A search for an unexpected asymmetry in the production of $e^+ \mu^-$ and $e^- \mu^+$ pairs in proton-proton collisions recorded by the ATLAS detector at $\sqrt s = 13~\mathrm{TeV}$

This search, a type not previously performed at ATLAS, uses a comparison of the production cross sections for $e^+ \mu^-$ and $e^- \mu^+$ pairs to constrain Beyond Standard Model physics processes. It uses $139~\mathrm{fb}^{-1}$ of proton-proton collision data recorded at $\sqrt s = 13~\textrm{TeV}$ at the LHC. Targeting sources of new physics which prefer the opposite $e^\pm\mu^\mp$ combination to the Standard Model, the search contains two broad signal regions which are used to provide model-independent constraints on the ratio of cross sections at the $2\%$ level. It also has two special selections targeting supersymmetric models and leptoquark signatures. Observations using one of these are able to exclude singly-produced smuons with masses up to 640 GeV in a model in which the only other light sparticle is a neutralino, when the $R$-parity violating coupling $\lambda_{231}^{'}$ is close to unity. Observations using the other exclude scalar leptoquarks with masses above 1500 GeV when $g_\textrm{1R}^{eu}=g_\textrm{1R}^{\mu c}>0.6$.