ANOMALY IN THE TIME DISTRIBUTION OF NEUTRINOS FROM A PULSED-BEAM STOPSOURCE

Analysis of the charged and neutral current reactions C-12 (nu(e),e(-) ) N-12 and C-12 (nu,nu') C-12 induced by neutrinos from pi(+)- and mu (+)-decays at rest reveals an anomaly in the time distribution after a ll pi(+) have decayed: the measured time constant for subsequent event s differs substantially from the value of 2.2 mu s corresponding to th e mu(+) lifetime. This anomaly cannot currently be explained by backgr ound processes or errors in the experimental set-up. A satisfactory de scription of the time spectrum is achieved by assuming it has two comp onents, one exponential with a 2.2 mu s time constant, the other a Gau ssian signal of 83 +/- 28 events at 3.6 mu s after beam-on-target. A s peculative explanation, but one fully consistent with ah the data, is that these delayed events originate from the decay of a slowly moving (beta similar to 0.02) massive neutral particle produced in the beam s top. Further measurements to improve statistical significance are nece ssary.