Of Particular Significance

A Neutrino Success Story

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/09/2012

Almost all the news on neutrinos in the mainstream press this past few months was about the OPERA experiment, and a possible violation of Einstein’s foundational theory of relativity. That the experiment turned out to be wrong didn’t surprise experts. But one of the concerns that scientists have about how this story turned out and was reported in the press is that perhaps many non-experts may get the impression that science is so full of mistakes that you can’t trust it at all. That would be a very unhappy conclusion — not just unhappy but in fact a very dangerous conclusion, at least for anyone who would like to keep their economy strong, their planet well-treated and their nation well-defended.

So it is important to balance the OPERA mini-fiasco with another hot-off-the-presses neutrino story that illustrates why, even though mistakes in individual scientific experiments are common, collective mistakes in science are rare. A discipline such as physics has intrinsic checks and balances that significantly reduce the probability of errors going unrecognized for long. In the story I’m about to relate, one can recognize how and why scientists start to come to consensus.  Though quite suspicious of any individual experiment, scientists generally take a different view of a group of experiments that buttress one another.

The context of this story, though much less revolutionary than a violation of Einstein’s speed limit, still represents a milestone in our understanding of neutrinos, which has been advancing very rapidly over the past fifteen years or so. When I was a starting graduate student in the late 1980s, almost all we knew about neutrinos was that there were at least three types and that they were much lighter than electrons, and perhaps massless. Today we know much, much more about neutrinos and how they behave. And in just the last few months and weeks and days, one of the missing entries in the Encyclopedia Neutrinica appears to have been filled in.

The Question

What’s in question isn’t so easy to explain; it’s a tricky bit involving neutrino oscillations.  I wrote a pedagogical article on neutrino types and neutrino oscillations (click here if you want to read it in its entirety), which I recommend you read if you want to understand in a bit more detail what I’m about to say.  Here I’m going to make a long story way too short, by simply saying that you can classify the three different types of neutrinos in two different ways.  In the “weak-type” classification, the three neutrinos are named “electron-neutrino”, “muon-neutrinos” and “tau-neutrinos”.  Their anti-particles are called anti-neutrinos with the same prefixes.  The names are given according to whether they appear in processes involving electrons, or muons, or taus (or their anti-particles). (Click here for a reminder about what electrons, muons and taus are.) In particular, if an anti-neutrino is emitted in a process involving an electron, that anti-neutrino is called an electron-antineutrino; if the process instead involves a muon, then the anti-neutrino is a muon-antineutrino.

The physics question that is the subject of this article is the following: as an electron-antineutrino travels along on its own, can it convert (or “oscillate”) into a muon-antineutrino or tau-antineutrino?  (Or can the reverse process occur, with, say, a muon-neutrino or antineutrino converting to an electron-neutrino or anti-neutrino?)  Certain types of neutrino oscillations have previously been observed, in the context of various types of neutrinos traveling through the sun or through the earth — but not this particular one.

I’m oversimplifying quite a bit here, but the full story is long and rather technical.  A very small bit of the technical stuff: In the equations that physicists use for neutrinos, a quantity appears that for historical reasons is called θ13.  If it is zero, then electron-antineutrinos will remain as they are.  If it is not zero, then they will “oscillate”.  (Again, click here for my article describing what oscillation means in this context.)  So when you read other people’s articles about this question, you will often see it phrased as asking whether θ13 is or isn’t zero.

How could you figure out, through an experiment, whether this type of oscillation occurs?   The trick is to create very large numbers of electron-antineutrinos (for instance, in a nuclear reactor such as one finds in a nuclear power plant). Next, set up a detector, a certain distance away, that is able to detect  electron-antineutrinos (but it won’t be able to detect the other types, for technical reasons having to do with the low energy carried by anti-neutrinos from a reactor.)  Not that it will detect very many; most of the anti-neutrinos will pass right through the detector, and indeed right through the entire earth, unobserved.  That’s because neutrinos and anti-neutrinos are unaffected by the electromagnetic force or the strong nuclear force, and can interact with matter only through the weak nuclear force, which is very weak indeed in these contexts (and by gravity, which is ridiculously weak and of no consequence here.)  But because there are so many anti-neutrinos produced by a reactor, a small but measurable number will hit something inside a suitably designed detector, allowing them to be observed.

