Difference between two species of emu hides a test for lepton flavour violation

Abstract

We argue that an LHC measurement of some simple quantities related to the ratio of rates of e$^{+}$μ$^{-}$ to e$^{-}$μ$^{+}$ events is surprisingly sensitive to as-yet unexcluded R-parity violating supersymmetric models with non-zero λ′$_{231}$ couplings. The search relies upon the approximate lepton universality in the Standard Model, the sign of the charge of the proton, and a collection of favourable detector biases. The proposed search is unusual because: it does not require any of the displaced vertices, hadronic neutralino decay products, or squark/gluino production relied upon by existing LHC RPV searches; it could work in cases in which the only light sparticles were smuons and neutralinos; and it could make a discovery (though not necessarily with optimal significance) without requiring the computation of a leading-order Monte Carlo estimate of any background rate. The LHC has shown no strong hints of post-Higgs physics and so precision Standard Model measurements are becoming ever more important. We argue that in this environment growing profits are to be made from searches that place detector biases and symmetries of the Standard Model at their core — searches based around ‘controls’ rather than around signals.