Question: why do you think neutrinos might hold the key to our understanding of why the earth is made up of matter and not antimatter?

Susan Cartwright answered on 20 Jun 2015:

There is a set of three conditions, first laid down by the Russian theoretical physicist and political dissident Andrei Sakharov, that must be satisfied for the big bang to generate an asymmetry between matter and antimatter (which it clearly managed to do, given that there doesn’t appear to be any primordial antimatter in our universe). One of these is known in the jargon as “C and CP violation”, which basically means that there has to be a measurable difference in the reactions of particles and antiparticles.

In the quark sector, there are some reactions which have this property, specifically decays of the K0 (bound state of d and anti-s quarks) and anti-K0 (s and anti-d) mesons, and the equivalent mesons with b quarks replacing the s quarks, which are called B0 and anti-B0. However, the amount of difference, which is well known, is much much too small (by many powers of 10) to create a large enough asymmetry to produce what we see.

The only other place in the Standard Model where there is a good chance of reactions existing that distinguish between particles and antiparticles in the necessary way is the neutrinos, and so far, though we haven’t seen an effect directly, the necessary conditions all seem to be there (for example, it would not work if the three different types of neutrinos didn’t all mix together – for a long time it appeared that there might be only two mixings, not three, but a few years ago our experiment T2K, together with experiments using antineutrinos from nuclear reactors, showed that the third possible mixing (if you have three types, 1 2 and 3, there are three possible mixings, 12, 23 and 31) is also non-zero). So the stage is set for neutrinos to play the starring role. I hope they do. We don’t know yet.