The Hunt for a New Neutrino

Physicists are hot on the trail of a new fundamental particle, whose discovery would not only revolutionize particle physics and require major revisions to current theories, but might also help resolve astrophysical mysteries.

Janet Conrad, Massachusetts Institute of Technology

Photo credit: Daniel Winklehner

Neutrinos are perhaps the most exotic particles known to science. They are everywhere—an estimated billion of them fill every cubic meter of space, and trillions more are emitted by our sun every second—but you can’t see them. Neutrinos are a form of matter, but they interact with normal matter so rarely and so weakly that it’s hard to detect them. Scientists have known how to produce neutrinos in reactors and accelerators for more than 50 years. And yet, even after multiple discoveries and three Nobel Prizes, scientists still know very little about them. But these elusive little wisps of matter may hold the key to the next major advance in our understanding of basic physics and the nature of the universe.

Neutrinos come in three types or flavors. And over the past two decades, scientists have discovered an effect called neutrino oscillations, where neutrinos morph from one flavor to another. This is a quantum mechanical effect that can only occur if neutrinos have mass, in contradiction to the current theories that describe fundamental particles and their properties, known as the Standard Model. Maybe the model can still be patched up—this is yet to be seen. But the discovery was a clear hint that understanding neutrinos may require new physics.

The surprises in neutrino physics have not stopped with neutrino mass. More recent experiments have detected additional anomalies in neutrino oscillations. The hints of unexplained neutrino behavior come from several directions, and are reasonably consistent. The first are accelerator based experiments that search for oscillations from one flavor of neutrino to another. The second are reactor-based experiments that search for one flavor of neutrinos to oscillate away—so called “disappearance” experiments. The last are beta-decay sources that also show neutrino disappearance. Over the last decade, as oscillations have gone from speculation to established fact, evidence has accumulated that cannot be explained by simple extensions of current theory.

The discovery of a new fundamental particle—just when physicists thought they had finished cataloging such things—would completely revolutionize particle physics and require significant revisions to current theories.

One solution that might explain these anomalies would be the existence of neutrinos that simply don’t interact with normal matter at all—so-called “sterile” neutrinos—with the suggestion that neutrinos can oscillate both between different types and between regular and sterile forms. That would be startling enough.

But even more recent experiments suggest still additional anomalies: they seem to show that neutrinos are changing flavor with a completely different (higher) frequency than has been observed before. If this result is confirmed, it would mean that an additional fundamental particle, in the form of a new and more massive neutrino or set of neutrinos, is causing these oscillations.

It’s important to note that the newly observed anomalies or unexpected oscillations of neutrinos, although they have been observed in several ways with several types of experiments, are not yet definitive. That’s not surprising, because it requires either a much more intense source of neutrinos, or a very long observing period (many years), as well as an ideal physical setup of the neutrino source and a detector, gauged to match the character or frequency of the expected oscillations.

Just such a definitive experiment is what we propose to do. Our experiment has several unique elements:

  • A re-designed, very compact accelerator utilizing state-of-the-art technology that will be a very strong source of neutrinos, thus shortening observing time;
  • Placement of the neutrino source immediately next to a massive 1,000 ton detector, with the precise geometry—a travel distance from neutrino source to target of about 16 meters—to observe the anomalous oscillations that would signal a new neutrino particle;
  • An international collaboration of scientists from Europe, the U.S. and Japan that will enable use of an existing neutrino detector, called KamLAND, in a deep underground laboratory in Japan, thus saving the time and expense of constructing such a facility from scratch.

The discovery of a new fundamental particle—just when, with the recent discovery of the Higgs particle, physicists thought they had finished cataloging such things—would completely revolutionize particle physics and require significant revisions to current theories. A new neutrino might also help solve astrophysical mysteries, providing a possible candidate for the dark matter that comprises most of the universe: we can’t see dark matter, but can observe its gravitational effects on clusters of galaxies. Some theorists think that a new neutrino could also help to explain other fundamental mysteries, such as why there is such an imbalance in the presence of matter and anti-matter particles (antimatter particles are relatively rare). In short, a new neutrino, if it exists, would create all sorts of excitement and open new research directions in basic physics and astrophysics.


Photo: A beam of H2+ particles (two protons and an electron) in a vacuum chamber at an MIT test facility. The beam excites a small amount of air bled into the chamber, producing a glow that shows the presence of the particle beam. Deep underground in Japan, this beam will be used to generate an intense source of neutrinos, whose behavior can then be measured precisely.