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 $\sqrt{s}=13$ 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 physics processes beyond the Standard Model. It uses $139 $\text{fb}$^{-1}$ofproton$-$protoncollisiondatarecordedat$\sqrt{s} = 13$ TeV at the LHC. Targeting sources of new physics which prefer final states containing $e^{+}{\mu }^{-}$ to $e^{-}{\mu }^{+}$, 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. The search also has two special selections targeting supersymmetric models and leptoquark signatures. Observations using one of these selections are able to exclude, at 95% confidence level, 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 ${\lambda }_{231}^{\mathrm{\prime }}$ is close to unity. Observations using the other selection exclude scalar leptoquarks with masses below 1880 GeV when $g_{\text{1R}}^{eu}=g_{\text{1R}}^{\mu c}=1$, at 95% confidence level. The limit on the coupling reduces to $g_{$\text{1R}$}^{eu}=g_{$\text{1R}$}^{\mu c}=0.46$ for a mass of 1420 GeV.