In the past months we’ve talked a lot about topics that receive more attention than they deserve. Today I want to talk about a topic that doesn’t receive the attention it deserves. That’s a 20 years old anomaly in neutrino physics which has been above the discovery threshold since 2018, but chances are you’ve never even heard of it. So what are neutrinos, what’s going on with them, and what does it mean? That’s what we’ll talk about today.
I really don’t understand why some science results make headlines and others don’t. For example, we’ve seen loads of headlines about the anomaly in the measurement of the muon g-2 and the lepton anomaly at the Large Hadron Collider. In both of these cases the observations don’t agree with the prediction but neither is statistically significant enough to count as a new discovery, and in both cases there are reasons to doubt it’s actually new physics.
But in 2018, the MiniBooNE neutrino experiment at Fermilab confirmed an earlier anomaly from an experiment called LSND at the Los Alamos National Laboratory. The statistical significance of that anomaly is now at 6 σ. And in this case it’s really difficult to find an explanation that does not involve new physics. So why didn’t this make big headlines? I don’t know. Maybe people just don’t like neutrinos?
But there are lots of reasons to like neutrinos. Neutrinos are elementary particles in the standard model of particle physics. That they are elementary means they aren’t made of anything else, at least not for all we currently know. In the standard model, we have three neutrinos. Each of them is a partner-particle of a charged lepton. The charged leptons are the electron, muon, and tau. So we have an electron-neutrino, a muon-neutrino, and a tau-neutrino. Physicists call the types of neutrinos the neutrino “flavor”. The standard model neutrinos each have a flavor, have spin ½ and no electric charge.
So far, so boring. But neutrinos are decidedly weird for a number of reasons. First, they are the only particles that interact only with the weak nuclear force. All the other particles we know either interact with the electromagnetic force or the strong nuclear force or both. And the weak nuclear force is weak. Which is why neutrinos rarely interact with anything at all. They mostly just pass through matter without leaving a trace. This is why they are often called “ghostly”. While you’ve listened to this sentence about 10 to the fifteen neutrinos have passed through you.
This isn’t the only reason neutrinos are weird. What’s even weirder is that the three types of neutrino-flavors mix into each other. That means, if you start with, say, only electron-neutrinos, they’ll convert into muon-neutrinos as they travel. And then they’ll convert back into electron neutrinos. So, depending on what distance from a source you make a measurement, you’ll get more electron neutrinos or more muon neutrinos. Crazy! But it’s true. We have a lot of evidence that this actually happens and indeed a Nobel Prize was awarded for this in 2015.
Now, to be fair, neutrino-mixing in and by itself isn’t all that weird. Indeed, quarks also do this mixing, it’s just that they don’t mix as much. That *neutrinos mix is weird because neutrinos can only mix if they have masses. But we don’t know how they get masses.
You see the way that other elementary particles get masses is that they couple to the Higgs-boson. But the way this works is that we need a left-handed and a right-handed version of the particle, and the Higgs needs to couple to both of them together. That works for all particles *except the neutrinos”. Because no one has ever seen a right-handed neutrino, we only ever measure left-handed ones. So, the neutrinos mix, which means they must have masses, but we don’t know how they get these masses.
There are two ways to fix this problem. Either the right-handed neutrinos exist but are very heavy, so we haven’t seen them yet because creating them would take a lot of energy. Or the neutrinos are different from all the other spin ½ particles in that their left- and right-handed versions are just the same. This is called a Majorana particle. But either way, something is missing from our understanding of neutrinos.
And the weirdest bit is the anomaly that I mentioned. As I said we have three flavors of neutrinos and these mix into each other as they travel. This has been confirmed by a large number of observations on neutrinos from different sources. There are natural sources like the sun, and neutrinos that are created in the upper atmosphere when cosmic rays hit. And then there are neutrinos from manmade sources, particle accelerators and nuclear power plants. In all of these cases, you know how many neutrinos are created of which type at what energy. And then after some distance you measure them and see what you get.
What physicists then do is that they try to find parameters for the neutrino mixing that fit to all the data. This is called a global fit and you can look up the current status online. The parameters you need to fit are the differences in masses which determines the wavelength of the mixing and the mixing angles, that determine how much the neutrinos mix.
By 2005 or so physicists had pretty much pinned down all the parameters. Except. There was one experiment which didn’t make sense. That was the Liquid Scintillator Neutrino Detector, LSND for short, which ran from 1993 to 98. The LSND data just wouldn’t fit together with all the other data. It’s normally just excluded from the global fit.
In this figure, you see the LSND results from back then. The red and green is what you expect. The dots with the crosses are the data. The blue is the fit to the data. This excess has a statistical significance of 3.8 \sigma. As a quick reminder, 1 \sigma is a standard deviation. The more sigmas away from the expectation the data is the less likely the deviation is to have come about coincidentally. So, the more \sigma, the more impressive the anomaly. In particle physics, the discovery threshold is 5 \sigma. The 3.8 sigma of the LSND anomaly wasn’t enough to get excited, but too much to just ignore.
15 years ago, I worked on neutrino mixing for a while, and in my impression back then most physicists thought the LSND data was just wrong and it’d not be reproduced. That’s because this experiment was a little different from the others for several reasons. They detected only anti-neutrinos created by a particle accelerator and the experiment had a very short baseline of only 30 meters, shorter than all the other experiments.
Still, a new experiment was commissioned to check on this. This was the MiniBooNE experiment at Fermilab. That’s the Mini Booster Neutrino Experiment and it’s been running since 2003. As you can tell by then the trend of cooking up funky acronyms had taken hold in physics. MiniBooNE is basically a big tank full of mineral oil surrounded with photo-detectors which you see in this photo. The tank waits for neutrinos from the nearby Booster accelerator, which you see in this photo.
For the first data analysis in 2007, MiniBoone didn’t have a lot of data and the result seemed to disagree with LSND. This was what everyone expected. Look at this headline from 2007 for example. But then in 2018 with more data MiniBooNE confirmed the LSND result. Yes, you heard that right. They confirmed it with 4.7 σ, and the combined significance is 6 σ.
What does that mean? You can’t fit this observation by tweaking the other neutrino mixing parameters. There just aren’t sufficiently many parameters to tweak. The observations is just incompatible with the standard model. So you have to introduce something new. Some ideas that physicists have put forward are symmetry violations, or new neutrino-interactions that aren’t in the standard model. There is also of course still the possibility that physicists misunderstand something about the experiment itself, but given that this is an independent reproduction of an earlier experiment, I find this unlikely. The most popular idea, which is also the easiest, is what’s called “sterile neutrinos”.
A sterile neutrino is one that doesn’t have a lepton associated with it, it doesn’t have a flavor. So we wouldn’t have seen it produced in particle collisions. Sterile neutrinos can however still mix into the other neutrinos. Indeed, that would be the only way sterile neutrinos could interact with the standard model particles, and so the only way we can measure them. One sterile neutrino alone doesn’t explain the MiniBooNE/LSND data though. You need at least two or more, or something else in addition. Interestingly enough, sterile neutrinos could also make up dark matter.
When will we find out. Indeed, seeing that the result is from 2018, why don’t we know already. Well, it’s because neutrinos… interact very rarely. This means it takes a really long time to detect sufficiently many of them to come to any conclusions.
Just to give you an idea, the MiniBooNe experiment collected data from two thousand and two to two thousand and seventeen. During that time they saw an excess of about five hundred events. 500 events in 15 years. So I think we’re onto something here. But glaciers now move faster than particle physics.
This isn’t a mystery that will resolve quickly but I’ll keep you up to date, so don’t forget to subscribe.