$K_L\to2\mu$ decay

A brief survey of the possible explanations of $K_L\to2\mu$ decay is given. Some of these explanations cannot be rejected on the basis of the existing experiments, and special experiments must be set up to test them. If the problem of $K_L\to2\mu$ decay is not brought about by experimental errors, the most plausible explanation appears to be the existence of a new interaction between kaons and muons.