“I know all about neutrinos, and my friend here knows about everything else in astrophysics.” -John Bahcall
Neutrinos are the most poorly understood particles in the standard model. Remember the standard model?
The standard model of elementary particles tells us what the fundamental constituents of matter and forces are in our Universe. We have the force carriers — photons, gluons, W’s and Z’s, and the Higgs — that are responsible for every force in the Universe except gravity. We’ve got the six quarks (up, down, strange, charm, bottom, and top), that are responsible for the proton, the neutron, and nearly all of the normal matter in the Universe. We’ve got three charged leptons: the electron being the most common, as well as its two heavier cousins, the muon and the tau. And we’ve got three neutral, light leptons: the neutrinos. The standard model is all of these particles and their antimatter counterparts, which have the same mass but the opposite charge.
Neutrinos (and their anti-matter counterparts, antineutrinos) show up in many cases, such as whenever you have radioactive beta decay (above). The standard model tells us that, for instance, whenever we turn a neutron into a proton, we also need to spit out an electron (to conserve charge) and an antineutrino (to conserve lepton number). We can also go the other way, and turn to the Sun.
The first step of nuclear fusion in a star is to take two protons, collide them together and add energy, and produce a deuteron (a proton and neutron bound together). To do that, we’ve got to conserve charge, which means spitting out a positron (an antielectron), and to conserve lepton number, which means spitting out a neutrino.
And that’s what we expect the Sun to do: to spit out a bunch of neutrinos. In fact, since we can calculate how many neutrinos the Sun ought to be producing at any given time (based on the amount of energy emitted by the Sun), it was very exciting for us when we realized that we could detect them!
Only, there was a problem. Based on how well we’re able to detect them, we knew how many we were supposed to see. And what we actually saw was only one-third of what we expected!
Now, remember our standard model. We have three different types of neutrino: the electron-neutrino, the muon-neutrino, and the tau-neutrino. The Sun only makes electron neutrinos, which is the only type we were looking for. But there’s a theory that goes beyond the standard model, that says that if neutrinos have mass, they can change from one species into another!
This idea is called neutrino oscillations! And when we started examining how neutrinos from the Sun oscillate (solar neutrinos), and how neutrinos from cosmic rays (passing through the atmosphere, or atmospheric neutrinos) oscillate, we can learn what the mass difference is between these three different types of neutrino!
And everything seems well and good. Atmospheric neutrinos tell you one type of mixing, while solar neutrinos tell you another type. From this, we can infer some useful information about the masses of the three neutrinos, and everything is consistent.
But then this other experiment comes along, and tries to shatter our simple picture.
It’s actually kind of old; back in the late 90’s, an experiment called LSND — Liquid Scintillator Neutrino Detector — sought to measure neutrino oscillations from antineutrinos emitted from a radioactive source. And they did measure it! And — for about 10 years — everyone thought there was something wrong with their data.
Why? Because it both looks and sounds crazy. The data they got was inconsistent with there being only three neutrinos (as we see from the decay of the Z-boson), and with cosmological models of the Universe (that place tight bounds on the total mass of neutrinos).
So along came another experiment, MiniBooNE, which tried to verify or refute LSND. And when they did almost the same nuclear decay experiment — for neutrinos instead of antineutrinos — they found results that were consistent with the Solar and Atmospheric neutrino oscillations, and which were inconsistent with LSND. Case closed, right?
Except, just to check, they decided to re-do the experiment using antineutrinos. Now — and this is important — it shouldn’t matter! Some of the most important conservation laws we have tell us that particles and antiparticles should have the same lifetimes and masses, and this should hold true for neutrinos and antineutrinos.
But that isn’t what the data says. In fact, the antineutrino data agrees with LSND!
The big thing that this slide shows is that there really is this weird effect, saying that something bizarre is going on in the neutrino sector! And it’s the same thing that LSND saw!
What is it?
Well, there are a lot of ideas. There could be an extra, “sterile” neutrino out there, although cosmology places tight restrictions on that. There could be a fundamental difference between neutrinos and anti-neutrinos, which we don’t (at present) understand at all. Or there could be some physics that’s completely off the radar that explains this, but it looks like the good ol’ standard model (and the simplest modifications to it) is woefully inadequate to explain what we’re seeing.
This is still, of course, preliminary data, and isn’t significant enough to bet the farm on. But if these results hold up over time (and accumulation of more data), this could point us in a nu new direction, telling us that this is where our current understanding of physics breaks down! Check out Mark’s take on this (he was at the talk) as well as Sarah’s thoughts.
At this point, I’d personally want to see better statistical significance before I believe this result, but it’s definitely suggestive enough that I want to know what’s behind this apparent neutrino/antineutrino asymmetry!