It’s been a banner year for neutrinos. Last month two physicists won the Nobel Prize for determining that the elusive fundamental particles can switch between three types, or flavors. Now the same finding has netted its discoverers the 2015 Breakthrough Prize in Fundamental Physics—a $3-million award launched three years ago by billionaire venture capitalist Yuri Milner. There is one key difference between the two honors, though: the Breakthrough Prize will be split among 1,370 physicists.
Seven leaders of five experiments as well as all of the co-authors of the scientific papers reporting the experiments’ groundbreaking results will become Breakthrough laureates. The five teams will share the prize money ($600,000 to each), with two thirds of those purses going to the leaders and one third to the collaborators. The huge pool of winners is a record for the Breakthrough Prizes. (The largest group until now was the 51 scientists who shared the award last year for the discovery that the universe is accelerating.) This is also a huge departure from the way science prizes are traditionally awarded—each of the science Nobels is famously limited to three laureates. “We think it’s important to recognize that an awful lot of hard work by a lot of people goes into these experiments,” says physicist Edward Witten, chair of the Breakthrough physics prize Selection Committee and one of the inaugural Breakthrough Prize winners. “We thought it was important to symbolically include all the participants who were involved.”
The contrast between the Breakthroughs and Nobels is especially stark given that both prizes this year celebrate the same achievement—a circumstance that is purely coincidental, Breakthrough officials say. The Breakthrough laureates were notified this summer, well before the Nobel announcement was made. “It’s a very good feeling” to have gained recognition from both committees this year, says Arthur McDonald of Queens University in Ontario, who won the Nobel and the Breakthrough for his work at the Sudbury Neutrino Observatory (SNO) in Canada. “We didn't set out to win prizes; we set out to do solid physics, but these prizes are an indication that the scientific community regards our results as being really significant.” Takaaki Kajita at the University of Tokyo, who shared the physics Nobel with McDonald this year and will also receive the Breakthrough Prize, says the awards are a great honor. “I'm really glad in particular for the Breakthrough Prize for the recognition of the whole collaboration.”
SNO and Kajita’s experiment, Super-Kamiokande (Super-K) in Japan, were the first projects to find that neutrinos of one flavor can change, or “oscillate,” into another. In 1998 Super-Kamiokande used a 50,000–metric ton tank of very pure water buried a kilometer underground to catch neutrinos born in interactions between cosmic rays (high-energy charged particles from space) and atoms in Earth’s atmosphere. Neutrinos are difficult to pin down—they tend to fly right through matter, and even across Earth and our bodies, without hitting anything. Occasionally, one will collide with an atom of water in the detector, releasing a bright ring of light that Super-K can identify. The experiment looked for muon neutrinos, the predominant flavor that results from cosmic-ray interactions, coming from two directions—the sky above and the ground beneath the experiment. (The latter would have been created in the atmosphere on the other side of the planet and then traveled through Earth to reach Super-K.) The researchers found more neutrinos coming from above than below, and concluded that the muon neutrinos originating on the other side of the planet, which had to cover a longer distance to reach Super-K, had more time to turn into the other two flavors, electron and tau.
A few years later SNO observed a similar phenomenon in neutrinos coming from the sun. The fusion reactions in our star’s core produce large numbers of electron-flavored neutrinos. SNO saw fewer electron neutrinos than expected, however, and more muon and tau neutrinos, in its detector made of heavy water (in which the isotope deuterium replaces regular hydrogen). Again it seemed that neutrinos were swapping identities on their way from the sun to Earth. More experiments followed that found further evidence of neutrino oscillations: the Kamioka Liquid Scintillator Antineutrino Detector (KamLAND), along with the KEK to Kamioka (K2K) and Tokai to Kamioka (T2K) Long-Baseline Neutrino Oscillation experiments in Japan as well as the Daya Bay Reactor Neutrino Experiment in China. The leaders and teams behind all these projects are also sharing this year’s Breakthrough Prize.
The revelation that neutrinos can change flavor was not just odd—it also contradicted the long-held notion that they have no mass. For them to oscillate, they must have different, and thus nonzero, masses. The discovery contradicted the reigning theory of particle physics, the Standard Model, which has no explanation for how neutrinos could have gotten their mass. “Our world can be explained very well by the Standard Model, but that model assumes neutrinos are massless,” says Yoichiro Suzuki of the University of Tokyo, co-leader of Super-Kamiokande. “This means the current Standard Model must be expanded.”
Neutrino oscillations thus opened up a whole new field of research to look into just how much mass neutrinos have and why. In the decade and a half that has followed the original experiments, scientists have analyzed neutrino oscillations more and more precisely, allowing them to place upper limits on the total mass of neutrinos. Physicists can now say that two of the neutrino masses are similar, and that the third is either significantly larger or significantly smaller. Determining which of these options is the case should help point to an explanation for how they got their mass in the first place—a mystery whose solution may involve processes physicists have never seen before. Several current and upcoming experiments take aim at just these problems. “These days neutrinos are very hot,” says Wang Yifang of the Institute of High Energy Physics in Beijing, a co-leader of the Daya Bay experiment. “We are at a very special moment and we are extremely lucky to be able to participate in this great advancement of science.”