Will A New Neutrino Change The Standard Model?

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Since the discovery of the Higgs boson, physicists have searched and searched for any hint of new particles. That search has been fruitless, until perhaps now.

Today on Space Time Journal Club, we'll look at a paper that reports a compelling hint of a new particle outside the standard model, the sterile neutrino.

Regular neutrinos are a bit aloof. They don't interact by the electromagnetic or strong nuclear forces, only by the weak nuclear force and gravity. They are so weakly interacting that they pass through matter like it isn't there. To have a 50-50 chance of stopping any given neutrino, you'd need a wall of lead one light-year thick.

If regular neutrinos are aloof, then sterile neutrinos are complete loners. They don't even interact via the weak interaction. Even so, detection of a sterile neutrino would be incredibly important. Besides being the first expansion of the standard model family since the Higgs boson, sterile neutrinos are a candidate for dark matter. And their existence would have had a huge influence on the expansion of the early universe. We can detect regular neutrinos by watching for the rare interaction between a neutrino and an atomic nucleus in some huge volume of matter. An entire glacier in the ice cube experiment, or a huge vat of oil in the experiment we're about to discuss. Those regular neutrinos are spotted when they interact with matter via the weak nuclear force.

So, how on earth do you spot a sterile neutrino that doesn't even undergo that interaction? Well, by being extremely clever, obviously. This may have been achieved, as reported in the 2018 paper Observation of a Significant Excess of Electron-Like Events in the MiniBoon Short Baseline Neutrino Experiment. Catchy name. Before we get to that, we're going to need to go a few inception layers deep to set up the knowledge. We're going to drop through the standard model of particle physics, electric charge and anti-matter, the bizarreness of quantum chirality and the Higgs mechanism, and finally, to why all of this points to sterile neutrinos. Hold on to your butts. First, the standard model of particle physics. As we'll see in upcoming episodes, these particles are divided into the bosons, which carry the fundamental forces, and the fermions, which comprise matter. The latter include the quarks, up-down, which comprise protons and neutrons, and the more exotic top, bottom, strange and charm, as well as the antimatter versions of these. And then there are the leptons, the ubiquitous electron and its heavier cousins, the muon and taon, and again, each with its antimatter counterpart. Neutrinos are also leptons, and each of the heavier lepton flavors has a neutrino version. These have far lower mass, and unlike quarks leptons, they have no electric charge, hence neutrino, or little neutral one.

Each flavor of neutrino also has an antimatter counterpart. So let's drop down into antimatter. An antimatter version of a particle has the same mass and the opposite electric charge. So an electron has a charge of negative one, and an anti-electron has a charge of plus one. Neutrinos don't have charge, so what's the difference between a neutrino and an anti-neutrino? Well, there's actually another property that gets reversed in antiparticles, and that brings us to the next level, chirality. The physical interpretation of chirality is pretty abstract. To explain, we need to start with helicity. Helicity is just the direction of a particle's spin relative to its direction of motion. Helicity can be right-handed, which means clockwise rotation, or left-handed, or anti-clockwise. Like helicity, chirality can be...

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