  We extend our investigation of backgrounds to new physics signals, following
CMS's data-driven search for supersymmetry at the LHC. The aim is to use
different sets of cuts in gamma + 3-jet production to predict the irreducible Z
+ 3-jet background (with the Z boson decaying to neutrinos) to searches with
missing transverse energy + 3-jet signal topologies. We compute ratios of Z +
3-jet to gamma + 3-jet production cross sections and kinematic distributions at
next-to-leading order (NLO) in alpha_s. We compare these ratios with those
obtained using a parton shower matched to leading-order matrix elements
(ME+PS). This study extends our previous work [arXiv:1106.1423 [hep-ph]] on the
Z + 2-jet to gamma + 2-jet ratio. We find excellent agreement with the ratio
determined from the earlier NLO results involving two instead of three jets,
and agreement to within 10% between the NLO and ME+PS results for the ratios.
We also examine the possibility of large QCD logarithms in these processes.
Ratios of Z + n-jet to gamma + n-jet cross sections are plausibly less
sensitive to such corrections than the cross sections themselves. Their effect
on estimates of Z + 3-jet to gamma + 3-jet ratios can be assessed
experimentally by measuring the gamma + 3-jet to gamma + 2-jet production ratio
in search regions. We partially address the question of potentially large
electroweak logarithms by computing the real-emission part of the electroweak
corrections to the ratio using ME+PS, and find that it is 1% or less. Our
estimate of the remaining theoretical uncertainties in the Z to gamma ratio is
in agreement with our earlier study.
