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