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

Abstract

A search for long-lived heavy neutrinos (N) in the decays of B mesons produced in proton-proton collisions at √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 < cτ_N < 104 mm, where τ_N is the N proper mean lifetime. Signal events are defined by the signature B → lBNX; N → l^±π^∓, where the leptons lB 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.