SEARCH FOR THE HYPOTHETICAL PI-]MU-X-DECAY

The KARMEN collaboration has reported the possible observation of a hi therto unknown neutral and weakly interacting particle x, which is pro duced in the decay pi(+) --> mu(+)x with a mass m(x) = 33.9 MeV. We ha ve searched for this hypothetical decay branch by studying muons from pion decay in flight with the LEPS spectrometer at the pi E3 channel a t PSI and find branching ratios BR(pi(-) --> mu(-) (x) over bar) < 4 x 10(-7) and BR(pi(+) --> mu(+)x) < 7 x 10(-8) (95% C.L.).