CP symmetry

Charge conjugation (C) swaps every particle for its antiparticle; parity (P) holds the world up to a mirror. On their own, the weak force breaks both. But what if you do both at once — turn matter into antimatter and reflect it in a mirror at the same time? That combined operation is called CP, and for a long time it looked as though nature might respect this double flip even though it shamelessly violated each piece separately.

Here is why CP felt like the better symmetry. The weak force produces only left-handed neutrinos and only right-handed antineutrinos. Apply C alone to a left-handed neutrino and you get a left-handed antineutrino — which does not exist, so C fails. Apply P alone and you get a right-handed neutrino — which also does not exist, so P fails. But apply both, C and P together, and a left-handed neutrino becomes a right-handed antineutrino — and that particle does exist. So CP neatly maps a real particle to another real particle, and for most processes it holds. CP symmetry says, in effect, that the laws governing matter are the same as the laws governing antimatter once you also account for the mirror.

And yet, in 1964, CP was found to be violated too — by a tiny amount, first in the decays of neutral kaons, later confirmed in B mesons. Matter and antimatter are not perfect mirror images after all; the universe has a faint built-in preference. This small CP violation is one of the most consequential facts in particle physics, because it is one of the necessary conditions for the universe to have ended up made of matter rather than equal parts matter and antimatter. The amount of CP violation in the Standard Model, though, appears far too small to fully explain that cosmic imbalance — a major open puzzle.