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.
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.
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.
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.