Because the way electron-antineutrinos interact with matter via the weak nuclear force is very well understood, we can predict very accurately how many of them from the reactor would be observed in the detector if they do not oscillate.  If oscillation does occur, then fewer will be observed than expected, because a fraction of the electron-antineutrinos, having converted into another type of antineutrino, will be unobservable by the detector.

Actually, the best way to carry out this measurement is not quite the way I just described. What you really want to do is measure the number of electron-antineutrinos observed in two (or more) detectors, located at different distances from the same reactor; see Figure 1. That way, even if you’ve made a mistake in your understanding of the reactor, or of your detector technology, it won’t matter; when you take the ratio of how many antineutrinos your nearby detector observes compared to how many your farther detector observes, those mistakes will cancel out and won’t affect your measurement of whether oscillation is absent or present, and thus of whether θ13 is zero or not.

Fig. 1: Sketch of a reactor neutrino oscillation experiment. A nuclear reactor produces electron anti-neutrinos, which stream outward. Two (or more) identical detectors which can only detect electron-anti-neutrinos are placed at different distances from the reactor. The ratio of the number of observed neutrinos in the far detector to the number observed in the near detector can be predicted very precisely in the case that no oscillation taking place, and if the observed ratio is less than this, that indicates that oscillation is occurring as the neutrinos transit between the two detectors.

The Answers

In the last few months and weeks, several different experiments have reported on measurements relevant to this question. All of them are finding that θ13 is not zero, and all are consistent with θ13 being something like 0.16 radians (which is about 9 degrees — small, but clearly not zero.)  That number may move around a bit with more data, but is very unlikely to drop below 0.08 radians.

The first dramatic evidence was presented in early March, from the Daya Bay experiment based in China — a reactor experiment similar to but a bit more elaborate than the sketch shown in Figure 1.  Their result, quite convincing by itself, disagrees with the hypothesis of zero oscillation by more than 5 standard deviations, an amount that by the standards of particle physics convincingly rules out the possibility that θ13 =0.

But in saying “this is convincing”, I am assuming they didn’t make any mistakes. Should we believe this neutrino experiment more than we believed OPERA? After all, OPERA claimed a 6 standard deviation result (though I complained it should have been more like 4, if they’d been a bit more conservative in their statistical analysis.)  That turned out to be irrelevant, because they’d made an error.

It’s worth noting that one mark against OPERA’s result was that it was particularly implausible and difficult to fit into existing knowledge. Daya Bay’s result is a lot more plausible and not in any way inconsistent with other things  we know.

But still, if we only had Daya Bay’s measurement, then indeed there would be good reason to be patient and cautious. The measurement they are making is difficult. They are observing only a 6% deficit in the number of electron-antineutrinos compared to the number they expected — a small effect.  You may well be tired of hearing me say that first-time experimental measurements commonly turn out to be wrong, but I’ll say it again: Daya Bay isn’t exempt from this caveat.

However, already before Daya Bay produced its result, three pre-cursors to this experiment had reported hints over the past few months that are consistent with what Daya Bay now sees. These were T2K (in Japan), MINOS (in the U.S.A.), and Double Chooz (in France). Double Chooz is also a reactor experiment (as in Figure 1), while T2K and MINOS are accelerator experiments (similar in some ways to OPERA) in which high-energy muon-neutrinos are produced in one laboratory and a detector in a laboratory hundreds of kilometers away tries to measure whether a significant fraction of them convert to electron-neutrinos while in flight — a process also sensitive to θ13.

