Abstract
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 $\ensuremath{\gamma}+3$-jet production to predict the irreducible $Z+3$-jet background (with the $Z$ boson decaying to neutrinos) to searches with ${\mathrm{E\ensuremath{\llap{\not\;}}}}_{T}+3$-jet signal topologies. We compute ratios of $Z+3$-jet to $\ensuremath{\gamma}+3$-jet production cross sections and kinematic distributions at next-to-leading order (NLO) in ${\ensuremath{\alpha}}_{s}$. We compare these ratios with those obtained using a parton shower matched to leading-order matrix elements ($\mathrm{ME}+\mathrm{PS}$). This study extends our previous work [Bern et al., Phys. Rev. D 84, 114002 (2011)] on the $Z+2$-jet to $\ensuremath{\gamma}+2$-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 $\mathrm{ME}+\mathrm{PS}$ results for the ratios. We also examine the possibility of large QCD logarithms in these processes. Ratios of $Z+n$-jet to $\ensuremath{\gamma}+n$-jet cross sections are plausibly less sensitive to such corrections than the cross sections themselves. Their effect on estimates of $Z+3$-jet to $\ensuremath{\gamma}+3$-jet ratios can be assessed experimentally by measuring the $\ensuremath{\gamma}+3$-jet to $\ensuremath{\gamma}+2$-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 $\mathrm{ME}+\mathrm{PS}$ and find that it is 1% or less. Our estimate of the remaining theoretical uncertainties in the $Z$ to $\ensuremath{\gamma}$ ratio is in agreement with our earlier study.
Highlights
The Large Hadron Collider has produced more than two years of data from highenergy collisions
Data from the first year of running have been analyzed in a wide variety of searches, to seek new physics beyond the Standard Model, and to understand the underlying mechanism of electroweak symmetry-breaking
We provided the theoretical input needed for using the photon process to estimate the Z one, and for assessing the remaining theoretical uncertainties in this procedure. These results were used by the CMS collaboration to provide the theoretical uncertainty in their search for new physics based on topologies with large missing transverse energy and three or more jets [4]
Summary
The Large Hadron Collider has produced more than two years of data from highenergy collisions. The CMS collaboration has used [2, 3] W -boson and photon production in association with jets in order to estimate the METZJ background in setting limits on the constrained minimal supersymmetric standard model and on simplified models of new physics [4] In such a data-driven approach, the unknown background is estimated by combining an experimental measurement of a reference process (which may be the same process in a different kinematic region) with a theoretical factor expressing the ratio between the two processes. We provided the theoretical input needed for using the photon process to estimate the Z one, and for assessing the remaining theoretical uncertainties in this procedure These results were used by the CMS collaboration to provide the theoretical uncertainty in their search for new physics based on topologies with large missing transverse energy and three or more jets [4]. In the appendix we describe how we modified SHERPA so that Z bosons and photons are treated on an equal footing
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