Laboratory constraints on a 33.9 MeV/c(2) isosinglet neutrino: Status and perspectives

An anomaly in the time behaviour of the signals observed by the KARMEN Collaboration may be interpreted as the possible decay signature of a 33.9 MeV/c(2) mainly sterile neutrino. This note discusses the parameter space still open for the mixing of this hypothetical particle with the neutrinos of known leptonic flavour, considering the experimental results which became available recently, as well as those to be expected from forthcoming measurements. It is concluded that if no positive signature is observed, the envisaged laboratory experiments are not expected to close entirely the parameter space of mixing amplitudes. However, a proper reassessment of the ALEPH upper bound on the nu(tau) neutrino mass including the possibility of tau flavour mixing would certainly help in reducing the parameter space left open.