Remarks on the KARMEN anomaly

Abstract

A recently reported anomaly in the time structure of signals in the KARMEN neutrino detector suggests the decay of a new particle x, produced in π + → µ + x with mass m x = 33.9 MeV. We discuss the constraints and difficulties in interpreting x as a neutrino. We show that a mainly-sterile neutrino scenario is compatible with all laboratory constraints, within narrow limits on the mixing parameters, although there are problems with astrophysical and cosmological constraints. This scenario predicts that appreciable numbers of other x-decay events with different origins and time structures should also be observable in the KARMEN detector.Such x-decay events should also be found in the LSND experiment and may be relevant to the search forν µ →ν e oscillations.(Published in Phys. Lett. B352 (1995) 365; erratum added June 1995)The KARMEN collaboration, which studies the interactions of neutrinos from the stopped π + decay chain at RAL, has recently reported an anomaly [1] in the timedependence of their signals. This anomaly suggests the production of a new weaklyinteracting neutral particle (call it x) in the initial π + decays, which travels with well determined velocity β x = v x /c ≃ 1/60 and decays in the detector after a mean flight path of 17.5 m. The distinctive feature of the x-events is their timing, apparently at a well determined interval 3.6µs after the arrival and prompt decay of the pion pulse (which determines β x ); however, the visible energy in the detector scintillator shows no anomaly, so x decays apparently deposit visible energy similar to typical neutrino interactions.The present note briefly discusses the interpretation of x as a massive neutrino. We show that a mainly-sterile neutrino scenario is compatible with all laboratory constraints, for either Dirac or Majorana options, within rather narrow bounds on the mixing parameters. There are some problems with astrophysical and cosmological constraints, but it is interesting nevertheless to explore the further implications for laboratory experiments which can test this interpretation directly. This scenario predicts that appreciable numbers of other x-decay events with different origins and time structures should also be observable in the KARMEN detector. Such x-decay events should also be found in the LSND experiment at LAMPF [2] and may be relevant to the ongoing search forν µ →ν e oscillations [3].If we postulate no other new particles below the pion mass, then the precise time structure [1] requires x production to go via one of the two-body modes π + → µ + x or π + → e + x . However the latter implies mass m x = 137.2 MeV (determined from β x ) and hence anomalously large visible x-decay energy, with mean value < T vis > ≃ 51MeV for x → µeν → eeννν or < T vis > ≃ 88 MeV for x → eeν, compared to neutrino interactions that typically give T vis ∼ 11−35 MeV [1]. We must therefore presume that x is produced via π + → µ + x, with m x = 33.9 MeV determined from β x . Assuming no new weak interactions, the standard decay mode is x → e − e ...

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