Particle Physics


Neutrino Physicists win Nobel, but Neutrino Mysteries Remain

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.

View from the bottom of the SNO acrylic vessel and photomultiplier tube array with a fish-eye lens. This photo was taken immediately before the final, bottom-most panel of photomultiplier tubes was installed. Photo courtesy of Ernest Orlando, Lawrence Berkeley National Laboratory.

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.

Go Deeper

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.

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Don Lincoln

    Don Lincoln is a senior experimental particle physicist at Fermi National Accelerator Laboratory and an adjunct professor at the University of Notre Dame. He splits his research time between Fermilab and the CERN laboratory, just outside Geneva, Switzerland. He has coauthored more than 500 scientific papers on subjects from microscopic black holes and extra dimensions to the elusive Higgs boson. When Don isn’t doing physics research, he spends his time sharing the fantastic world of science with anyone who will listen. He has given public lectures on three continents and has authored many magazine articles, YouTube videos and columns in the online periodical Fermilab Today. His most recent book "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind" tells the tale of the Large Hadron Collider, the physics and the technology required to make it all work, and the human stories behind the hunt for the Higgs boson.

    • Hosam Otaibi

      very nice article.
      how can we differentiate between the three types of neutrinos

      • A lot of people are worried about him (strassler).
        Everything he was doing on social media sites stopped on May 6th (his blog, facebook, etc.)

        • He quit his professorship at Rutgers in 2013 and has been an itinerant professor, wandering between universities. His last known whereabouts was Harvard and the gig completed in August. Maybe he’s doing physics trying to find a next spot?

    • Stan Sykora

      Nice article, Don. Somewhat off topic,
      I would like to quip in with a comment on the para where you say:
      “… 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 …”
      I believe that we should not fool the public 🙂 As scientists, we do not “explain why” and therefore we should not employ that kind of wording. In the best case, we simply replace one question with another.
      In this case: “why is matter and antimatter unbalanced in the part of Universe that surrounds us” with “why do neutrinos and anti-neutrinos oscillate in different ways”. Science is great at creating chains of -why-because- elements, but such chains are necessarily always terminated by a “why”, and yours is an excellent example of that fact. Whenever we say “science explains why” it is a bona-fide lie, science just tries to elongate those chains as much as possible (mind you, I do say that religions are better off, they are not at all).

      • Hi Stan,

        Naturally, I disagree. I don’t think anyone takes quite as an absolute interpretation of the term as you do. For instance, if we find that there is some sort of matter/antimatter asymmetric field in the universe, we can definitively say that we know why the asymmetry exists. We cannot claim that we then immediately understand what the cause of that underlying field is.

        I would claim that I can say quite authoritatively why you look like some admixture of your mother and father. This is clearly a case of genetics, rooted in the biochemistry of DNA and gene expression. You might then ask why DNA is the molecule that governs heritability and this will lead to a long conversation about the chemistry of gene repair, stability of molecules, atomic physics, etc. It will inevitably end up in my turf of quarks and leptons and matter and dark matter and many unknown things.

        But I don’t think that you need to know everything there is to know about the model from which the Standard Model originates to say that we know why Stan is an admixture of mom and dad.

        In short, I don’t disagree you in some sort of over-the-top purist’s sense. But not in a sensible-use-of-language sense.

        • Stan Sykora

          Well, I see your point, of course, and when I was younger it was my point also – who cares about the terminal “why”, just look how many of the intermediate “because” we have discovered and settled. Nowadays, I am not so sure. Popularization media today tend to hype up the fundamental “why”s, and then hammer down the “scientists have proofed” cliche, and plain folks either buy it, or else see the fallacy and consequently, and sadly, reject all science. In any case, I really fail to see which “why” would be solved should you manage to show that “matter/antimatter imbalance” is a consequence of yet to be discovered “neutrino/antineutrino behavioral differences”. It would be extremely interesting, I agree, and you would have really earned your physicist’s salary, I agree too, but, I am sorry, you would have not removed any fundamental “why”. The two propositions look to me equally puzzling, nor is one of them much simpler than the other. Genetics is another story: it points towards information theory which is no part of Standard Model, and not even a part of Physics. My opinion, naturally (I err quite often).

