# Search for heavy neutral leptons in decays of $W$ bosons produced in 13 TeV $pp$ collisions using prompt and displaced signatures with the ATLAS detector

## TL;DR

This paper searches for heavy neutral leptons produced in W boson decays at the LHC, using prompt and displaced signatures in ATLAS data, to explore neutrino mass models and potential new physics.

## Contribution

It introduces a combined prompt and displaced decay signature analysis for HNLs in W boson decays at 13 TeV, extending sensitivity to HNL masses between 4.5 and 50 GeV.

## Key findings

- Set new constraints on HNL mixing angles for masses 4.5-50 GeV.
- Utilized both prompt and displaced decay signatures for comprehensive search.
- Analyzed 32.9-36.1 fb$^{-1}$ of LHC data with the ATLAS detector.

## Abstract

The problems of neutrino masses, matter-antimatter asymmetry, and dark matter could be successfully addressed by postulating right-handed neutrinos with Majorana masses below the electroweak scale. In this work, leptonic decays of $W$ bosons extracted from 32.9 fb$^{-1}$ to 36.1 fb$^{-1}$ of 13 TeV proton-proton collisions at the LHC are used to search for heavy neutral leptons (HNLs) that are produced through mixing with muon or electron neutrinos. The search is conducted using the ATLAS detector in both prompt and displaced leptonic decay signatures. The prompt signature requires three leptons produced at the interaction point (either $\mu\mu e$ or $e e\mu$) with a veto on same-flavour opposite-charge topologies. The displaced signature comprises a prompt muon from the $W$ boson decay and the requirement of a dilepton vertex (either $\mu\mu$ or $\mu e$) displaced in the transverse plane by 4-300 mm from the interaction point. The search sets constraints on the HNL mixing to muon and electron neutrinos for HNL masses in the range 4.5-50 GeV.