Search for heavy neutral leptons decaying into muon-pion pairs in the MicroBooNE detector

TL;DR

This paper reports the first search for heavy neutral leptons decaying into muon-pion pairs using the MicroBooNE liquid-argon TPC, setting new upper limits on their mixing parameters in the 260-385 MeV mass range.

Contribution

It introduces a novel search for HNLs in a liquid-argon TPC and provides the first upper limits on their mixing parameters in this experimental setup.

Findings

Set upper limits on |U_{μ4}|^2 for Dirac and Majorana HNLs.

Analyzed data corresponding to 2.0×10^{20} protons on target.

Focused on HNL masses between 260 and 385 MeV.

Abstract

We present upper limits on the production of heavy neutral leptons (HNLs) decaying to $\mu \pi$ pairs using data collected with the MicroBooNE liquid-argon time projection chamber (TPC) operating at Fermilab. This search is the first of its kind performed in a liquid-argon TPC. We use data collected in 2017 and 2018 corresponding to an exposure of $2.0 \times 10^{20}$ protons on target from the Fermilab Booster Neutrino Beam, which produces mainly muon neutrinos with an average energy of $\approx 800$ MeV. HNLs with higher mass are expected to have a longer time-of-flight to the liquid-argon TPC than Standard Model neutrinos. The data are therefore recorded with a dedicated trigger configured to detect HNL decays that occur after the neutrino spill reaches the detector. We set upper limits at the $90\%$ confidence level on the element $\lvert U_{\mu4}\rvert^2$ of the extended PMNS mixing matrix in the range $\lvert U_{\mu4}\rvert^2<(6.6$-$0.9)\times 10^{-7}$ for Dirac HNLs and $\lvert U_{\mu4}\rvert^2<(4.7$-$0.7)\times 10^{-7}$ for Majorana HNLs, assuming HNL masses between $260$ and $385$ MeV and $\lvert U_{e 4}\rvert^2 = \lvert U_{\tau 4}\rvert^2 = 0$.