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The Sudbury Neutrino Observatory

The Sudbury Neutrino Observatory studied neutrinos from the sun, which do not actually undergo neutrino oscillations.

Courtesy of Lawrence Berkeley National Laboratory

Did the Nobel committee get the physics wrong?

In an unusual paper, a leading theoretical physicist says that the citation for the 2015 Nobel Prize in Physics is wrong. The two winners, who led enormous experiments that studied particles called neutrinos, deserved the prize, says Alexei Smirnov of the International Centre for Theoretical Physics in Trieste, Italy. But the Nobel committee's pithy 12-word description of their findings misstates what one of the experiments did.

"Certainly, he is right that the citation is essentially wrong," says Giorgio Gratta, a neutrino physicist at Stanford University in Palo Alto, California, who was not involved in either of the prize-winning experiments. However, Olga Botner, a neutrino physicist at Uppsala University in Sweden and a member of the Nobel committee, says that "[t]he citation for the Nobel Prize is by necessity short and cannot reflect all details of the discoveries being recognized."

Born in certain nuclear interactions and nearly massless, neutrinos barely flirt with ordinary matter. They come in three types or "flavors"—electron, muon, and tau—and, weirdly, can change from one type into another, so that an electron neutrino can change into a muon neutrino and back again. Such back-and-forth "neutrino oscillations" prove that neutrinos have mass. Were neutrinos massless, they would have to move at light speed, at least in a vacuum, according to Einstein's theory of relativity. If that were the case, time for them would stand still, and change would be impossible.

The 2015 physics Nobel honored leaders of two experiments "for the discovery of neutrino oscillations, which shows that neutrinos have mass." Takaaki Kajita, a particle physicist at the University of Tokyo, and his colleagues used a gargantuan subterranean particle detector in Japan called Super-Kamiokande (SuperK) to study high-energy muon neutrinos generated as cosmic rays strike the atmosphere. In 1998, they reported that those raining down from above outnumbered those coming up through Earth, suggesting that some of those making the longer journey through the planet were oscillating along the way into electron and tau neutrinos, which SuperK couldn't detect.

Arthur McDonald of Queen’s University in Kingston, Canada, and colleagues used a detector in a mine called the Sudbury Neutrino Observatory (SNO) to study lower-energy neutrinos coming from the sun, where they are born in nuclear interactions as electron neutrinos. The team employed two techniques: one that could count only electron neutrinos and another that was sensitive to all types. In 2001 and 2002, the SNO reported that electron neutrinos accounted for just 34% of all the neutrinos emanating from the sun, suggesting that some were changing flavors along the way. Together, the SuperK and SNO results prove that neutrinos oscillate, according to the Nobel committee.

Except that the SNO results show no such thing, argues Smirnov in a paper posted 8 September to the arXiv preprint server. The SNO results proved that electron neutrinos from the sun change their type, but they do so through a different bit of physics that is essentially independent of neutrino oscillations, Smirnov says. The Nobel committee got that wrong not only in the short prize citation, but also in its longer technical explanation of the prize, he says. "No question the experiment deserves to be awarded a Nobel Prize," Smirnov says. "It's just a question of what they actually saw."

Neutrino oscillations occur because, bizarrely, a neutrino with a definite flavor—such as an electron neutrino—doesn't have a well-defined mass. That is, physicists cannot say the electron neutrino has one mass, the muon neutrino has another mass, and the tau neutrino a third. Instead, thanks to quantum weirdness, each is a different combination of three different "mass states," which are themselves made of different combinations of the three flavors. Mathematically, the mass states mesh together in one way to make an electron neutrino, another to make a muon neutrino, and a third way to make a tau neutrino—like puzzle pieces that can be assembled in different ways to make three different objects.

Crucially, thanks to their different masses, the three mass states evolve differently in time, so how they mesh also changes. For example, for a muon neutrino the mass states' muon components reinforce each other while their electron and tau components cancel one another out. After a while, the mass states' tau parts will reinforce each other while the other parts cancel out, transforming the muon neutrino into a tau neutrino. Wait longer, and the mass states' muon parts will reinforce again, turning the tau neutrino back into a muon neutrino. This mechanism requires multiple mass states whirring at different rates, and it explains the SuperK results.

In contrast, the SNO results involve the subtle influence of matter on neutrinos. Electron neutrinos emerge from nuclear interactions deep within the sun into an environment rich with electrons. Interactions with those electrons change the neutrinos’ mass states and their flavor makeup, much as interactions with matter can slow a photon to a crawl. As a result of that "matter effect," electron neutrinos in the heart of the sun consist of only one mass state, and that mass state consists of only one flavor: electron.

As the neutrino makes its way out of the sun, however, the electron density falls and its effects on the mass state wane. So the state's usual flavor combination of electron, muon, and tau emerges. Thus, the electron neutrinos from the sun change flavor in a way that doesn't involve back-and-forth neutrino oscillations, but simply reflects the changing electron density, Smirnov says. Such "adiabatic flavor conversion" doesn't even require that the neutrinos have mass, he says, as the one mass state involved could have zero mass once the neutrinos escape the distorting environment of the sun. SNO researchers described their results correctly and did not claim an observation of neutrino oscillations, Smirnov says.

Some physicists say Smirnov is sticking to a particularly precise definition of neutrino oscillations. "He's right about the physics," says Kate Scholberg, a neutrino physicist at Duke University in Durham, North Carolina. "But I personally think it's okay to have the citation for neutrino oscillations because that was the common usage" at the time of the SNO results.

However, Smirnov says that even after the SNO results, "five or six" explanations of how neutrinos work remained viable, including the possibility that neutrinos decay or that they undergo exotic new interactions. The picture of neutrinos with three flavors and three mass states came into tight focus only after another experiment, the Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) in Toyama, Japan, observed oscillations of electron antineutrinos from nuclear reactors in 2002, Smirnov says. For that reason, he says, KamLAND might have shared in the Nobel Prize.

Regardless, the committee might have given McDonald and the SNO a simpler citation, some physicists says. Researchers first detected electron neutrinos from the sun in the late 1960s, but measured less than half the amount predicted by solar models, a controversial discrepancy known as the solar neutrino problem. SNO showed that, contrary to the expectations of many physicists, the solar models were correct, but that the neutrinos were changing flavor on their way to Earth. "It's really clear that SNO deserved the Nobel Prize because they solved the solar neutrino problem," says Patrick Huber, a theoretical physicist at Virginia Polytechnic Institute and State University in Blacksburg.

Why bring up the citation if everybody agrees that the SNO and McDonald deserve the Nobel Prize? Many younger physicists don’t understand that the SNO results and the solution of the solar neutrino problem do not involve neutrino oscillations, he says. That's plausible, Scholberg says, given that most neutrino physicists work on experiments designed to study neutrino oscillations: "Probably a lot of [younger physicists] don't know about solar neutrinos because it's not what they work on every day."