Search for heavy neutrinos mixing with tau neutrinos

We report on a search for heavy neutrinos (upsilon (4)) produced in the decay D-s --> tau upsilon (4) at the SPS proton target followed by the decay upsilon (4) --> upsilon (tau)e(+)e(-) in the NOMAD detector. Both decays are expected to occur if upsilon (4) is a component of upsilon (tau). From the analysis of the data collected during the 1996-1998 runs with 4.1 x 10(19) protons on target, a single candidate event consistent with background expectations was found. This allows to derive an upper limit on the mixing strength between the heavy neutrino and the tau neutrino in the upsilon (4) mass range from 10 to 190 MeV. Windows between the SN1987a and Big Bang Nucleosynthesis lower limits and our result are still open fur future experimental searches. The results obtained are used to constrain an interpretation of the time anomaly observed in the KARMEN1 detector.