A search for an unexpected asymmetry in the production of $e^+ μ^-$ and $e^- μ^+$ 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 139 fb^-1  of proton–proton collision data recorded at √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 640 GeV 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 1880 GeV when g^eu_1R = g^μc_1R = 1, at 95% confidence level. The limit on the coupling reduces to g^eu_1R = g^μc_1R = 0.46  for a mass of 1420 GeV.