CP (charge conjugation – parity) is a symmetry that relates particles to antiparticles. CP violation occurs when there is a difference in the way that particles and antiparticles interact. It is believed to be necessary to explain the excess of matter over antimatter observed in the universe.

 

CP violation was discovered by the American physicists James Watson Cronin (1931–2016) and Val Logsdon Fitch (1923–2015) as a result of an experiment in 1964 which examined the decay of kaons (see meson) and showed that a reaction run in reverse does not retrace exactly the path of the original reaction. For this discovery Cronin and Fitch were awarded the 1980 Nobel Prize in Physics.

 

CP violation in kaons

When kaons decay, they break into a charged pion, a neutrino and either an electron or its antimatter counterpart, a positron. In the absence of CP violation, the number of electrons and positrons created in these decays would be equal. However, scientists have observed that the scales tip slightly toward decay into electrons. This provides proof that CP violation can lead to an excess of matter over antimatter. If this process occurred in the early universe, all of the positrons would annihilate upon encountering electrons. But after all of the positrons had disappeared, some matter would remain.

 

This result gives credence to the theory that CP violation allowed all of us to exist. But the effects of the process that causes an excess of matter from kaon decays are too small to complete the picture. The observed difference is orders of magnitude away from explaining asymmetry in the universe. This is one of the reasons that scientists are so interested in observing CP violation in other places, like neutrinos.