Which particle physics symmetry violation earned Cronin and Fitch the Nobel?

Physicists had assumed that the universe cannot tell matter from antimatter. Swap every particle for its antiparticle and reflect everything in a mirror - the operations called C and P - and the laws ought to come out identical. In 1964 James Cronin and Val Fitch, working at Brookhaven with neutral kaons, found that they do not. About one decay in five hundred came out the way it was not supposed to. They received the Nobel Prize in 1980.

The reason this matters is not bookkeeping. If matter and antimatter were made in exactly equal amounts at the Big Bang they would have annihilated each other completely and left a universe of light and nothing else. Andrei Sakharov set out in 1967 the conditions under which a surplus of matter could survive, and one of them is precisely that the laws must treat matter and antimatter slightly differently. CP violation is why there is anything.

The uncomfortable part is the size. The CP violation built into the Standard Model, which is now measured with great precision in kaons and B mesons, is far too small - by many orders of magnitude - to account for the amount of matter actually present. Something else must be going on, and finding it is one of the open problems the field is built around.