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 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 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 λ₂₃₁ ^′ is close to unity. Observations using the other selection exclude scalar leptoquarks with masses below 1880 GeV 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 1420 GeV.