Much more impressively, just a couple of weeks after Daya Bay’s result was announced, the RENO experiment in Korea (which is a reactor experiment similar to Daya Bay, and had been competing with them, hoping to beat them to the punch) announced its result. What RENO finds is completely consistent with what Daya Bay has measured, and is even just a bit more [after improved analysis, just a bit less] statistically significant.

In short, we have very quickly seen two experiments measure θ13  independently, using different nuclear reactors and somewhat different experimental designs, and subject to different potential problems. They both make the measurement as illustrated in Figure 1, as a ratio between what is observed by nearby detectors and what is observed by farther ones; such a technique is reassuringly cushioned from many types of possible errors. They get essentially the same answer (about 0.15 or so in radians). And they were preceded by three experiments that collectively were all already hinting that θ13 was larger than 0.08 radians or so.

Is it possible that they are all wrong, and that θ13 is actually zero, or at least much smaller than 0.15 radians? In principle, yes; that possibility is still large enough to be worth remembering. The measurements are not simple. Is it likely? Not very. Not with so much consistent evidence, some of it strong, coming from so many different experiments.

The Implications

So science has probably made a step forward. It is highly probable that we now know something about neutrinos that we did not know last year. Over time, as RENO and especially Daya Bay (which, because of its design, will pull ahead of its rivals) collect more data, their results will become more  precise and will likely become more and more convincing. The new information will then go gradually from “highly probable” to “near certain.” We’ll be watching this process over the next couple of years, though it will be slow and unexciting and will engender little public comment — unless it stumbles unexpectedly.

In comparing this story to the speedy neutrino saga, we get to see both sides of how science works. Sometimes experiments conflict with each other; first one experiment claims a new result, then others knock it down. So it was with OPERA and ICARUS.   But sometimes they agree, which provides some confidence in the result. So it is with Daya Bay and RENO, as well as their predecessors.  Both the OPERA/ICARUS story and the Daya Bay/RENO story show science working well, preventing falsehoods from leaking into scientific knowledge, and assuring that only those potential facts that have run a gauntlet of cross-checks are allowed into the textbooks of our children.

One more thing: should you, personally, care about the scientific result of this neutrino success story? Whether θ13 is zero or not will not keep you awake at night, and the new result does not potentially challenge a hundred years of theoretical physics, as OPERA’s result might have done had it been correct. But still, it is significant for several reasons.

  • a short-term scientific implication: a non-zero θ13 makes possible a class of experiments looking for CP violation in neutrinos. (I described what CP violation means in a post discussing its appearance in the physics of quarks. ) There are many physicists who believe that CP violation in neutrinos may give deep insights into why the universe has more matter than anti-matter (i.e. more electrons than positrons, and more protons and neutrons than anti-protons and anti-neutrons.) So that experimental program will now go ahead.
  • a long-term scientific implication: the measurement of θ13  gives us some insight into the processes that might be responsible for neutrinos having the masses that they do.   I call this long-term because we don’t know what neutrino masses are yet!  (We just know some differences of the squares of masses.)  But many theories of neutrino masses would have predicted  θ13 much smaller than measured by Daya Bay and RENO, so those theories can now be discarded.
  • a long-term, long-shot technological implication: it seems likely to me that someday neutrinos will be a technological tool. The applications will be limited because huge numbers of neutrinos are needed if one is use them to measure anything, but still, neutrinos are the only known measurable particles that can pass through large quantities of rock and potentially convey to us information about what’s in there. And one can’t dream broadly about neutrino-based technology without a more complete knowledge of how neutrinos behave.  This most recent information fills in one of the key missing pieces.
  • other long-term, long-shot scientific implications: we need a full understanding of neutrinos if we are to use the neutrinos coming from supernovas to learn better how supernovas work (and remember how important supernovas are — their precursor stars are the furnaces in which many nuclei of the heavier chemical elements are forged, and the supernovas themselves blow those elements across wide expanses of interstellar space, eventually to help form the cores of rocky planets and the bodies of human beings.)  There are likely to be other contexts, too, in which this knowledge is crucial for gaining understanding of phenomena in which neutrinos play a central role.

