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

A bstract A search for long-lived heavy neutrinos (N) in the decays of B mesons produced in proton-proton collisions at s 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 < c τ N < 10 4 mm, where τ N is the N proper mean lifetime. Signal events are defined by the signature B → ℓ B NX; 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 c τ 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.