Sensitivity of the ATLAS experiment to long-lived particles with a displaced vertex and $E_{T}^{miss}$ signature at the HL-LHC

This note presents the estimated sensitivity of a search for long-lived particles decaying within the tracking volume to multiple outgoing charged particles, using a signature of a displaced vertex and missing transverse momentum, in the upgraded ATLAS detector for the High-Luminosity LHC. The dataset is taken as 3000 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}=14$ TeV. The replacement of the existing tracking detector with a full-silicon inner tracker (ITk) significantly changes the search sensitivity, which is estimated using extrapolations of the reconstruction capabilities for displaced tracks and displaced vertices from a combination of the performance of the current detector and a simulation of the proposed upgraded detector. Results are projected in the context of gluino $R$-hadron pair production. For long-lived gluinos which decay to SM quarks and a 100 GeV stable neutralino, ATLAS should have the sensitivity to discover $R$-hadrons with lifetimes from 0.1 to 10 ns with masses up to 2.8 TeV. In the absence of long-lived gluino production, the 95% CL upper limit on gluino masses will reach 3.4 TeV.