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See detailMissing energy and jets for supersymmetry searches
Bern, Zvi; Diana, Giovanni UL; Dixon, L. J. et al

in Physical Review D (2013), 87(3), 034026

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 γ+3-jet production to ... [more ▼]

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 γ+3-jet production to predict the irreducible Z+3-jet background (with the Z boson decaying to neutrinos) to searches with E̸T+3-jet signal topologies. We compute ratios of Z+3-jet to γ+3-jet production cross sections and kinematic distributions at next-to-leading order (NLO) in αs. 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 [Bern et al., Phys. Rev. D 84 114002 (2011)] on the Z+2-jet to γ+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 γ+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 γ+3-jet ratios can be assessed experimentally by measuring the γ+3-jet to γ+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 γ ratio is in agreement with our earlier study. [less ▲]

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See detailDriving missing data at next-to-leading order
Bern, Zvi; Diana, Giovanni UL; Dixon, L. J. et al

in Physical Review D (2011), 84(11), 114002

The prediction of backgrounds to new-physics signals in topologies with large missing transverse energy and jets is important to new-physics searches at the LHC. Following a CMS study, we investigate ... [more ▼]

The prediction of backgrounds to new-physics signals in topologies with large missing transverse energy and jets is important to new-physics searches at the LHC. Following a CMS study, we investigate theoretical issues in using measurements of γ+2-jet production to predict the irreducible background to searches for missing energy plus two jets that originates from Z+2-jet production where the Z boson decays to neutrinos. We compute ratios of γ+2-jet to Z+2-jet production cross sections and kinematic distributions at next-to-leading order in αs, as well as using a parton shower matched to leading-order matrix elements. The former ratios rely on the first next-to-leading-order calculation of γ+2-jet production at a hadron collider. We find that the ratios obtained in the two approximations are quite similar, making γ+2-jet production a theoretically reliable estimator for the missing energy plus two jets background. We employ a Frixione-style photon isolation, but we also show that for isolated prompt-photon production at high transverse momentum the difference between this criterion and the standard-cone isolation used by CMS is small. [less ▲]

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See detailLeft-handed W bosons at the LHC
Bern, Z.; Diana, Giovanni UL; Dixon, L. J. et al

in Physical Review D (2011), 84(3), 034008

The production of W bosons in association with jets is an important background to new physics at the LHC. Events in which the W carries large transverse momentum and decays leptonically lead to large ... [more ▼]

The production of W bosons in association with jets is an important background to new physics at the LHC. Events in which the W carries large transverse momentum and decays leptonically lead to large missing energy and are of particular importance. We show that the left-handed nature of the W coupling, combined with valence quark domination at a pp machine, leads to a large left-handed polarization for both W+ and W- bosons at large transverse momenta. The polarization fractions are very stable with respect to QCD corrections. The leptonic decay of the W± bosons translates the common left-handed polarization into a strong asymmetry in transverse momentum distributions between positrons and electrons and between neutrinos and antineutrinos (missing transverse energy). Such asymmetries may provide an effective experimental handle on separating W+jets from top-quark production, which exhibits very little asymmetry due to C invariance, and from various types of new physics. [less ▲]

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