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The irreducible background from Z(nunu)+jets, to beyond the Standard Modelsearches at the LHC, can be calibrated using gamma+jets data. The methodutilises the fact that at high vector boson pT, the event kinematics are thesame for the two processes and the cross sections differ mainly due to theboson-quark couplings. The method relies on a precise prediction from theory ofthe Z/gamma cross section ratio at high pT, which should be insensitive toeffects from full event simulation. We study the Z/gamma ratio for final statesinvolving 1, 2 and 3 hadronic jets, using both the leading-order parton showerMonte Carlo program Pythia8 and a leading-order matrix element program Gambos.This enables us both to understand the underlying parton dynamics in bothprocesses, and to quantify the theoretical systematic uncertainties in theratio predictions. Using a typical set of experimental cuts, we estimate thenet theoretical uncertainty in the ratio to be of order 7%, when obtained froma Monte Carlo program using multiparton matrix-elements for the hard process.Uncertainties associated with full event simulation are found to be small. Theresults indicate that an overall accuracy of the method, excluding statisticalerrors, of order 10% should be possible.

Using gamma+jets Production to Calibrate the Standard Model Z(nunu)+jets Background to New Physics Processes at the LHC