Answer to Question #125413 in Physics for Bareerah Islam

Question

What do you think of neutral pion decay shows cp violation? Support yours answer by experimental aspects?

Answer 1

The neutral pion is spin 0, it has negative parity P, and positive charge conjugation C, and it decays electromagnetically, mostly into two photons via the anomaly, and the decay does not violate CP. In fact all the known decay modes of neutral pions are, essentially, into two photons and since electromagnetic interactions conserve CP, decays of the neutral pion into three photons are forbidden and are not in fact observed.

There are Dalitz decays in which one of the two photons is internally converted into an electron+positron pair, and there are rarer double Dalitz decays in which both photons are converted. Extremely rarely there is a direct decay of the neutral pion to a single electron+positron pair, but so far as I know, none of the known and observed decays of the neutral pion violates CP.

CP violation is seen in heavier neutral meson systems such as the neutral kaon system, where famously two strongly interacting neutral spin 0 mesons that were heavier than the pions were discovered and were actually seen experimentally to have exactly the same mass, spin and charge, but also to decay into either two or into three pions. This was originally called the τ−θ puzzle. The two particles, based just on mass and spin and electric charge should have been the same particle. But they were clearly not the same. The puzzle was resolved by realizing that the weak interactions do violate both P and CP, and that these two particles were antiparticles of eachother. Later, regeneration of the state decaying into three pions was seen experimentally, that is, oscillations in the neutral kaon system were detected, by Val Fitch and James Cronin, indicating that a complex CP violating phase must exist and that these states were mixtures of CP-even and CP-odd components. The CP-even state decayed to two pions, the CP-odd state decayed to three pions, and two different lifetimes were involved.

These were both weak decays and so the states were unexpectedly long lived for strongly interacting resonances. That led to the nomenclature by which these states were called “strange” mesons.