A search for long-lived particles with large ionization energy loss in the ATLAS silicon pixel detector using 139 fb$^{−1}$ of $\sqrt{s}$ = 13 TeV $pp$ collisions

The results from a search for long-lived, heavy charged particles with the ATLAS Run-$2$ dataset are presented. As charged particles move through a material, the amount of energy they lose via ionization depends only on their mass for a given momentum. This property can be used to search for new Beyond the Standard Model particles in $ \sqrt{s}=13$ $TeV$ proton-proton collisions at the Large Hadron Collider. This search is conducted using data collected by the ATLAS detector, with an integrated luminosity of $139$ fb$^{−1}$ . Events are required to have large missing transverse energy and an isolated, high-quality track with large transverse momentum and large ionization energy loss. The analysis tests the data for evidence of supersymmetric particles including gluinos, charginos, and sleptons. The observed yields in the signal regions are consistent with Standard Model expectation for low reconstructed track mass. At high mass, there is an excess of observed events over data, with the largest deviation in a mass region spanning 1100 to 2800 $GeV$, where $7$ events are observed and $0.7$ $\pm$ $0.4$ events are predicted.