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

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 all pi(+) have decayed: the measured time constant for subsequent events differs substantially from the value of 2.2 mu s corresponding to the mu(+) lifetime. This anomaly cannot currently be explained by background processes or errors in the experimental set-up. A satisfactory description of the time spectrum is achieved by assuming it has two components, one exponential with a 2.2 mu s time constant, the other a Gaussian signal of 83 +/- 28 events at 3.6 mu s after beam-on-target. A speculative 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 stop. Further measurements to improve statistical significance are necessary.