Blast from the past II: Constraints on heavy neutral leptons from the BEBC WA66 beam dump experiment

We revisit the search for heavy neutral leptons with Big European Bubble Chamber in 1982 proton beam dump experiment at CERN, focussing on those heavier than kaon and mixing only tau neutrino, as these are far less constrained their counterparts smaller mass or other mixings. Recasting previous terms of this model including additional production decay channels yields strongest bounds to date, up mass. This applies also our updated electron neutrino.