Search for long-lived particles using displaced vertices of oppositely charged leptons in 140 fb${}^{-1}$ of pp collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Abstract

A search is presented for long-lived particles decaying into an oppositely charged lepton pair, ${\mu }^{+}{\mu }^{-}$, $e^{+}{e}^{-}$, or $e^{\pm }{\mu }^{\mp }$, that form a vertex within the inner tracking system of the ATLAS detector at the Large Hadron Collider, displaced from the primary proton-proton interaction region. The analysis uses the 140 fb${}^{-1}$ of Run-2 data collected at $\sqrt{s}=13$ TeV by the ATLAS experiment in 2015-2018. The results of the analysis are interpreted in the context of three benchmark models covering masses from 0.1 to 2.2 TeV and a range of mean proper lifetimes times the speed of light from 1 to 10000 mm. The first model is a generic $Z^{\prime }$ boson pair-produced by a new heavy scalar, with the $Z^{\prime }$ decaying into lepton pairs. The remaining two models are $R$-parity violating supersymmetric models in which the lightest neutralino ${\tilde{\chi }}_1^0$ decays into ${\ell }^{+}{\ell }^{{}^{\prime }-}\nu$ ($\ell ,{\ell }^{{}^{\prime }}=e$, $\mu$). The models differ by the mode of production of the ${\tilde{\chi }}_1^0$, which can be produced via the decay of pairs of gluinos or of pairs of charginos and neutralinos (${\tilde{\chi }}_1^{\pm }{\tilde{\chi }}_1^0$, ${\tilde{\chi }}_1^{\pm }{\tilde{\chi }}_2^0$, or ${\tilde{\chi }}_2^0{\tilde{\chi }}_1^0$). Although each benchmark sample includes pair-produced LLPs, only a single vertex is required to be reconstructed. No dilepton displaced vertex candidate is observed and the results are presented as upper limits on the production cross-sections. This analysis sets leading limits on the production cross-sections for multiple models, including parameter space that has never been directly probed.

40 pages in total, author list starting page 23, 7 figures, 1 table. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/SUSY-2020-08/

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