A search for long-lived neutralinos decaying to di-lepton displaced vertices in proton-proton collisions at $\sqrt{s} = 13$ TeV

A search for a long-lived neutral massive particle decaying to a pair of oppositely charged leptons $ee$, $e\mu$, $\mu\mu$ is presented using 140.1 fb$^{-1}$ of $pp$ collision data at center-of-mass energy of 13 TeV, collected with the ATLAS detector. The displaced leptons are required to form a vertex that is displaced from the primary $pp$ collision and within the volume of the inner detector tracking system. The requirements of a minimum displacement of 2 mm and a minimum invariant mass 12 GeV effectively suppress backgrounds from Standard Model processes. The remaining backgrounds from cosmic ray muons and random track crossings are estimated using data-driven techniques to be less than one background event. No events containing a displaced di-lepton vertex were observed yielding upper limits for the production cross-section of two supersymmetric models that could produce a long-lived neutralino: pair production of gluinos $\tilde{g}$ and pair production of electroweakinos $\tilde{\chi}^0_1$, $\tilde{\chi}^0_2$, and $\tilde{\chi}^{\pm}_1$. These are the first limits on the production cross-section for these long-lived neutralino models.