Maybe there are other implications that neutrino experts remember that I’m forgetting right now. But suffice it to say that our children will likely care, indirectly, about this measurement, quite possibly for reasons that we cannot yet imagine.

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40 Responses

  1. The electron antineutrinos at Daya Bay are detected by inverse Beta decay. Let’s say that one, out 10 to the 30th, neutrinos are intercepted in the detector by this process (just a ballpark guess). So, some 10, to the 32nd, neutrinos would have to pass through the detector for 100 neutrinos to be detected by inverse Beta decay. But there are other neutrino interactions like elastic scattering. I don’t know how frequent this process is compared to inverse Beta decay, but if it was occuring in the rock between the near and far detector the electron anti-neutrinos from the reactor might be deflected at an angle such to miss the far detector. But, I assume, the Daya Bay collaboration would have included this in their calculations.

    Is it permissable to use html coding for super/sub scripts in these posts?

  2. The korean experiment may not be a good “independent” validation of the chinese one. It has been reported that the korean has requested a visit to Daya Bay in seeking “collaborations”. During the visit, koreans learned some key designed tricks and then ceased contacts with chinese. Koreans could be the winner of this game since their test is in a smaller scale thus faster to build. But angried chinese accelerate their construction and test after they heard the korean’s plan on an “independent” study.

    1. Interesting claim. (Note the Daya Bay experiment in China is very international, including many Americans; I don’t know much about the RENO collaboration working in Korea.)

      Well, there are different types of independence. Independent designs that lead to the same answer are definitely more convincing. But independent operation of the same type of detector is still independence as long as the experiments were done well and bias was avoided. So a more important question is whether the RENO experimenters are reliable and used proper methods to avoid bias from Daya Bay’s result. This I can’t evaluate.

      Over time, the Daya Bay experiment, with its 8 detectors, will have better methods for clarifying the measurement than its competitors, and I expect their results will become more convincing than anything we know now.

  3. Matt, thanks for that guidance! Being the founder and Chief Scientist of an AI company doesn’t preclude me from having my own special brand of stupidity.

    BTW, I think this site is wonderful!! Next to reading the Notes appendix to Brian Greene’s The Hidden Reality, you are my best source of clear information about the actual elements of quantum physics, the scientific method (an eye opner for me), and the meaning of many phenomena that I thought I understood but really didn’t (like virtual particles).

    1. You do not have permission to post video on this site. You are the only person who has done so; everyone else posts a link to their own site. This site is not an advertising clearing house for everyone to post their stuff. Do you see my professional colleagues posting their papers here?

      And after your personal and completely inappropriate (as well as unprofessional) attack on me via twitter, you are banned from this website. Don’t bother to put anything else up; I’ll take it down.

  4. Great article.

    Personally however I don’t find experiments where the conclusions are drawn from the absence of neutrinos all that convincing. There could be other reasons why they fail to arrive.

    To be convinced that the oscillations are real I would like to see an experiment which detects the extra neutrinos that are “created”(not really created but don’t know how to better describe it) by oscillations. For example if both detectors in the experiment above were able to measure not only electron anti-neutrinos but also muon anti-neutrinos and the amount of extra muon anti-neutrinos detected in the second detector compared to the first one also matched predictions of the oscillations theory that would be much stronger evidence for oscillations.

    Where there any such experiments which successfully detected types of (anti)neutrinos in a man-made beam which were not present there at the time of creation, but which were expected to be present later on based on oscillations theory?

    I would like to learn more about weak interactions, is there any textbook you could recommend? (I mean actual physics, not on the popular level, but the more approachable the better)

  5. This a great article, but it drove me crazy. You start out saying “you can classify the three different types of neutrinos in two different ways. In the “weak-type” classification,” but, as far as I can see, you never got back to the second way of classifying neutrinos. I read along for a few paragraphs and then started skipping ahead to find the contrast information implied by this lead in.

