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

TL;DR

This paper re-analyzes CHARM experiment data to establish new constraints on Heavy Neutral Leptons, especially those mixing with tau neutrinos, in the mass range 380 MeV to 1.6 GeV, expanding the known limits.

Contribution

It provides the first limits on HNLs that mix exclusively with tau neutrinos within a specific mass range, based on reinterpreting existing experimental data.

Findings

Constraints on HNLs mixing with tau neutrinos established.

Limits cover HNL masses from 380 MeV to 1.6 GeV.

Data extends the parameter space constraints for HNL searches.

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.