It was originally assumed that parity must be conserved in all particle interactions, but it was demonstrated that parity does not have to be conserved in beta decay.

Parity is a rather subtle concept and has no classical analogue. It is concerned with the behaviour of wave function under space inversion. Two kinds of parity actually correspond to two different kinds of quantum wave function for a particle.

Symmetric wave function. Wave functions for which the value at the point (-x, -y, -z) is the same as at the point (x, y, z) are known as symmetric wave functions.

Antisymmetric wave function. Wave functions for which the value at the point (-x, -y, -z) is minus the value as at the point (x, y, z) are known as symmetric wave functions.

A parity transformation (also called parity inversion) is the flip in the sign of one spatial coordinate. This is equivalent to studying the mirror image of the original system. It was originally assumed that parity must be conserved in all particle interactions, but it was demonstrated that parity does not have to be conserved in beta decay.

It had been found that the kaon, K (positive or negative), could decay in two different ways, one that produced three pions, the other only two pions. In order to account for this anomaly, Lee and Yang (physicists who were awarded the Nobel prize in 1957) postulated that parity is not conserved in processes of this type. This interaction (the decay of kaon particle) is known as the weak interaction.

Another transformation that is not conserved by the weak interaction is charge conjugation, usually symbolised by the letter C, under which all the particles in a system are replaced by their corresponding antiparticles, without making any other change.

References:

Nuclear and Reactor Physics:

J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).

J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.

W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.