    I couldn’t find it. Did I miss it? If I missed it, I apologize. If you didn’t touch on it, then, for us non-scientists, you would have been better to have simply said “Neutrinos are generally sorted out by using the “weak-type” classification method…etc.”

    1. That’s why I gave you the link to the older article; the two classification schemes are discussed in detail there. Sorry, I though that was clear enough from the linking…

      1. My previous comment was meant to be taken ironically – I think that Look-Elsewhere is getting over used. I know that the RENO result was a measurement of a single parameter.


        David Perkin.

  6. It seems we have a long way to go before we really figure out what is going on with neutrinos. Are we sure that nothing else is created when neutrinos
    “oscillate”? I mean when a muon “decays” to an electron a muon-neutrino and an electron anti-neutrino are also ALWAYS created, preseving electron and muon family number. Is it possible that when a muon neutrino “oscillates” into an electron neutrino other particles (particles that we haven’t detected) are also created that preserves family number?

    1. Yes we are sure, and no it isn’t possible. During oscillation, the energy of the oscillating object is preserved. If anything were spit off as one neutrino converted to another, the energy would decrease, and oscillation back to the original neutrino type would be impossible. Moreover, the quantum coherence which makes oscillation possible would be broken, and observations would differ very much from what is expected from the theory of oscillations.

  7. Matt, I was wondering about neutrino mass. I’m really confused on this point. Some sources say that electron, muon and tau neutrinos have masses < 2.2 eV, 170 KeV, and 15.5 MeV respectively. Other sources say that the sum of the 3 neutrino type masses < .3 eV. What's the deal here? Is the first Listing experimental and the second by theory?

    1. No, it’s all experimental. Theoretically, neutrinos could be as heavy as top quarks.

      Assume first that something like the Standard Model is right and there really are only three types of neutrinos. Then there are three ways to look for neutrino masses

      a) Study the decays of particles that produce neutrinos; even though you don’t observe the neutrinos, in principle the motion of the particles you observe are dependent on the masses of any neutrinos produced. But extreme precision is difficult to obtain. This is where the first set of numbers come from; it’s more or less what we knew when I was a graduate student 20 years ago. But the way they’re stated is well out of date, and doesn’t account for the fact that the neutrinos that have definite mass are defined in the mass-type classification, and cannot be labeled “electron-“, “muon-” or “tau-“, which represent the weak-type classification. http://profmattstrassler.com/articles-and-posts/particle-physics-basics/neutrinos/neutrino-types-and-neutrino-oscillations/

      b) Study the effect of neutrinos on the early universe by studying the cosmic microwave background radiation. This gives you the second number. The value you quote is quite recent; I haven’t studied it so I don’t know if it leaves any wiggle room.

      c) Study neutrino oscillations (again, see http://profmattstrassler.com/articles-and-posts/particle-physics-basics/neutrinos/neutrino-types-and-neutrino-oscillations/ , but a warning that this article is very introductory and just hints at the full complexity and richness of neutrino oscillation phenomena.) This gives you differences of the squares of masses, but not the masses themselves. The differences of masses squared are very small, so either neutrino masses are very small or the neutrino masses are almost identical. This information was obtained over the last 15 years.

      Combining all of the information together, we are growing increasingly confident that all three neutrinos have mass not higher than .1 eV. Much more may be learned in the coming decade.

      Once you allow for the possibility of additional neutrinos beyond the ones we know about — sterile neutrinos, in particular, which means particles that (unlike the known neutrinos) don’t feel weak nuclear forces — then the story can get a lot more complicated. I don’t know a good way to summarize it. But until we have evidence that suggests the three-neutrino story can’t explain the data, that simplest version will be the working hypothesis that people try to test.

  8. Nice article, Matt. I guess somehow neutrinos are, in a sense, the losers in the struggle for flavor identity… something about quarks make them the winners and so their matrix is more nearly diagonal. What I wonder is whether the theta_13 implies how far theta_12 or theta_23 must be off of 45 degrees. Or something like that. Thanks again.

