The most powerful force in the study of physics is not gravity or electromagnetism.
It is not love, or courage.
It is accounting.
“But that doesn’t quite add up” has produced heartache and exhilaration in equal measure, and the tantalization created by blank space in the universe’s checkbook has pushed few areas of science to such heights as that of neutrino research, starting with Wolfgang Pauli’s desperate invention of the neutrino in 1930 to balance the books of Beta Decay.
Physics was in a desperate state that year. Try as they might, scientists couldn’t get the energy of beta decay reactions to come out quite right. Puzzled at the continuous spectrum of energies of the electrons produced when a neutron changes into a proton, even scientific titans like Niels Bohr were moved to hypothesize that maybe, after all, Conservation of Energy, one of the bedrocks of modern science, was not a thing. Frantic to save Conservation, Pauli hypothesized that the energy was carried away by an undetectable neutral particle which he called the neutron and which we now know as the neutrino.
It was an inspired bit of madness from one of science’s most original minds, but the fact that this particle would be all but impossible to detect raised a number of eyebrows. Pauli himself bet a magnum of champagne against his mystery particle ever being detected. It seemed a safe bet – neutrinos overwhelmingly ignore the matter they pass through. Right now, you’ve trillions of them passing through your body, but if you live to be a hundred, you’d be lucky if one of them interacts with your atoms.
Neutrinos are profoundly indifferent as well to the electromagnetic force, so all the fancy magnetic field tricks that we used to investigate the nature of, say, the proton or the electron are more or less useless. The only thing that a neutrino responds to is the weak force governing Beta Decay, and so the only way to detect one, it seemed, would be to build a device that could register the after-effects of a rare neutrino-atom interaction. These devices are typically massive, using tons upon tons of purified liquid or solid in the hope of capturing a few measurable neutrino interactions, and involve engineering and chemical innovation at the highest level, which is why it took a full quarter century before the first neutrinos were actually detected, by Reines and Cowan in 1956.
That discovery kicked off a mad dash of neutrino hunting with results that continue to be astonishing and deeply troubling. Until the 1970s, the leading figures in the hunt for neutrinos and their properties were overwhelmingly men: Ettore Majorana, who theorized that the neutrino might be its own antiparticle, Bruno Pontecorvo, who predicted the strange phenomenon of neutrino oscillations, and John Bahcall, whose model of the sun predicted a neutrino output that, when combined with Ray Davis’s exquisite measuring of solar neutrino emissions, produced a puzzle that motivated a generation to search for and confirm Pontecorvo’s oscillation hypothesis.
Physics has famously the worst record for gender parity of the modern sciences. Less than ten percent of full professorships in physics departments are filled by women, and so it would come as no surprise had neutrino physics continued on its path of male-dominated research to the present day. And yet, it didn’t. In the early 1970s, the tip of the gender wedge entered neutrino studies in the form of Linda Stutte, who came to Fermilab in 1972, and Wyatt Merritt, who arrived in 1973. They were both adept at writing the crucial analytic software needed to make sense of neutrino detector data, while Stutte was also a crack hand at tuning Neutrino Area beams and played a role in constructing the SELEX project’s Cerenkov light detector, which is one of the major tools scientists have in detecting neutrino events.
Stutte and Merritt paved the way for the arrival of Heidi Schellman and Regina Rameika in the early Eighties. Schellman, originally a programmer for the Stanford Linear Accelerator, started at Fermilab on the CCFR (Chicago-Columbia-Fermilab-Rochester) experiment where Merritt was a postdoc. One of only four women on the project, which observed neutrino scattering off the quarks of a proton to learn more about proton structure, Schellman used her experience to advise the coming waves of women neutrino researchers. When she went to the E665 muon scattering experiment, she showed Janet Conrad how to strip equipment from CCFR to make new devices, a talent which would stand them both in good stead when they cannibalized E665 in turn to create components for their new project, Fermilab’s NuTeV, which studied what neutrinos might reveal about physics beyond the reigning Standard Model.
Meanwhile, Rameika, after a start measuring hyperon magnetic moments, switched to neutrino research in 1993, and contributed to the first direct observation of a tau neutrino interaction during her time at DONUT (Direct Observation of the Nu Tau – one gets the impression, glancing at neutrino studies, that new projects are proposed just to have a chance at inventing whimsical new acronyms – I’m looking at you, SNIF). From there, she became a project manager at MicroBooNE, which developed the liquid argon detection methods that will be employed in DUNE (Deep Underground Neutrino Experiment), a massive collaboration of a thousand neutrino physicists that aims at settling some of the deep lingering mysteries of the neutrino.
