Neutrinos are the most enigmatic of the subatomic fundamental particles. Ghosts of the quantum world, neutrinos interact so weakly with ordinary matter that it would take a wall of solid lead five light-years deep to stop the neutrinos generated by the sun. In awarding this year’s Nobel Prize in physics to Takaaki Kajita (Super-Kamiokande Collaboration/University of Tokyo) and Arthur McDonald (Sudbury Neutrino Observatory Collaboration/Queen’s University, Canada) for their neutrino research, the Nobel committee affirmed just how much these “ghost particles” can teach us about fundamental physics. And we still have much more to learn about neutrinos.
Neutrinos are quantum chameleons, able to change their identity between the three known species (called electron-, muon- and tau-neutrinos). It’s as if a duck could change itself into a goose and then a swan and back into a duck again. Takaaki Kajita and Arthur B. McDonald received the Nobel for finding the first conclusive proof of this identity-bending behavior.
In 1970, chemist Ray Davis built a large experiment designed to detect neutrinos from the sun. This detector was made up of a 100,000-gallon tank filled with a chlorine-containing compound. When a neutrino hit a chlorine nucleus, it would convert it into argon. In spite of a flux of about 100,000 trillion solar neutrinos per second, neutrinos interact so rarely that he expected to see only about a couple dozen argon atoms after a week’s running.
But the experiment found even fewer argon atoms than predicted, and Davis concluded that the flux of electron-type neutrinos hitting his detector was only about a third of that emitted by the sun. This was an incredible scientific achievement and, for it, Davis was awarded a part of the 2002 Nobel Prize in physics.
Explaining how these neutrinos got “lost” in their journey to Earth would take nearly three decades. The correct answer was put forth by the Italian-born physicist Bruno Pontecorvo, who hypothesized that the electron-type neutrinos emitted by the sun were morphing, or “oscillating,” into muon-type neutrinos. (Note that the tau-type neutrino was postulated in 1975 and observed in 2000; Pontecorvo was unaware of its existence.) This also meant that neutrinos must have mass—a surprise, since even in the Standard Model of particle physics, our most modern theory of the behavior of subatomic particles, neutrinos are treated as massless. So, if neutrinos could really oscillate, we would know that our current theory is wrong, at least in part.
In 1998, a team of physicists led by Takaaki Kajita was using the Super Kamiokande (SuperK) experiment in Japan to study neutrinos created when cosmic rays from space hit the Earth’s atmosphere. SuperK was an enormous cavern, filled with 50,000 tons of water and surrounded by 11,000 light-detecting devices called phototubes. When a neutrino collided with a water molecule, the resulting debris from the interaction would fly off in the direction that the incident neutrino was traveling. This debris would emit a form of light called Cerenkov radiation and scientists could therefore determine the direction the neutrino was traveling. By comparing the neutrinos created overhead, about 12 miles from the detector, to those created on the other side of the Earth, about 8,000 miles away, the researchers were able to demonstrate that muon-type neutrinos created in the atmosphere were disappearing, and that the rate of disappearance was related to the distance that the neutrinos traveled before being detected. This was clear evidence for neutrino oscillations.
Just a few years later, in 2001, the Sudbury Neutrino Observatory (SNO) experiment, led by Arthur B. McDonald, was looking at neutrinos originating in the sun. Unlike previous experiments, the SNO could identify all three neutrino species, thanks to its giant tank of heavy water (i.e. D2O, two deuterium atoms combined with oxygen). SNO first used ordinary water to measure the flux of electron-type neutrinos and then heavy water to observe all three types. The SNO team was able to demonstrate that the neutrino flux of all three types of neutrinos agreed exactly with those emitted by the sun, but that the flux of electron-type was lower than would be expected in a no-oscillation scenario. This experiment was a definitive demonstration of the oscillation of solar neutrinos.
With the achievements of both the SuperK and SNO experiments, it is entirely fitting that Kajita and McDonald share the 2015 Nobel Prize in physics. They demonstrated that neutrinos oscillate and, therefore, that neutrinos have mass. This is a clear crack in the impressive façade of the Standard Model of particle physics and may well lead to a better and more complete theory.
The neutrino story didn’t end there, though. To understand the phenomenon in greater detail, physicists are now generating beams of neutrinos at many sites over the world, including Fermilab, Brookhaven, CERN and the KEK laboratory in Japan. Combined with studies of neutrinos emitted by nuclear reactors, significant progress has been made in understanding the nature of neutrino oscillation.
Real mysteries remain. Our measurements have shown that the mass of each neutrino species is different. That’s why we know that some must have mass: if they are different, they can’t all be zero. However, we don’t know the absolute mass of the neutrino species—just the mass differences. We don’t even know which species is the heaviest and which is the lightest.
The biggest question in neutrino oscillation physics, though, is whether neutrinos and antimatter neutrinos oscillate the same way. If they don’t, this could explain why our universe is composed solely of matter even while we believe that matter and antimatter existed in equal quantities right after the Big Bang.
Accordingly, Fermilab, America’s premier particle physics laboratory, has launched a multi-decade effort to build the world’s most intense beam of neutrinos, aimed at a distant detector located 800 miles away in South Dakota. Named the Deep Underground Neutrino Experiment (DUNE), it will dominate the neutrino frontier for the foreseeable future.
This year’s Nobel Prize acknowledged a great step forward in our understanding of these ghostly, subatomic chameleons, but their entire story hasn’t been told. The next few decades will be a very interesting time.
Don Lincoln on neutrinos
Fermilab: Neutrinos: Nature’s Identity Thieves? (Video)
In this video, Don Lincoln explains the evidence for neutrino oscillation and how physicists around the world are working to better understand this complex mystery.
Fermilab: Neutrinos: Nature’s Ghosts? (Video)
In this video, Don Lincoln provides a brief introduction to neutrinos.
Picks for further reading
NOVA: The Ghost Particle
Find articles, a program transcript, teachers’ guide, and more resources associated with NOVA’s 2006 documentary “The Ghost Particle.”
Of Particular Significance: Neutrino Types and Neutrino Oscillations
Theoretical physicist Matt Strassler explains why there is more than one way to classify neutrinos.
Symmetry: How do you solve a puzzle like neutrinos?
Learn more about the wide variety of ways in which physicists are trying to probe the mysteries of neutrinos.