Introduction

The KARMEN Collaboration originally reported [1] on a `long-standing discrepancy' between their measured and expected time distributions of neutrino induced reactions originating from neutrinos produced from p⁺ and m⁺ -decays at rest in the primary target of the ISIS facility at RAL. Further evidence of an excess of events over the expected exponential time distribution characterized by the muon lifetime, and clustered around 3.6 ms after beam-on-target was reported more recently [2].

The speculative explanation given in ref. [1] was that a new massive, neutral particle (X) was produced in the rare pion decay process

Various hypotheses have been put forward as to the nature of the X-particle. Barger et al. [3] suggest a mainly isosinglet (sterile) neutrino solution decaying, predominantly into e⁺e^-n_e. They also show the constraints on an isodoublet solution from other experiments but reject this solution on the grounds of cosmological and astrophysical arguments, as well as on the ALEPH n_t mass limit. However, the validity of this latter bound was recently questioned when three generation mixing is considered [4]. Choudary and Sarkar [5] consider a supersymmetric solution where the X-particle is the lightest neutralino and decays radiatively.