Alexandra Day

Alexandra is a rising senior at Wellesley College, where she is majoring in physics and minoring in math. She is a Fellow in the Madeleine Korbel Albright Institute for Global Affairs and is President of Wellesley's Society of Physics Students. She will be working in the Office of International Relations at the European Center for Nuclear Research (CERN) this summer and is excited to be working on the DAEdALUS neutrino experiment with Professor Conrad in the fall!

In an abandoned gold mine nearly a mile below the surface of Lead, South Dakota, scientists Ray Davis and John Bahcall were poised to make history. The year was 1964, and they were searching for solar neutrinos, elusive particles that were expected to come from the Sun but which had never been observed. To increase their chances of seeing solar neutrinos, Davis and Bahcall constructed a massive yet extraordinarily sensitive neutrino detector in this underground cavern. Their results showed that the Sun indeed makes neutrinos, but the data raised a deeper mystery: the solar neutrino problem.

As the name suggests, solar neutrinos are produced in the Sun and travel outwards through the solar system. Physicists expected neutrinos to come from the Sun as a byproduct of solar core nuclear fusion, the basic process all stars use to generate energy. Detecting neutrinos would provide direct evidence of this process, and would further probe the details of the fusion mechanisms at work. With this goal in mind, Davis and Bahcall began designing their experiment. Because neutrinos virtually never interact with matter, the two physicists needed to distinguish these interactions from other signals from unwanted stray particles—particles that are showering the planet constantly—that the detector might pick up.

To minimize these false signals, the physicists placed their detector 4,850 feet underground and surrounded it with a 300,000-gallon tank of water to prevent non-neutrino particles from entering the detector. The detector itself was a 100,000-gallon tank of a special chemical compound designed to change in response to neutrino interactions. By examining these changes, scientists could analyze the events and determine if they had observed a neutrino interaction. Yet even with such a large detector, Bahcall's model predicted that the experiment would only detect one interaction per day out of the hundreds of trillions of solar neutrinos flying through the Earth.

When the scientists first analyzed the experimental results, they saw something puzzling. Instead of seeing one interaction per day, Davis' apparatus only detected about one interaction every three days. So while the experiment found the solar neutrinos that supported the nuclear fusion hypothesis, the inconsistency between the prediction and the observation was troubling. Despite significant discussion among scientists, no one could explain the anomaly, which became known as the Solar Neutrino Problem. In short, Davis and Bahcall's experiment wasn't remarkable because of what it found. It was remarkable because of what it didn't find.

In the decades following the Davis-Bahcall experiment, many other experiments confirmed their results. The number of observed solar neutrinos was consistently lower than expected. What was going on? It turns out that there are several types, or flavors, of neutrinos. The Sun's nuclear fusion process only produces electron neutrinos, and the Davis-Bahcall experiment was designed to detect only this specific flavor. However, subsequent experiments inferred other flavors of solar neutrinos, called muon neutrinos and tau neutrinos. Where did these extra flavors come from? Physicists developed a strange answer: those muon neutrinos and tau neutrinos originated in the Sun as electron neutrinos, and then—somewhere on their 93-million mile journey to Earth—they changed flavor. Thus the elusive, ghostlike neutrino gained another fantastic property: the ability to shape-shift.

Particle physicists refer to this transformation as "flavor oscillation," and it has huge implications for our understanding of neutrinos. Most importantly, flavor oscillation requires that neutrinos have mass. If neutrinos can change flavor over a distance, then they must have some sense of time. According to Einstein's theory of special relativity, that's impossible for a massless particle, which moves at the speed of light and observes everything as happening at the same time. Changes with time mean neutrinos must have mass, which challenges the long-held Standard Model hypothesis that neutrinos have zero mass. An added complication is that no one has yet been able to measure a non-zero neutrino mass. Understanding the impact of this discovery is the task of today's particle physicists.

André de Gouvêa of Northwestern University wants to understand how neutrinos acquire mass in the first place. He has spent considerable time working on a model to explain the extraordinary lightness of neutrinos. It's called the Seesaw Model, and it posits that the ultra-light neutrinos have heavier partners and their masses are related—when one goes up, the other goes down. This model pairs ordinary neutrinos, often called left-handed, which means they spin counterclockwise as they travel, with a different kind of neutrino, often called right-handed, that spins clockwise. There's just one problem—no one has seen a right-handed neutrino.

Many theorists interpret the theory to mean that right-handed neutrinos must be incredibly heavy. They ought to be so heavy, in fact, that any accelerator we could build would not have enough energy to produce them. According to de Gouvêa, this isn't an encouraging state of affairs. "If you have a good idea that you can't test experimentally," he explains, "then it's just an idea; it's not a theory." To help remedy this situation, he has worked on developing a different interpretation of the seesaw model — one that can be experimentally verified.

Although right-handed neutrinos are often assumed to be very heavy, de Gouvêa points out that there's no reason why this has to be true. In fact, right-handed neutrinos might be very light, even lighter than the ultra-light left-handed neutrinos. If this were the case, then it would drastically change our understanding of neutrino masses and it might make it easier for us to observe them. de Gouvêa is currently working with experimental physicists to develop this theory in the hope that that they will soon be able to observe right-handed neutrinos.

Regardless of how neutrino mass originates, its very existence violates the Standard Model. Although this seems discouraging, de Gouvêa points out that this is actually a good thing. "We're looking for physics beyond the Standard Model," he says. "There are very few things that this model fails to explain. And one of them is neutrino masses." If we can understand neutrino mass, he says, then we might be able to create a new framework of knowledge to replace the Standard Model. "When we build a new one," he says of theories, "It doesn't mean that the old one is wrong or bad. It just means that the old one is applicable for a certain set of circumstances." The Standard Model could still be used to describe many phenomena—just not neutrino masses.

Like their colleagues in the experimental realm, theoretical particle physicists are working to interpret the newest experimental data and create new frameworks of knowledge. Updating or replacing the Standard Model would be an incredibly significant event in particle physics and physics in general, and it would ultimately lead to deeper understanding and insight about the world in which we live. "By understanding these phenomena better," de Gouvêa explains, "we will end up in a much better place than we are today."