Let’s take a break from people to talk about those mysteries a moment. The first of those is definitely the phenomenon of neutrino oscillation. There are three types of neutrino (maybe four, depending on whom you ask): electron, muon, and tau. Early detectors only could pick up electron neutrinos, but for some reason the devices were constantly only registering a third of the expected neutrino yield. The solution to that puzzle came from an old suggestion of Pontecorvo, that, in transit, neutrinos phase back and forth between their different flavors, so what left the sun as an electron neutrino might be a muon neutrino by the time it hits the Earth. This behavior in turn calls for the neutrino to possess mass, but if it does, that contradicts some assumptions of the Standard Model of particle physics which has been otherwise remarkably accurate in describing the universe.
Not only that, but, unlike every other particle we know about, there’s a good chance that neutrinos are their own anti-particle. And not only not only that, but neutrinos might disturb charge-parity symmetry, a principle of physics that says that, if you replace the matter of an experiment with anti-matter, the physics should work out the same. That might not be the case for neutrinos, and if they do violate CP symmetry, that difference in behavior could be the explanation for why we have more matter in the universe than anti-matter.
There are plenty of mysteries to investigate, and in the 1990s women took up an astonishing number of key positions in that effort. After her time at NuTeV with Schellman, Janet Conrad, who won the 1999 Presidential Early Career award in recognition of her promise, went on to become co-spokesperson for MiniBoone, which used a 40 foot sphere of baby oil to plumb the mystery of neutrino oscillations. Muon neutrinos, when they hit carbon atoms in the baby oil, produce muons and Cerenkov light, but electron neutrinos produce electrons and a different pattern of Cerenkov light. By studying the different light patterns produced in her oil vat, Conrad was able to investigate the particulars of muon neutrinos transforming into electron neutrinos.
Jenny Thomas studied Neutrinoless Double Beta Decay at NEMO-III and SuperNEMO, a phenomenon that, if observed, would provide evidence that neutrinos violate CP symmetry, before becoming co-spokesperson for MINOS (about which more later) and heading the CHIPS project, which uses detectors submerged in water-filled abandoned mines as a low-cost but equally effective alternative to larger scale projects.
Stepping from the sub-terrestrial to the galactic, Kate Scholberg started as a hunter of neutrinos produced during supernova events and is carrying that knowledge to DUNE, where she is working to ensure that the detector is able to detect low-energy neutrinos from stellar explosions. She also is the coordinator of SNEWS, which is a massive worldwide automated network of neutrino detector systems geared towards collecting neutrino data from supernovas, which would give us deep insights into the particulars of how stars die. And if that’s not enough, she also is the spokesperson for and originator of COHERENT, which looks at how low-energy neutrinos interact not just with individual nucleons, but with entire nuclei, another small-scale effort (only about 60 collaborators) that is tackling a thoroughly unique phenomenon. (She was, in addition, part of the Super-K team that used atmospheric neutrinos to provide the first evidence of neutrino oscillations, an accomplishment that won last year’s Nobel Prize – yeah).
Physics has famously the worst record for gender parity of the modern sciences. Less than ten percent of full professorships in physics departments are filled by women.
Debbie Harris, after working in beamline monitoring for NuMI, the neutrino beam used by MINERvA, a project that measures how neutrinos interact with different nuclei and whether low-energy neutrinos interact with matter, was promoted to project manager of MINERvA and then Co-Spokesperson, a position she’s held for the last seven years. And recently, with the next woman on our parade of awesome people, she established the Neutrino Physics Center at Fermilab to coordinate neutrino scientists’ efforts.
Bonnie Fleming, who began at Fermilab in 1997, worked on MicroBooNE with Rameika. After MicroBooNE, she was nominated as the deputy chief research officer for DUNE, taking a leading role in the next generation’s largest-yet neutrino experiment. MicroBooNE also produced Sam Zeller, who won the 2012 Department of Energy Early Research Award and is using it to develop multi-kiloton liquid argon neutrino detection facilities that will be needed at the DUNE project
And then there’s NOvA, where Patricia Vahle, Mayly Sanchez, and Kanika Sachdev have smashed protons into graphite targets to produce pions that decay into muon neutrinos, measuring how those neutrinos interact with liquid scintillator detectors. They are working on the ranking of neutrino masses, and on whether neutrino and antineutrino oscillation follow the same pattern. If they don’t, that’s evidence for CP violation, and thus an answer to the basic question of Why There Is Something Versus Nothing in the universe. Vahle also works in the phenomenon of muon antineutrino disappearance, while Sanchez is Co-Spokesperson for ANNIE, a project that counts the neutrons that emerge when neutrinos interact with water, and Sachdev is improving liquid argon detection and measuring how neutrinos react with detectors made of different materials.
