Abstract
<jats:title> A <jats:sc>bstract</jats:sc> </jats:title> <jats:p> A search for long-lived heavy neutral leptons produced in B-meson decays and decaying to a μ <jats:sup> <jats:italic>±</jats:italic> </jats:sup> π <jats:sup>∓</jats:sup> final state is performed with data collected by the LHCb experiment in proton-proton collisions at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5 fb <jats:sup> <jats:italic>−</jats:italic> 1 </jats:sup> . The results are interpreted in both lepton-number-conserving and lepton-number-violating scenarios. No significant excess is observed. Constraints are placed on the squared mixing element | <jats:italic>U</jats:italic> <jats:sub>μN</jats:sub> | <jats:sup>2</jats:sup> to the active muon neutrino, under the assumption that couplings to other lepton flavours are negligible, in the mass range of 1 <jats:italic>.</jats:italic> 6–5 <jats:italic>.</jats:italic> 5 GeV. </jats:p>