A search for an unexpected asymmetry in the production of And pairs in proton–proton collisions recorded by the ATLAS detector at 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 $139\phantom{\rule{0.2em}{0ex}}{\text{fb}}^{-1}$ of proton–proton collision data recorded at $\sqrt{s}=13$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 640GeV in a model in which the only other light sparticle is a neutralino when the R-parity-violating coupling ${\lambda }_{231}^{\prime }$ is close to unity. Observations using the other selection exclude scalar leptoquarks with masses below 1880GeV 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 1420GeV.