Neutrinos and the 2015 Nobel Prize in Physics - Sixty Symbols

It is the Nobel Prize news, and I was waiting, waited like yesterday evening, nothing. Then I realised they don't make the phone call till really close to the announcement, so charged up my phone, but still nothing. Yeah, wait, it's written on the whiteboard behind you. Say their name. Takaki Kajita and Arthur MacDonald are the two people. One of them is called, I've forgotten his name. Well, I guess the first thing to say is there are a lot of physicists out there. I don't know them personally, no, no. Have you heard of them before? No, I hadn't heard of them. And the second thing to say is that actually it's the nature of the subject that we quite often know what's going on, but we don't necessarily know who the people are who are associated with what's going on. So Takaki Kajita, you could probably surmise he's in charge of the Super Kamiokande detector in Japan, and Arthur B MacDonald, who is in charge of the Sudbury Neutrino Observatory in Sudbury, Canada. It must be a very difficult thing to do, just because, especially with these big experimental projects, there are many people involved. And it's not like, you know, when you've got the Higgs boson, you can point to Higgs. But here, the science behind it had lots of people involved in it, the experiments had lots of people involved in it. I guess, generally speaking, you try and figure out who was the person who really made it happen, and that's the person you try and give the prize to. My assumption is that they were both highly influential, if not pioneers, in determining each of their own experiments. How the layout of the experiment, what the experimental aims would be, and then they were in charge of it, and they probably led the science direction. It was for demonstrating that neutrinos have mass. Many of us have heard of neutrinos, they're part of the standard model of particle physics. Neutrinos are these pathetic little particles that are hardly there at all. They're elusive. They only interact via the weak interaction, so they don't have any charge, they don't have any electromagnetic interactions, they don't have any strong interactions. So it means that every second through, say, if I take my thumbnail here... And so you can have, you know, a million of them passing through your little finger every second, because the sun's producing them in huge numbers, and that's the number from the sun that are going through your little finger every second, and you're completely unaware that they're there. And for a long time, these particles were thought to be massless. They went at the speed of light. But more recently, there have been sort of indications that they might have mass, and these really are the experiments that have demonstrated, at least indirectly, that there must be mass in neutrinos. It caught me by surprise. I wasn't expecting the neutrino oscillations to win. But when they did, it brought back a very specific memory, in that I actually remember the day that the super cameo candy results came out in 1998. And I remember it because I was in Cambridge doing my PhD, and it was one of those days where there was just a buzz. You know, people came out in the hallway and said, Oh, did you hear? We discovered neutrino oscillations, and people were talking about it. And there was so much of a buzz that it was decided that someone would give a special seminar just so we could all get together, discuss the results and what their significance was. So Douglas Goff, one of the professors, gave a seminar on neutrino oscillations, and it just so happens that I actually had the notes from that seminar, because it was back in the days where I actually took very comprehensive notes in seminars because I was a very conscientious PhD student. In the standard model, there are quarks and there are leptons, and in the leptons, there are three flavours. There's the electron, the muon, and the tau. And associated with those three flavours, there are neutrinos. So there's the electron neutrino, the muon neutrino, and the tau neutrino, and they've basically got the same properties, except with the electron, the muon, and the tau, the masses increase as they go from electron to muon to tau. You would normally think of a particle as just being it. It's an electron, once an electron, always an electron. Maybe it will annihilate and create photons, but you don't think of it as just switching to something else. For quite a long time, there have been a series of experiments looking at neutrinos from the sun, because the nuclear reactions in the centre of the sun are producing loads and loads of neutrinos. If you try hard enough, you ought to be able to detect them down on Earth. There'd been this sort of long-running problem that the detectors on Earth weren't detecting as many neutrinos as they ought to. They were detecting about a third as many as all the theories of what should be going on in the centre of the sun told you should be created. And so that either meant that our understanding of what's going on in the middle of the sun is a little bit off, or it means that something's happening to the neutrinos, but we weren't really quite sure which. It could be some fairly subtle piece of the nuclear fusion reactions going on in the middle of the sun meant that we were kind of messing things up. Ponte... Ponte... Ponte... Ponte Carlo. Bruno. Bruno. He realised that, actually, they could actually begin to change between themselves. So the electron could maybe turn into a muon, and the muon into an electron, or a muon into a tau neutrino, and a tau neutrino into a muon neutrino. And he developed an understanding of how this could happen. The key thing that this then meant was that we had a way of beginning to understand why is it that we're not seeing all these neutrinos from the sun, as many as we thought there should be, because these are electron neutrinos from the sun. So if those electron neutrinos, as they propagate to the Earth, are converting into something else, then we won't see them as electron neutrinos on the Earth, so we'll see a reduced number. What this Sudbury experiment did is it detected the same deficit in the electron neutrinos that all the other experiments had detected. So it showed that, actually, yes, those neutrino electrons being produced by the sun aren't there in the numbers they should be there in. But because it also detected all the other neutrinos as well, it was able to say, but actually they're all there, and it looks like those electron neutrinos have been converted into the mu neutrinos and the tau neutrinos. So the total number of neutrinos is right, even though the number of electron neutrinos is down. And that, again, is a very strong piece of evidence that, actually, if the total number is right but one particular flavour is wrong, that means somehow that particular type of neutrino must have been turned into the other types of neutrino. This is the notebook that I used to take notes and seminars in in the very beginning of my PhD. It didn't last very long. These are notes from the 18th of June, 1998, on a seminar on neutrino masses. And this was sparked by the announcement of the detection of neutrino oscillations by Superkamya Kendi in Japan. So I can imagine this was happening in other universities, other physics departments around the world. Maybe not a formal seminar. We happened to have an expert who could stand up and talk for an hour on the background and the implications. But when something like this comes out, very often you'll bother the person next door who knows more about it than you do, or maybe gather in a little group and just really hash over what it means. These neutrinos are indeed changing flavours as they go from one to another. This has a massive knock-on effect, massive being the appropriate word, because now the only way this can happen is that the neutrinos themselves have a mass. So they are not moving at the speed of light, they've got a mass, they're moving very close to the speed of light. If you start finding out that neutrinos actually have mass, this takes you into this thing that particle physicists call physics beyond the standard model. We've now gone beyond the standard model of particle physics. We have to now begin to understand the origin of the mass of the neutrinos, and this remains something that we're all trying to get our head around. We still don't know the individual masses of these neutrinos, but we know that they have got a mass. Very occasionally the neutrino will bump into one of the chlorine atoms in this carbon tetrachloride, this cleaning fluid, and transmute that chlorine atom into an argon atom. And all the particles are created in this big bone. Well, I imagine that, I wasn't there. And so you get a huge number of electrons and neutrinos.