knew in her field was headed off to join large collaborations of scientists working with particle accelerators to search for the top quark, the most massive of all elementary particles predicted by the Standard Model, which had not yet been detected. Conrad, going against the tide, had decided to strike out in search of the sterile neutrino, starting a postdoc at Columbia University in 1993.
“I remember making this decision in my life and having a colleague say, ‘Janet, you’re too good to be doing that,’” Conrad says. “But it’s a particle whose personality I like; it’s a very independent particle.”
In 1995, Conrad joined the physics faculty at Columbia and began building her research group. She also planted the seeds for a new particle detector designed specifically to prove or disprove the LSND results: the Mini Booster Neutrino Experiment, or MiniBooNE. Her idea was to send beams of neutrinos down a 500-meter tunnel into a huge spherical tank, about 12 meters in diameter, lined with 1,200 light sensors and filled with 800 tons of mineral oil. Though they rarely interact with other matter, neutrinos that did happen to collide with a carbon atom in the mineral oil would leave behind energy traces, making it possible to identify their flavor. Neutrinos are actually known to oscillate from one flavor to another as they move through space, but as long as they oscillate between the three standard flavors, the total number should remain stable. A dip in the total would suggest that some have transformed into sterile neutrinos, which are even more difficult to detect; an increase in the total would suggest that some sterile neutrinos have transformed into the other flavors. By counting the type and number of neutrinos MiniBooNE detected at high energies, Conrad and her team could look for signs of any weird, unpredicted excesses, in line with LSND’s results at low energies.
Conrad secured funding to build the detector at Fermilab, where the Booster—a particle accelerator with a 474-meter circumference—would produce the neutrinos to be analyzed. In 2007, she and an expanding MiniBooNE collaboration reported their first results: there did not appear to be an excess of electron neutrinos, at least in the high energy ranges that the LSND results at low energy ranges predicted. These initial results seemed to refute the existence of a fourth, sterile neutrino. But MiniBooNE did record a mysterious excess of electron neutrinos at lower energies—a finding that the researchers could not explain.
The hunt for the sterile neutrino is far from over, since multiple experiments have turned up conflicting results. One source of those results is the IceCube Neutrino Observatory, based at the Amundsen-Scott South Pole Station. IceCube is made up of more than 5,000 light sensors, hung from vertical strings that stretch down more than 2,450 meters into the Antarctic ice. The detector is designed to pick up traces of neutrinos originating not from accelerators on Earth but from extreme sources in the cosmos, such as the cores of exploding stars and the centers of active galaxies. When they pass through ice, they produce muons, electrically charged secondary particles that emit light. Analyzing the light picked up by IceCube’s sensors lets scientists count the neutrinos and determine the angle at which they pass through the ice. Conrad is among 300 scientists who are looking for signs of sterile neutrinos, along with other neutrino-related phenomena, in the particles streaming through the detector. (In September 2017, for example, they traced a high-energy cosmic neutrino to its source, a blazar about 3.7 billion light-years away.)
In 2016, IceCube’s search for the sterile neutrino came up empty: scientists had found no sign of the particle among 100,000 neutrino events picked up by the detector. They concluded with 99 percent certainty that the particle does not exist in the range they could explore.
And yet there’s still a chance that it is out there. In June, Conrad and her MiniBooNE colleagues announced that the experiment had again detected an excess of electron neutrinos in the low energy range, and this time it was clear the results were not a statistical fluke but a likely sign of something beyond the three main neutrino flavors.
“With the suggestion of evidence for sterile neutrinos, she’s lobbed a bombshell into the subject, and now we’ll have to see whether it explodes,” says Wilczek. “If her indication holds up, it will shake up some of our ideas about how to achieve a unified theory of the fundamental forces. It will mean that we’re not as close to solving the problem as some of us think we are.”
“It’s not the sterile neutrino we were looking for, but it may be one anyway,” Conrad says of the new results. “And it’s a clear sign that wow, this is something we do not understand, which is a fun, frustrating place to be.”
Conrad and her group are at the head of a widening hunt. “We were really out there in the farthest reaches of the frontier,” she says. “Nobody cared about us; we were in our own corner. And as we and other experiments have taken more data, and there are more signals that look like there could be an extra neutrino, people have gotten a lot more interested.”
