200 research outputs found
Next-to-leading order diphoton+2-jet production at the LHC
We present results from a recent calculation of prompt photon-pair production
in association with two jets to next-to-leading order (NLO) at the LHC. The
virtual contribution is evaluated using the BlackHat library, a numerical
implementation of on-shell methods for one-loop amplitudes, in conjunction with
SHERPA. We study four sets of cuts: standard jet cuts, a set of Higgs-related
cuts suggested by ATLAS, and corresponding sets which isolate the kinematic
region where the process becomes the largest background to Higgs production via
vector-boson fusion.Comment: 10 pages, 4 figures, Presented at 11th International Symposium on
Radiative Corrections (RADCOR 2013), 22-27 September 2013, Lumley Castle
Hotel, Durham, U
Next-to-Leading Order W + 5-Jet Production at the LHC
We present next-to-leading order QCD predictions for the total cross section
and for a comprehensive set of transverse-momentum distributions in W + 5-jet
production at the Large Hadron Collider. We neglect the small contributions
from subleading-color virtual terms, top quarks and some terms containing four
quark pairs. We also present ratios of total cross sections, and use them to
obtain an extrapolation formula to an even larger number of jets. We include
the decay of the boson into leptons. This is the first such computation
with six final-state vector bosons or jets. We use BlackHat together with
SHERPA to carry out the computation.Comment: RevTex, 27 pages, 7 figures, v2 minor corrections and corrected
reference
Left-Handed W Bosons at the LHC
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
anti-neutrinos (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.Comment: 32 pages, revtex, 17 figures, 3 tables, v2 minor corrections to ME+PS
results, no changes to conclusions, added reference
Driving Missing Data at Next-to-Leading Order
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 gamma + 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 gamma +
2-jet to Z + 2-jet production cross sections and kinematic distributions at
next-to-leading order in alpha_s, as well as using a parton shower matched to
leading-order matrix elements. We find that the ratios obtained in the two
approximations are quite similar, making gamma + 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.Comment: 27 pages, 9 figures, 3 tables, RevTex, v2 minor corrections and added
reference
Missing Energy and Jets for Supersymmetry Searches
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.Comment: 32 pages, 10 figures, 8 tables, RevTe
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One-Loop Multi-Parton Amplitudes with a Vector Boson for the LHC
In this talk, we present the first, numerically stable, results for the one-loop amplitudes needed for computing W; Z + 3 jet cross sections at the LHC to next-to-leading order in the QCD coupling. We implemented these processes in BlackHat, an automated program based on on-shell methods. These methods scale very well with increasing numbers of external partons, and are applicable to a wide variety of problems of phenomenological interest at the LHC
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Precise Predictions for W + 3 Jet Production at Hadron Colliders
We report on the first next-to-leading order QCD computation of W + 3-jet production in hadronic collisions including all partonic subprocesses. We compare the results with CDF data from the Tevatron, and find excellent agreement. The renormalization and factorization scale dependence is reduced substantially compared to leading-order calculations. The required one-loop matrix elements are computed using on-shell methods, implemented in a numerical program, BlackHat. We use the SHERPA package to generate the real-emission contributions and to integrate the various contributions over phase space. We use a leading-color (large-N{sub c}) approximation for the virtual part, which we confirm in W + 1,2-jet production to be valid to within three percent. The present calculation demonstrates the utility of on-shell methods for computing next-to-leading-order corrections to processes important to physics analyses at the Large Hadron Collider
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