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