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 $e^+ μ^−$ and $e^− μ^+$ pairs to constrain physics processes beyond the Standard Model. It uses 139fb$^{−1}$ 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+μ− to $e^− μ^+$, 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 $λ'_{231}$ is close to unity. Observations using the other selection exclude scalar leptoquarks with masses below 1880GeV when $g_{1R}^{eu}$=$g_{1R}^{μc}=1$, at 95% confidence level. The limit on the coupling reduces to $g_{1R}^{eu}=g_{1R}^{μc}=0.46$ for a mass of 1420GeV.

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