Of neutrino oscillations and coming full circle

Technicians in a boat examine the detectors at the Kamiokande neutrino observatory in Japan. [Credit: Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo]

I’m happy to say that I’m secure enough to start asking dumb questions again.

Dumb questions, of course, are circumstantial. By “dumb”, we really mean “a naïve question the answerer thinks the questioner should know, based on the answerer’s own knowledge base”. Sure, there are truly ignorant questions — often based on willful disinterest on the questioner’s part — but I’m talking about those questions born of a genuine desire for knowledge that can make impatient insiders roll their eyes. Sometimes those naïve questions are dismissed (I confess I’ve sometimes done that, though I’m trying to break the habit). However, we have to remember that we are all strangers and sojourners in areas beyond our own expertise.

In 1997 or thereabouts, I asked a dumb question of John Bahcall. (My memory is oddly hazy on the larger features of this encounter; I may be wrong on both the speaker and the year, though I remember the shape of the auditorium. Brains are odd.) Bahcall was a prominent astrophysicist, probably best known for his work on the solar neutrino problem: the discrepancy between the number of neutrinos expected from nuclear reactions in the Sun’s core, and the number of neutrinos detected in Earth-based experiments. He spoke that day about the missing neutrinos, and how a particular idea — neutrino oscillations — could be the solution.

Neutrinos come in three flavors (and yes, that’s the technical term): electron, mu, and tau. Those are based on the type of particle — electrons, muons, or tauons — that they are associated with in particle reactions. (Possibly there are other flavors of neutrino, but that’s a story for another day.) Most neutrino detectors at that time, and many today, look for electron neutrinos, since those are the easiest to detect and are produced in common nuclear reactions, including hydrogen fusion in the Sun.

According to the Standard Model of particles and interactions, neutrinos are massless particles that maintain their identity forever, barring collisions with other particles. However, measurements of solar neutrinos found only 1/3 of the expected number, hinting that either the detectors were all misbehaving, the model of nuclear fusion in the Sun’s core was totally wrong, or that neutrinos somehow changed flavor between the Sun and Earth. Researchers ruled out the misbehavior option (thanks to several different detector designs that wouldn’t misbehave the same way), and the overwhelming success of the Standard Solar Model in every other way made it hard to accept that neutrinos would be its downfall.

If neutrinos have mass, however, they could change flavor in transit between the Sun and Earth. That change is known as neutrino oscillation, and by 1997 the idea was gaining favor. To test the idea, physicists had to build detectors capable of finding mu and tau neutrinos, a process underway when I heard Bahcall speak. In 1998, researchers from the Super Kamiokande experiment in Japan announced they had successfully detected the effect of neutrino oscillation, solving the solar neutrino problem, and vindicating the claims of those like Bahcall who worked on the issue for decades.

Now we finally get to my dumb question: I had never heard of neutrino oscillation before the seminar, and knew even less about particle physics than I do today. I also didn’t know that low-ranking people (undergraduates and early-career grad students) were not supposed to ask questions in a public forum like that, by custom. We were supposed to ask questions privately of our own professors, so as not to shame everyone for our lack of knowledge. We certainly were not supposed to ask questions of famous scientists like John Bahcall. I was completely ignorant of all that, and asked my question anyway.

I asked, “Would neutrinos oscillate from mu neutrinos back into electron neutrinos?” Recall this was in 1997, before neutrino oscillations were found. I didn’t fully understand what was involved in this: physicists were just starting to come to grips with the idea that the Standard Model was wrong about neutrinos, and what that meant. Bahcall was polite and answered (again, if my memory is correct) that it was too soon to tell. He didn’t seem particularly interested in the question, which was fair: he was mostly concerned with solving the solar neutrino problem. (Addendum: I should mention that I had several pleasant conversations with Bahcall after this experience, including one not long before he died in 2005. He was always approachable and willing to talk with me.)

However, just two weeks ago, my dumb question was answered: the T2K experiment, which uses the same facility as the Super Kamiokande detector, found evidence for oscillation between mu neutrinos and electron neutrinos. Of course, by 2013 we knew that kind of oscillation must be happening. The MINOS (Main Injector Neutrino Oscillation Search) experiment, among others, has been measuring the disappearance of mu neutrinos as they travel through the Earth between Fermilab in Illinois and Soudan in northern Minnesota. However, as with the solar neutrino problem, it isn’t enough to see neutrinos vanish: we have to see what they change into.

Measuring neutrino oscillations today isn’t just about showing it happens, a sign of how far we’ve come since 1997. We now are trying to figure out what the mass is of each neutrino flavor, which is a challenging problem. (It’s made more complicated by the fact that neutrinos are quantum combinations rather than pure entities like electrons; for more on that, please see my earlier post.) Particle physicists also want to know why neutrinos have the masses they do, since the Higgs mechanism that grants mass to electrons and quarks isn’t responsible for neutrinos. The Sun’s missing neutrinos may have been the beginning of the path, but the end lies in the mists.

Today I wouldn’t be afraid of asking my dumb question, or one like it, even if the person I’m asking is famous. We have to ask questions, and not worry about being perceived as stupid. Ultimately, after all, science is asking questions of the Universe, even if it isn’t always forthcoming with the answers — or may not give us the answers we like.

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2 Responses to “Of neutrino oscillations and coming full circle”


  1. 1 dwellscho July 30, 2013 at 13:21

    Matthew, this is a *very* good essay, one of your best, because it is talking about how scientists think and act. -Don


  1. 1 Morsels for the mind – 2/8/2013 › Six Incredible Things Before Breakfast Trackback on August 2, 2013 at 21:04
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