Atomic Structure and Parity Nonconservation

Parity violation is the important key to observing the weak interaction in atoms. Since all the other forces in nature, so far as known, conserve parity, their effect is to put the atom in a state of definite parity. Any mixing of parity states would then be due to the weak interaction. Furthermore, as discussed above, these weak interactions must involve only the previously unseen neutral current form. 

It turns out that the atom has built into it very good tests to determine if it is in a state of definite parity. A selection rule for electric dipole ( 1) transitions is that the parity of the final state must be opposite from that of the initial state. Therefore, one cannot induce an 1 transition between two electronic energy levels of the same parity. But if these electronic levels have a small admixture of opposite parity state (due, for example, to the 

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