54 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
Recommended from our members
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
Ntuples for NLO events at hadron colliders
We present an event-file format for the dissemination of next-to-leading-order (NLO) predictions for QCD processes at hadron colliders. The files contain all information required to compute generic jet-based infrared-safe observables at fixed order (without showering or hadronization), and to recompute observables with different factorization and renormalization scales. The files also make it possible to evaluate cross sections and distributions with different parton distribution functions. This in turn makes it possible to estimate uncertainties in NLO predictions of a wide variety of observables without recomputing the short-distance matrix elements. The event files allow a user to choose among a wide range of commonly-used jet algorithms and jet-size parameters. We provide event files for a W or Z boson accompanied by up to four jets, and for pure-jet events with up to four jets. The files are for the Large Hadron Collider with a center of mass energy of 7 or 8 TeV. A C++ library along with a Python interface for handling these files is also provided and described in this article. The library allows a user to read the event files and recompute observables transparently for different pdf sets and factorization and renormalization scales
Next-to-Leading Order QCD Predictions for W+3-Jet Distributions at Hadron Colliders
We present next-to-leading order QCD predictions for a variety of
distributions in W+3-jet production at both the Tevatron and the Large Hadron
Collider. We include all subprocesses and incorporate the decay of the W boson
into leptons. Our results are in excellent agreement with existing Tevatron
data and provide the first quantitatively precise next-to-leading order
predictions for the LHC. We include all terms in an expansion in the number of
colors, confirming that the specific leading-color approximation used in our
previous study is accurate to within three percent. The dependence of the cross
section on renormalization and factorization scales is reduced significantly
with respect to a leading-order calculation. We study different dynamical scale
choices, and find that the total transverse energy is significantly better than
choices used in previous phenomenological studies. We compute the one-loop
matrix elements using on-shell methods, as numerically implemented in the
BlackHat code. The remaining parts of the calculation, including generation of
the real-emission contributions and integration over phase space, are handled
by the SHERPA package.Comment: 64 pages, 37 figures, v2 minor changes in table X, cross sections and
distributions unaffected; other minor corrections to text, references adde
Gene Therapy of Murine Solid Tumors with T Cells Transduced with a Retroviral Vascular Endothelial Growth Factor–Immunotoxin Target Gene
Solid tumor growth can be inhibited by targeting its neovasculature with vascular endothelial growth factor (VEGF)-toxin fusion proteins (FPs), but these agents have been limited by their inability to localize at the tumor site. In this study, we devised a gene therapy approach intended to deliver VEGF-toxin directly to tumor. Antigen-specific cytotoxic T lymphocytes (CTLs) served as vehicles to deliver a retroviral VEGF-toxin fusion protein to its specific leukemia cell target in vivo. A retroviral vector was constructed for gene therapy with VEGF positioned downstream of its 27-amino acid leader sequence, which promoted secretion of a catalytic immunotoxin containing either truncated diphtheria toxin or Pseudomonas exotoxin A. VEGF was chosen on the basis of the expression of VEGF receptor on endothelial cells in the tumor neovasculature. The VEGF FP was first expressed and secreted by mammalian NIH 3T3 cells. Intracellular expression of both VEGF and toxin was verified by immunofluorescence. In vitro, supernatants collected from transfected cells specifically inhibited the growth of VEGF receptor-expressing human umbilical vein endothelial cells (HUVECs), but not a control cell line. In vivo findings correlated with in vitro findings. A retroviral vector containing the target gene and a nerve growth factor receptor (NGFR) reporter gene was used to transiently transduce T15, a CD8(+) CTL line that specifically recognizes C1498, a lethal C57BL/6 myeloid tumor. Transduced T15 cells injected intravenously significantly inhibited the growth of subcutaneous tumor, whereas nontransduced controls did not. Together, these data indicate that gene therapy of T cells with retrovirus containing a VEGF-immunotoxin target gene may be a valid means of inhibiting a broad range of solid tumors dependent on angiogenesis
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