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Abstract

In this thesis, a search for new long-lived, massive particles decaying into oppositely charged ee, emu or mumu pairs with the ATLAS experiment at the Large Hadron Collider is presented. The search is based on data of proton–proton collisions at a centre-of-mass energy of 13 TeV recorded in 2016, corresponding to an integrated luminosity of 32.8 fb-1. The lepton pairs are required to form a common vertex within the inner tracking volume of the ATLAS detector, which is displaced from the primary collision vertex. The signal selection criteria suppress the background produced in collisions to a negligible level. High-energy cosmic-ray muons are the dominant source of the remaining background, which is estimated from the data to be 0.27 +/- 0.17 events. No vertices with invariant mass greater than 12 GeV are observed, in agreement with the background expectation. The result is interpreted in a simplified supersymmetric model in which the lightest neutralino produced via squark-antisquark production is long-lived and decays to l+l- + neutrino (l = e, mu) via R-parity violating couplings. Upper limits on the signal cross-section are derived for specific squark and neutralino masses and for neutralino lifetimes (c x tau) between 1 mm and 10 m.