          • Well, genetics are based in a combination of fundamental atomic theory and, to a lesser degree, information theory. Information theory tells how information should be organized, but only atomic theory tells how the molecules are put together.

            The point is that nothing but a grand unified theory can answer all “whys?”. And, in my opinion, this is an unobtainable ideal. (At least for the moment. Perhaps my many-great grandchild will have that final ‘aha’ epiphany and solve things once and for all.)

            • Stan Sykora

              And then? Brrr, end of Physics, end of life! Poor grand-grand-kids. I hope you are not serious.

            • I don’t know. He’s my GGGGGG-grandkid. I’m hoping intelligence will distill over the generations. Maybe he’ll figure out that GUT thing. Then time travel and he’ll come back and tell me the answer.

              Even if we have a GUT theory, it won’t be the end of physics. There is still a helluva lot of complex things to work out.

              I aspire to a GUT understanding…literally the end of fundamental physics. I have no real expectation of us achieving this understanding anytime soon.

            • Stan Sykora

              Then you are basically a religious man, regardless of what you say. Since, once All-That-Exists is reduced to a [necessarily finite] kind of a machinery, the final WHY will become totally imperative. Thinking that such a situation can never happen is my own escape route. Of course, we will never settle that, nor is there any need to. Topic exhausted. Thanks, I enjoyed the exchange.

            • Nope, not religious. I have no faith in any of this. It is just my suspicion of how it will all turn out. Now we will let the data guide us and let us know how right I am.

    • Tho Huynh

      Oscillation Neutrinos are not exact observation without Particles Field, because we did not know what happened Neutrinos or its oscillations within Millions of miles of its journey from Sun to Earth or within few hundred miles of its journey in experiment . . . . . .It is why I preferred PARTICLES FIELD as “a Map” to all particles for better observation . . . . . . . .Besides, experiment about light photon with 2 lids ring some bell, for example. . . . . . . . . . . .

    • Thoisuquanhco

      Oscillation Neutrinos are not exact observation without Particles Field, because we did not know what happened to Neutrinos or its oscillations within Millions of miles of its journey from Sun to Earth or within few hundred miles of its journey in experiment , we cannot depend on end results without explain how it happened before it led to Results about, or rhetoric for the means; especially, it is in science. . . . . .It is why I preferred PARTICLES FIELD as “a Map” to all particles for better observation . . . . . . . .Besides, experiment about light photon with 2 lids ring some bell, for example. In facts a lot of things can happen and could happened in Millions Miles, we must know and we should know. . . . . . . . . . .

    • Anonymous

      Don, now that we know neutrinos have mass are they a candidate for one of the particles that may compose dark matter or are they too light?

      • Ordinary neutrinos are too light. There is the possibility that there is a kind of neutrinos called sterile neutrinos that might work. Nobody has seen sterile neutrinos, but the two big LHC experiments are looking for them. (As are other, smaller experiments.)

        Ordinary neutrinos that we have observed and were created in the Big Bang make up only 0.3% the matter in the universe. Dark matter makes up 25%.

    • “When a neutrino hit a chlorine nucleus, it would convert it into argon.”

      If that is correct it is another proof of my GUT. (yes, I have one and it is not cranky)


      i have heard that neutrino can pass by anything even our earth

      • That is because atoms are mostly empty space
        See the dot above the letter i
        If that were the size of the nucleus of an atom it would mean the size of the whole atom is about as big as a football stadium.
        A neutrino is even smaller than the i dot.
        That’s why there is almost no chance of the neutrino hitting the nucleus.

    • George Raina

      The Weak Interaction transforms an electric charge in the diffraction pattern from one side to the other side, causing an electric dipole momentum change, which violates the CP and Time reversal symmetry. The Neutrino Oscillation of the Weak Interaction shows that it is a General electric dipole change and it is possible to any other temperature dependent entropy and information changing diffraction pattern of atoms, molecules and even complicated biological living structures.