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 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 Rparity-violating coupling λ 231 is close to unity. Observations using the other selection exclude scalar
leptoquarks with masses below 1880 GeV when geu 1R = g μc
1R = 1, at 95% confidence level. The limit on the
coupling reduces to geu
1R = g
μc
1R = 0.46 for a mass of 1420 GeV.