CP violation

Look at the universe in a mirror and swap every particle for its antiparticle: if the laws of physics were perfectly even-handed, the reflected world would obey exactly the same rules. For most of physics it does. But in certain rare weak decays it does not, matter and antimatter behave subtly differently. This tiny asymmetry, called CP violation, may be the reason you exist at all, because it is one of the ingredients needed to explain why the universe is made of matter and not an equal-parts mix that would have annihilated to nothing.

CP is the combination of two symmetries: C, charge conjugation, which swaps every particle for its antiparticle, and P, parity, which reflects space like a mirror. CP violation means a process and its mirror-reflected, particle-antiparticle-swapped counterpart occur at different rates. It was discovered in 1964 by Cronin and Fitch in the decays of neutral kaons, which decayed in a way that should have been forbidden if CP were exact. In the Standard Model, CP violation arises from a single irreducible complex phase in the CKM matrix that describes how quarks of different generations mix under the weak interaction, which is why CP violation requires at least three generations of quarks, a connection pointed out by Kobayashi and Maskawa before the third generation was even found.

CP violation is one of the three Sakharov conditions that any mechanism must satisfy to generate a matter-antimatter imbalance in the early universe. Here lies an honest and unsolved tension: the CP violation present in the Standard Model's CKM matrix is far too small to account for the observed excess of matter over antimatter, by many orders of magnitude. So while CP violation is real and measured, its Standard Model source is insufficient, and additional, undiscovered CP violation, perhaps in the neutrino sector, perhaps beyond the Standard Model entirely, is one of the most active frontiers in physics.