  9. “neutrinos are the only known measurable things that can pass through large quantities of rock and potentially convey to us information about what’s in there.”

    Perhaps it’s fair to note that a lot has been learned about the internal structure of the Earth from the study of seismic waves, which can reverberate after earthquakes on a timescale of months. Small scale artificially generated seismic waves are also used in prospecting for oil.

    1. Oh! Wow, I sure phrased that badly, didn’t I! Indeed, most of what we know about the earth uses seismic waves (waves in the rock itself). We also learn things from precise measurements of the earth’s gravitational field and magnetic field. Neutrinos can presumably could someday provide complementary information; for one thing, they can more easily be made into beams.

  10. “…perhaps many non-experts may get the impression that science is so full of mistakes that you can’t trust it at all”

    An editorial in the wall street journal used this OPERA example of an experiment disproving the long-standing scientific conclusion that the speed of light can’t be broken to show that “science” cannot be trusted (in particular for him, the scientific conclusion of climate change). Now that the experiment turned out to be wrong and the speed of light is still absolute i find it strange it’s somehow also taken to be a measure of how trustworthy “science” is.

    Perhaps “science” needs to stop being seen as a kind of blackbox in which “science” discovers this and “science” shows that. The particular example of the WSJ editorial is alarming in its logical fallacy, which would be self-evident had the writer used an example like “a black person mugged me once hence all black people can’t be trusted”, but blackbox thinking is the natural human tendency to make sense of things we are ignorant or fearful of in a crude and superficial manner.

    1. Unfortunately, the WSJ is owned by News Corp., and their agenda seems to be furthering the aims of certain special interest groups in our nation. They’ve published all kinds of garbage related to science recently – papers like this gem, which I consider to be a parody of the scientific method:


      I believe the key is a long term approach – we need to make sure people understand the scientific method when they are young. I fear that many adults are too close-minded to accept certain truths.

  11. I hope they switched the detectors 1 and 2 and measured the ratio again to cancel out any systematic instrumental error.

    1. Unfortunately these things are very large, so literally switching them isn’t generally possible. But there are numerous ways to check the detectors. Moreover, Daya Bay has 6 (soon to be 8) of them.

  12. Hi Matt: I´d like to know your opinión about two measurements made by John Webb and Victor Flambaum of the University of New South Wales in Sydney and by Lamoreaux along with colleague Justin Torgerson in Los Álamos.

    The first team found out how the light from distant quasars was absorbed by gas clouds. They claimed that Alpha constant had increased by a few parts in 10 raised to the power 5 in the past 12 billion years. It means that the value for the speed of light is not constant. The second team analyzed the Oklo natural reactor data for the energy spectrum of the neutrons present in the reactor. In this case the Alpha constant has decreased by more than 4.5 parts in 10 raised to the power 8. It also means that the value for the speed of light has not been steady through time.

    On the other hand, space time does not have a constant curvature but fluctuates. According to the uncertainty principle is not possible to measure those fluctuations, so in this perspective would be difficult to set up a universal value for the speed of light. What do you think about?

    1. You have to be careful about two aspects of this.

      First of all, this is not the first time such claims have been made — by some of the same people. They are good scientists, but all radical scientific claims need to be confirmed, directly or indirectly, by a number of other experiments or observations. This has not happened; efforts to find evidence for the same effect, by others, has failed. One must therefore be very suspicious of the results (not of the scientists, mind you; I’m confident they’re doing their best.)

      Second, your statement that “the value of the speed of light has not been steady through time” would not necessarily be true even if one of the two results you mention were correct. All you can say is that a certain combination of `constants’ has varied; to show that it is the speed of light that has changed requires showing that none of the other constants has been changing, and the data is not sufficient for that.

