Search for heavy neutral leptons in B-meson decays

A search for long-lived heavy neutral leptons produced in B-meson decays and decaying to a ${\mu }^{\pm }{\pi }^{\mp }$ 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 $5{\mathrm{f}\mathrm{b}}^{-1}$. 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 $\mathrm{\mid }{U}_{\mu N}{\mathrm{\mid }}^2$ to the active muon neutrino, under the assumption that couplings to other lepton flavours are negligible, in the mass range of $1.6$-$5.5$ GeV.