Constraints from the CHARM experiment on Heavy Neutral Leptons with tau mixing

Abstract

We re-analyze the results of the searches for Heavy Neutral Leptons (HNLs) by the CHARM experiment. We study HNL decay channel $N\to e^{+}{e}^{-}\nu /{\mu }^{+}{\mu }^{-}\nu$ and show that, in addition to the constraints on the HNL's mixings with ${\nu }_e$ or ${\nu }_{\mu }$, the same data also implies limits on the HNLs that mix only with ${\nu }_{\tau }$ and have masses in the range $380\text{ MeV}<m_N\lesssim 1.6\text{ GeV}$ - the region in the parameter space that was considered in the literature as a target for HNLs searches.

Comments: Journal version