Neutrinos are peculiar particles. They are very common, billions are passing through you all the time. Perhaps it is fortunate then that they very rarely interact with matter, and therefore do you no harm as they pass through. They rarely interact because, uniquely among all the fundamental particles we know of that make up the matter in the universe, they only experience two of the four fundamental forces. The have mass and energy, so they feel the gravitational force, and they also feel the weak force (that is the one carried by W and Z bosons). But they have no charge, so they are invisible to the electromagnetic force; and the strong nuclear force also ignores them.

For that reason – they rarely interact and so are hard to measure – neutrinos are one of the least well known particles in the Standard Model of particle physics. In some ways we already know more about the newly discovered Higgs boson than we do about neutrinos. For instance we know the Higgs mass to within an accuracy of a couple of percent, whereas until the late 1990s we thought neutrinos were massless. Even now that we know neutrinos have mass, we do not know how much, though we know it is very little.

One thing we know about neutrinos is that when they are produced, they are one of three definite types – an electron, muon or tau type. These types are known as flavours. Flavour is just a label for the type of particle they can produce when they interact. So an electron-neutrino can radiate a W boson and turn into an electron, for example. That’s fairly straightforward. But a weird thing, which we also know, is that though there are neutrinos of three different flavours, and neutrinos of three different masses, the correspondence between the masses and the flavours is not straightforward. They mix up. The heaviest neutrino is a mixture of three different flavours. And the electron neutrino is a mixture of three different mass neutrinos. The mixing between them is described by a matrix, with four different parameters that characterise how the mixing happens – basically what proportions of one kind of neutrino make up another.

We can test this model and measure those parameters by producing neutrinos in one place and measuring them in another. This works because the mixture of the different types of neutrino changes with time as a neutrino travels, and the way it changes is fixed by those parameters. The options for producing them include making them with an accelerator, or a reactor, or using the free source of neutrinos provided by cosmic rays hitting the upper atmosphere. (These cosmic rays produce many different kinds of particle, including neutrinos.) Once you have built a neutrino detector (they are very big and very sensitive) you will most likely try to use any neutrinos you can, wherever they come from.

In the past week two neutrino experiments published new and quite extensive analyses of data from the neutrinos they can see. The MINOS experiment uses a beam from Fermilab in Chicago, detectors nearby, and a “far” detector in the Soudan mine in Minnesota, 735km away. They have used neutrinos from their beam and from cosmic rays to constrain the difference in mass between two of the neutrinos. In fact on Tuesday 5 March, when their paper appeared on the arXiv, this was the best measurement of that mass difference. They also rule out various options for combinations of mixing-matrix parameter values, and measure the mixing angle between the second and third heaviest neutrinos. Two days later the T2K experiment published an extensive analysis of the same parameters, with a similar precision to MINOS results in the mass difference, and with a more precise measurement of that mixing angle. T2K measures neutrinos in detectors near the J-PARC facility in Tokai*, Japan (where the neutrinos are produced) and uses SuperKamiokande as its “far” detector.



One of the most important parameters, about which we know very little, is called “delta CP”. If this parameter differs from zero, then interactions involving neutrinos can distinguish matter from antimatter in a way that might provide a clue as to why antimatter is so rare in the universe.

Both set of results show progress on understanding a particularly tricky and important bit of the Standard Model of particle physics. They help (along with results from reactor experiments) to set the stage on which the discussions of future neutrino projects are being planned.

* Originally for some reason I’d said Sendai here … sorry. T2K stands for Tokai to Kamioka.

Jon Butterworth’s book, Smashing Physics, is out on 22 May. You can order it now!