      However, I do not want to sound overly discouraging. Scientists must continue to pursue efforts of this type. It should not be assumed that things we call “constants” actually are. Still, it is amazing how constant they have been; galaxies a billion light years away, and therefore a billion years old as we see them today, are apparently behaving according to the same laws, with the same constants, as do nearby galaxies, or our own. The effects that these scientists claim to see are tiny; and if they are wrong, then any such effects must be even tinier. This is one of the mysterious aspects of the universe, potentially explained by the theory of “inflation” (which suggests that the entire visible part of the universe was once a very small region, and was inflated to enormous size by the same type of gravitational effect that now causes the expansion of the universe to accelerate.)

      Now “According to the uncertainty principle is not possible to measure those fluctuations [of space-time], so in this perspective would be difficult to set up a universal value for the speed of light. What do you think about?” Indeed, the speed of light is only a straightforward concept in contexts where the quantum fluctuations of space-time are small — as they are in all of daily life and in almost all cosmological contexts during the past 13.6 billion years. When you approach situations where quantum fluctuations of space-time are large, you need a quantum theory of gravity to understand how to treat this issue. In string theory, a candidate theory of quantum gravity, there is no known problem with extending Einstein’s speed limit into the regime where quantum gravity is important — and what this means practically is that there is no sign that the basic geometry of space-time is distorted so badly by quantum fluctuations that the usual notions of locality and causality are disrupted. But it is hard to prove this it the case, because of the technical difficulties of the theory’s equations. And of course we don’t know string theory is the right theory of quantum gravity. So I think that the question of how to meld Einstein’s speed limit into the quantum gravity regime is still open.

      But you should not conclude that there is any problem with using Einstein’s speed limit where it applies, any more than there is a problem using Newton’s laws in the regimes where they are expected to work. In any physical process where quantum gravity is unimportant, Einstein’s speed limit is expected to be valid without any complications. And that includes all measurements that we are currently capable of making — at present we have no means of studying quantum gravity except inside our heads.

  13. Great article as ever, but I’m a little confused. In your previous neutrino article, you say that neutrinos do oscillate. In this article you say that it was an open question whether anti-neutrinos oscillate. Were we expecting anti-neutrinos to behave differently from neutrinos? Or were we uncertain about the extent of the oscillation?

    1. Oh, great question. The issue is who oscillates into who, and how much.

      Anti-neutrinos are not expected to behave differently from neutrinos, but of course one should double check this. It is also unclear whether neutrinos are their own anti-particles or not; here I say electron-anti-neutrino because that’s what is emitted along with an electron, as opposed to processes where a positron and an electron-neutrino are emitted, but the two particles might not, in fact, be distinct. There are plenty of things still to be learned about these particles.

      1. A guy my officemate knows at Fermilab said the same thing when we asked him, namely:

        “… neutrinos may be their own anti-particles. If that is the case, an anti-neutrino would be a neutrino with it’s spin aligned oppositely to that of the neutrino. (sent via my iPhone)”

        Apparently even particle physicists use iPhones.

        Anyway, if that were true, does that make them unique as far as anti-particles go?

        1. I know some particle physicists who stood on line to get an iPhone. Some of us (not me, and not a majority) are gadget freaks. But almost everyone in the field has a smart phone; I use its smart features all the time.

          A particle can only be its own anti-particle if it is electrically neutral. Photons, Z particles, and presumably Higgs particles are their own antiparticles; so would be gravitons, assuming (as everyone does, for very good reasons) that they exist too.

          However, neutrons, though electrically neutral, are not their own anti-particles. Anti-neutrons can be distinguished from neutrons; they have very different interactions with matter (in particular, with protons) than neutrons do.

          So the question with neutrinos is open. It could go either way.

      2. Matt,

        A little off-topic, but your reply that mentioned gravitons reminded me of my curiosity on that topic. I was wondering if you had plans to write an article on gravitons in the near future?

        1. There’s not much to say about gravitons. They almost certainly exist. They are to gravitational waves as photons are to electromagnetic waves. It may be centuries before we isolate them. They interact with all particles, with a strength related to a particle’s energy and momentum. That’s kind of the whole story, so far…

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