Search for long-lived heavy neutrinos in the decays of B mesons produced in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A search for long-lived heavy neutrinos (N) in the decays of B mesons produced in proton-proton collisions at $\sqrt{s}$ = 13 TeV is presented. The data sample corresponds to an integrated luminosity of 41.6 fb −1 collected in 2018 by the CMS experiment at the CERN LHC, using a dedicated data stream that enhances the number of recorded events containing B mesons. The search probes heavy neutrinos with masses in the range 1 < m N < 3 GeV and decay lengths in the range 10 −2 < cτN < 104 mm, where τN is the N proper mean lifetime. Signal events are defined by the signature B → ℓ BNX; N → ℓ ± π∓, where the leptons ℓ B and ℓ can be either a muon or an electron, provided that at least one of them is a muon. The hadronic recoil system, X, is treated inclusively and is not reconstructed. No significant excess of events over the standard model background is observed in any of the ℓ ± π∓ invariant mass distributions. Limits at 95% confidence level on the sum of the squares of the mixing amplitudes between heavy and light neutrinos, |V N|2, and on cτ N are obtained in different mixing scenarios for both Majorana and Dirac-like N particles. The most stringent upper limit |V N | 2 < 2.0 × 10 −5 is obtained at m N = 1.95 GeV for the Majorana case where N mixes exclusively with muon neutrinos. The limits on |V N | 2 for masses 1 < m N < 1.7 GeV are the most stringent from a collider experiment to date.