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.

RT Journal
T1 KL → 2μ DECAY
A1 Dolgov,A.D.
A1 Zakharov,V.I.
A1 Okun’,L.B.
PB Physics-Uspekhi
PY 1973
FD 10 Apr, 1973
JF Physics-Uspekhi
JO Phys. Usp.
VO 15
IS 4
SP 404-415
DO 10.1070/PU1973v015n04ABEH004989
LK https://ufn.ru/en/articles/1973/4/c/