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

TL;DR

This paper reanalyzes past CERN beam dump data to set new constraints on heavy neutral leptons, especially those heavier than kaons and mixing with tau neutrinos, providing the most stringent bounds to date.

Contribution

It offers the first comprehensive recast of the 1982 CERN experiment focusing on heavy neutral leptons with tau neutrino mixing, extending constraints up to the tau mass.

Findings

Established new bounds on heavy neutral leptons with tau neutrino mixing.

Provided updated constraints on mixing with electron neutrinos.

Achieved the strongest limits to date for certain heavy neutral lepton masses.

Abstract

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.