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

## TL;DR

This study searches for heavy neutral leptons in W boson decays at 13 TeV LHC collisions using prompt leptonic signatures, setting new constraints on their mixing with electron and muon neutrinos in the 8-65 GeV mass range.

## Contribution

First search for heavy neutral leptons in W boson decays at the LHC using prompt signatures, providing new limits on their mixing parameters in the 8-65 GeV mass range.

## Key findings

- No significant excess found over Standard Model backgrounds.
- Constraints on mixing parameters are set, excluding |U_e|^2 > 8×10^{-5} and |U_μ|^2 > 5×10^{-5}.
- Strongest limits are for masses 15-30 GeV, with |U_e|^2 < 1.1×10^{-5} and |U_μ|^2 < 5×10^{-6}.

## Abstract

The existence of right-handed neutrinos with Majorana masses below the electroweak scale could help address the origins of neutrino masses, the matter-antimatter asymmetry, and dark matter. In this paper, leptonic decays of W bosons from 140 fb$^{-1}$ of 13 TeV proton-proton collisions at the LHC, reconstructed in the ATLAS experiment, are used to search for heavy neutral leptons produced through their mixing with muon or electron neutrinos in a scenario with lepton number violation. The search is conducted using prompt leptonic decay signatures. The considered final states require two same-charge leptons or three leptons, while vetoing three-lepton same-flavour topologies. No significant excess over the expected Standard Model backgrounds is found, leading to constraints on the heavy neutral lepton's mixing with muon and electron neutrinos for heavy-neutral-lepton masses. The analysis excludes $|U_{e}|^2$ values above $8\times 10^{-5}$ and $|U_{\mu}|^2$ values above $5.0 \times 10^{-5}$ in the full mass range of 8-65 GeV. The strongest constraints are placed on heavy-neutral-lepton masses in the range 15--30 GeV of $|U_{e}|^2 < 1.1 \times 10^{-5}$ and $|U_{\mu}|^2 < 5 \times 10^{-6}$.