Search for the hypothetical π → μx decay

Abstract

The KARMEN collaboration has reported the possible observation of a hitherto unknown neutral and weakly interacting particle x, which is produced in the decay π + → μ + x with a mass m x = 33.9 MeV. We have searched for this hypothetical decay branch by studying muons from pion decay in flight with the LEPS spectrometer at the πE3 channel at PSI and find branching ratios BR(π − → μ − x) < 4 × 10 −7 and BR( π + → μ + x) < 7 × 10 −8 (95% C.L.).