New constraints on tau-coupled Heavy Neutral Leptons with masses $m_N = 280-970$ MeV

TL;DR

This study searches for tau-coupled Heavy Neutral Leptons in the 280-970 MeV mass range using the ArgoNeuT detector, setting new constraints on their mixing angles based on null results in decay signatures.

Contribution

It provides the first experimental constraints on tau-coupled Heavy Neutral Leptons in this mass range using the ArgoNeuT detector and NuMI beam data.

Findings

Zero candidate events observed, consistent with background.

Established new 90% CL constraints on tau-coupled Heavy Neutral Leptons.

Improved limits on mixing angles for masses 280-970 MeV.

Abstract

A search for Heavy Neutral Leptons has been performed with the ArgoNeuT detector exposed to the NuMI neutrino beam at Fermilab. We search for the decay signature $N \to \nu \mu^+ \mu^-$, considering decays occurring both inside ArgoNeuT and in the upstream cavern. In the data, corresponding to an exposure to $1.25 \times 10^{20}$ POT, zero passing events are observed consistent with the expected background. This measurement leads to a new constraint at 90\% confidence level on the mixing angle $\left\vert U_{\tau N}\right\rvert^2$ of tau-coupled Dirac Heavy Neutral Leptons with masses $m_N =$ 280 - 970 MeV, assuming $\left\vert U_{eN}\right\rvert^2 = \left\vert U_{\mu N}\right\rvert^2 = 0$.