Rethinking particle accelerators
Shortly after MiniBooNE reported its first results in 2007, Conrad left Columbia University and joined the MIT faculty.
“Changing jobs now and then is not a bad thing, but that’s not usually what tenured faculty do,” she says. “But part of the reason people change jobs is because it gives them new creative insights. And that’s really what happened to me.”
In MIT’s physics department, Conrad found a hive of theoretical and experimental ideas. And once she moved into her office in the main corridor of MIT’s Laboratory for Nuclear Science, she began developing a room-size particle accelerator.
Neutrino experiments had always required enormous accelerators to zing protons near the speed of light, at which point the particles might produce enough neutrinos for detectors like MiniBooNE to analyze. “Neutrino experiments and particle physics in general have basically been getting bigger and bigger,” Conrad says. “We take the same technology we already have, and we just keep expanding and multiplying it.”
Instead of going further in that direction, Conrad decided to look for ways to build a particle accelerator just as powerful as those that spanned several kilometers in a fraction of the space. Smaller accelerators, she reasoned, could be built inexpensively and placed near any large neutrino detector, whether in the middle of a prairie, such as at Fermilab, or deep beneath the mountains.
A design began to take shape after she attended a talk on cyclotrons—room-size apparatuses that fling charged particles out from their center via a magnetic field and accelerate them along radio frequency waves, much as surfers ride ocean waves.
“It’s a true MIT story in the sense that I was sitting here working away, had too much to do, and someone said, ‘Do you want to go over to hear about this new cyclotron?’’ Conrad recalls. “So I went, and was sitting there in the talk, and I was like, ‘That’s the accelerator of my dreams.’”
Since the 1930s, cyclotrons have been used to produce proton beams for experiments in nuclear physics. But the number of protons they could accelerate was limited, and as larger, more powerful accelerators came on the scene, cyclotrons were repurposed to spin up proton beams aimed at killing cancerous tumors. Conrad looked for ways to increase the number of protons that a cyclotron can accelerate, and she found a solution in the hydrogen molecule ion H2+, which is made from two protons held together by an electron. If these hydrogen molecules were pumped into a cyclotron, their electrons would essentially fly off, leaving two protons for every molecule—meaning twice as many could be available to produce neutrinos and other exotic particles.
She and her students are currently building a cyclotron accelerator at MIT, which she’s dubbed IsoDAR, for “isotope decay at rest,” the process by which the cyclotron’s protons would decay into neutrinos. Once constructed—ideally by 2022, if all goes well—the mini-accelerator is expected to fit into an area the size of a spacious living room. She also hopes to build a slightly larger, more powerful version, Daedalus, which would still be a fraction as big as current neutrino-generating accelerators—a size that could easily fit within the MIT dome. If these small accelerators were placed next to some of the world’s most sensitive detectors, Conrad believes, they could greatly advance the search for the sterile neutrino.
“They can go an order of magnitude further in exploring the space for sterile neutrinos, compared with any other experiment today,” she says. “To be able to do that, we’re trying to think very differently.”
An independent streak
Conrad will be the first to admit that she is not your average particle physicist. Especially early in her career, she would find herself one of few women in attendance at seminars and conferences.
“I would not be in this field if I were not okay being the only woman in some places,” she says. “There’s a certain advantage in being a woman in the sense that you’re obviously different already. You’re not the Standard Model physicist.”
This comes across especially in Conrad’s talks at meetings and conferences, where she often will gleefully attribute personalities to certain elementary particles, comparing quarks—which can be so loud as to obscure any other particle signal—to “mean girls” and the quiet, ever-present neutrino to the “girl next door.”
Lindley Winslow, an assistant professor of physics at MIT, remembers seeing Conrad speak for the first time, when she attended an annual physics meeting as a college junior. “It was really inspiring to see,” Winslow recalls. “Not only was it a woman giving a talk, but she was also being free to be cute. And she was having fun with it, and was totally doing it very much as a woman, not as a woman pretending to be a man.”
