  We argue that an LHC measurement of some simple quantities related to the
ratio of rates of e+mu- to e-mu+ events is surprisingly sensitive to as-yet
unexcluded R-parity violating supersymmetric models with non-zero lambda-prime
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.
