Search for long-lived particles decaying to oppositely charged lepton pairs with the ATLAS experiment at $\sqrt{s} = 13$ TeV

In this thesis, a search for new long-lived, massive particles decaying into oppositely charged $ee$, $e\mu$ or $\mu\mu$ 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 $0.27\pm0.17$ 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 $\ell^{+}\ell'^{-}\nu$ ($\ell=e,\mu$) via R-parity violating couplings. Upper limits on the signal cross-section are derived for specific squark and neutralino masses and for neutralino lifetimes ($c\tau$) between $1$ mm and $10$ m.