Search for Heavy Neutral Leptons in Decays of W Bosons Using a Dilepton Displaced Vertex in √s = 13 TeV pp Collisions with the ATLAS Detector

A search for a long-lived, heavy neutral lepton (𝒩) in 139  fb⁻¹ of √𝑠 = 13  TeV 𝑝⁢𝑝 collision data collected by the ATLAS detector at the Large Hadron Collider is reported. The 𝒩 is produced via 𝑊→𝒩⁢𝜇 or 𝑊→𝒩⁢𝑒 and decays into two charged leptons and a neutrino, forming a displaced vertex. The 𝒩 mass is used to discriminate between signal and background. No signal is observed, and limits are set on the squared mixing parameters of the 𝒩 with the left-handed neutrino states for the 𝒩 mass range 3  GeV<𝑚_𝒩<15  GeV. For the first time, limits are given for both single-flavor and multiflavor mixing scenarios motivated by neutrino flavor oscillation results for both the normal and inverted neutrino-mass hierarchies.