Meanwhile, Jen Raaf is taking argon detection to the next level, looking at how liquid argon detectors might be replaced by high-pressure gaseous Argon detectors. In liquids, as neutrinos react with atoms, they create a distinctive trail of electrons that travel to detectors. But traveling through a liquid can be slow, meaning that nearly simultaneous neutrino events can get muddled. Raaf hopes to use gaseous Argon to improve electron travel time and make for a crisper distinction between events.
The overlap between projects and people in neutrino physics is dizzying. Schellman worked at MINERvA with Harris, at NuTeV with Harris, Conrad, Zeller, and Fleming, at CCFR with Merritt, and is currently at DUNE with Everybody. Vahle, Harris, Thomas, and Rameika were all at MINOS (Main Injector Neutrino Oscillation Search), which aimed at making precise measurements of the “mixing angle” which dictates the course of neutrino oscillation by measuring muon neutrino disappearance and electron neutrino appearance rates.
Wherever you turn in neutrino studies, it seems there are three or four women there doing high profile work. My list of women researchers I reached out to for this piece included two dozen prominent names. And yet, some of the scientists I talked to point out, that prominence might be deceptive. Yes, there are a number of big names in the field, but according to Kanika Sachdev, an informal survey of NOvA and DUNE yields only about 14% women, above the national average for physics, but hardly a banner figure.
At the same time, there is concern about the direction of the field generally, about the move towards massive collective projects instead of creative small team efforts. As Jenny Thomas notes, “A new beam and planned detector are so expensive that they have to attract as many people as possible to justify the costs. There will be 1000 people on the DUNE experiment, so I wonder how that will be when the data needs analyzing. I don’t think there are enough measurements to do in neutrino physics to support that many people. Furthermore, the data collection rate is so slow, an experiment will need to run for a decade before it can match the data already collected by all the other experiments. There is no money for people to do smaller experiments, so it’s either DUNE or LHC. Those are now the options and it will crush creativity in our field.”
Those are important and sobering counterpoints to the impression that gender parity in physics is just around the corner, and that the only cost of big team science is a financial one. We should take a minute to wonder what more can be done to bring women into physics, and another to be thankful that we not only have magnificent large scale projects like DUNE, but innovative individual efforts like Thomas’s CHIPS and Scholberg’s COHERENT, driving forward investigation at multiple scales.
But… after having dragged myself and you over the course of this column through all the indignities inflicted upon Mileva Maric-Einstein, the early termination of Harriet Brooks’s promising career, and the institutional conspiracy against Lise Meitner’s work, I find it heartening to look at a burgeoning new field and see, within the space of four decades, a roster of innovative women scientists spring up that includes people like Heidi Schellman, Linda Stutte, Wyatt Merritt, Janet Conrad, Debbie Harris, Jenny Thomas, Jen Raaf, Kate Scholberg, Donna Naples, Regina Rameika, Patricia Vahle, Sam Zeller, Bonnie Fleming, Mayly Sanchez, and Kanika Sachdev. Things are not yet where they ought to be, but for the science-prone daughters of this generation, looking for heroes in whose footsteps they might follow, the world is a bit less lonely. Let’s just hope this crop of talent doesn’t solve all the problems in neutrino studies before those daughters get there.
Lead image by Reidar Hahn: Jen Raff early career award winner at Meson Test Beam Facility with LArIAT Detector. Via U.S. Department of Energy, Fermilab; public domain.
FURTHER READING: Ray Jayawardhana’s Neutrino Hunters (2013) is a relatively recent, brisk, and enjoyable romp through the history of the neutrino and the people who seek to unmask it, which serves as a good primer. Fair warning, though, that our only heroes to make an appearance are Kate Scholberg and Janet Conrad. Once you’ve finished that, head over to Fermilab where they’ve got nice pages set up for each of the experiments they’re running and links to more information.