Development of secondary vertex reconstruction using tracks in the ATLAS Muon Spectrometer for long-lived particle decays

Many promising beyond the Standard Model (BSM) theories predict invisible long-lived particles (LLPs) that travel macroscopic distances before decaying into visible Standard Model (SM) particles with a common spatial origin. Fast and accurate reconstruction of secondary vertices therefore plays a central role in ATLAS LLP searches. While significant advancements have been made in efficiently reconstructing Inner Detector (ID) tracks originating far from the primary Interaction Point (IP), traditional secondary vertexing algorithms still remain limited by the ID track reconstruction acceptance, significantly reducing sensitivity to LLP phase space where the bulk of expected decays lie beyond the Pixel detector. This note describes a novel secondary vertexing technique that leverages StandAlone muon tracks reconstructed exclusively in the ATLAS Muon Spectrometer (MS), which demonstrates the ability to efficiently reconstruct ultra-displaced dimuon vertices up to 8 meters from the IP. Its performance is characterized on a range of simulated BSM signatures and in Run 3 data through a tag-and-probe approach using J/ψ→μμdecays.Scientific contact person Juste Rozas, Aurelio, (aurelio.juste.rozas@cern.ch)