Conrad eventually recruited Winslow as her first postdoc at MIT and has worked hard to attract other women to MIT’s physics department, both at the graduate and faculty levels. Women made up just 13.7 percent of the Institute’s physics graduate students in 2007, but as head of the department’s admissions process, Conrad has helped bump that up.
“I’m incredibly proud of the fact that we’re 23 percent women in this next class,” she says. “My great hope is that we’ll get up to 33 percent.”
At the faculty level, Conrad sees a bigger challenge. MIT’s physics department—one of the largest in the country—has just 12 women on its faculty of about 100, four of whom joined just in the last few years.
“We’ve gone from eight to 12—that’s a large fractional increase,” she says. “But honestly, this is pretty far behind [other physics programs].” So she’s working to boost that number and regularly checks in with the women on the physics faculty now, who she says “are willing to go out and try things, and are not afraid to fall and pick themselves up again.”
This independent streak is something Conrad looks for in the students who want to join her research group. That’s partly because she makes it a point to send her students directly to the sites where neutrino experiments are located, such as Fermilab, where scientists operate MiniBooNE and MicroBooNE, and Madison, Wisconsin, where they receive and analyze IceCube data.
“If they’re in their little island here, they may be doing great work, but people don’t really know,” Conrad says. “It’s very important to be there and be central and be one of the key people who are very visible. And it gives you a much broader view of the world.”
Conrad tries herself to be present as much as possible at her experiments, particularly at Fermilab, where she travels so often that she and her husband have kept a second house in Illinois since Conrad was a graduate student. “I go there as often as I can, and Skype my group all the time,” she says. “They’ll get a thousand e-mails from me over one weekend, because my habit is to put in a thought—then the thought develops after an hour and I’ll send another e-mail. So they’ll come back and be like, ‘Omigod.’”
Indeed, Conrad’s life seems to be taken up with physics, and happily so. Although she likes gardening, and used to help her father raise champion dahlias, these days she sticks to growing low-maintenance daisies. “I don’t have the time to make a dahlia look the way I want it to,” she says, “and I don’t want shabby dahlias.”
For Conrad, physics is not just her job, but her hobby—all-consuming and, above all, fun. So she continues to lead the hunt for neutrino anomalies, and to rally her colleagues to join the pursuit. Having built the case that it’s possible all these hints line up, says Winslow, Conrad has convinced the community that it’s vital to continue the search to either confirm or refute the existence of sterile neutrinos. “With anyone who loves something this much, you don’t want to get in the way,” Winslow says. “We need to go out and look.
How to see a neutrino
In the 1960s, researchers developed bubble chambers to study the elusive neutrino. When a neutrino collides with a nucleus, it produces charged particles. If this happens inside a bubble chamber, which is filled with pressurized liquid, the charged particles leave a trail of freed electrons as they travel through the liquid. As liquid vaporizes around those electrons, microscopic bubbles form, documenting the collision site and the particles’ paths. Releasing the chamber’s pressure allows the bubbles to expand until they are large enough to be photographed. Although such photos are beautifully detailed, the process of capturing them is time- and labor-intensive.
Janet Conrad and several colleagues had an idea for building a detector that could collect digital records of neutrinos with similar precision but much greater efficiency. She became a founding member of a group of collaborators that designed and built the 170-ton MicroBooNE neutrino detector, which began recording neutrinos generated by Fermilab’s Booster accelerator in 2015. When neutrinos enter MicroBooNE’s high-voltage field cage, which is filled with liquid argon, they interact with the argon, creating charged particles. As these charged particles move through the detector, they free electrons in the argon, producing an ionization trail. The charged particles also excite argon, producing light.
The freed electrons drift to gold-coated wires installed on the low-voltage side of the field cage. When extremely sensitive light detectors known as cryogenic photomultiplier tubes detect the accompanying light—indicating that the electrons are drifting to the wires—each electron’s charge is recorded. That data can be used to reconstruct a 3-D image of the neutrino’s path.
MicroBooNE can record a million times as many neutrino events as a bubble chamber in the same amount of time. And deep learning can be used to analyze the vast number of digital images it produces. When you’re searching for evidence of a never-before-seen variety of an already elusive particle, the ability to collect and analyze huge volumes of data is critical.
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