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<inf>N</inf> < 3 GeV and decay lengths in the range 10−2 < cτ<inf>N</inf> < 104 mm, where τ<inf>N</inf> is the N proper mean lifetime. Signal events are defined by the signature B → ℓ<inf>B</inf>NX; N → ℓ±π∓, where the leptons ℓ<inf>B</inf> 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<inf>N</inf>|2, and on cτ<inf>N</inf> are obtained in different mixing scenarios for both Majorana and Dirac-like N particles. The most stringent upper limit |V<inf>N</inf>|2 < 2.0 × 10−5 is obtained at m<inf>N</inf> = 1.95 GeV for the Majorana case where N mixes exclusively with muon neutrinos. The limits on |V<inf>N</inf>|2 for masses 1 < m<inf>N</inf> < 1.7 GeV are the most stringent from a collider experiment to date. © 2025 Elsevier B.V., All rights reserved.