98 research outputs found

### One-loop amplitudes for W+3 jet production in hadron collisions

We employ the recently developed method of generalized $D$-dimensional
unitarity to compute one-loop virtual corrections to all scattering amplitudes
relevant for the production of a $W$ boson in association with three jets in
hadronic collisions, treating all quarks as massless.Comment: 26 pages, 5 figures, v2 to agree with published versio

### Semi-Numerical Evaluation of One-Loop Corrections

We present a semi-numerical algorithm to calculate one-loop virtual
corrections to scattering amplitudes. The divergences of the loop amplitudes
are regulated using dimensional regularization. We treat in detail the case of
amplitudes with up to five external legs and massless internal lines, although
the method is more generally applicable. Tensor integrals are reduced to
generalized scalar integrals, which in turn are reduced to a set of known basis
integrals using recursion relations. The reduction algorithm is modified near
exceptional configurations to ensure numerical stability. To test the procedure
we apply these techniques to one-loop corrections to the Higgs to four quark
process for which analytic results have recently become available.Comment: 33 pages, 10 figures, some typos fixed and references added, to
appear in Phys. Rev. D. v3 corrects a typo in eq. A2

### The t-tbar cross-section at 1.8 and 1.96 TeV: a study of the systematics due to parton densities and scale dependence

We update the theoretical predictions for the t-tbar production cross-section
at the Tevatron, taking into account the most recent determinations of
systematic uncertainties in the extraction of the proton parton densities.Comment: 12 pages, 1 figure, Late

### Multi-gluon one-loop amplitudes using tensor integrals

An efficient numerical algorithm to evaluate one-loop amplitudes using tensor
integrals is presented. In particular, it is shown by explicit calculations
that for ordered QCD amplitudes with a number of external legs up to 10, its
performance is competitive with other methods.Comment: 25 pages, results for quark loops added, accuracy analysis extended,
mistakes corrected, reference adde

### Higher-Order Corrections to Timelike Jets

We present a simple formalism for the evolution of timelike jets in which
tree-level matrix element corrections can be systematically incorporated, up to
arbitrary parton multiplicities and over all of phase space, in a way that
exponentiates the matching corrections. The scheme is cast as a shower Markov
chain which generates one single unweighted event sample, that can be passed to
standard hadronization models. Remaining perturbative uncertainties are
estimated by providing several alternative weight sets for the same events, at
a relatively modest additional overhead. As an explicit example, we consider Z
-> q qbar evolution with unpolarized, massless quarks and include several
formally subleading improvements as well as matching to tree-level matrix
elements through alpha_s^4. The resulting algorithm is implemented in the
publicly available VINCIA plugin to the PYTHIA 8 event generator.Comment: 72 pages, 78 figure

### Associated Top Quark-Higgs Boson Production at the LHC

We compute the O(alpha_s^3) inclusive cross section for the process pp ->
t-tbar-h in the Standard Model, at sqrt(s)=14 TeV. The next-to-leading order
corrections drastically reduce the renormalization and factorization scale
dependence of the Born cross section and increase the total cross section for
renormalization and factorization scales larger than m_t. These corrections
have important implications for models of new physics involving the top quark.Comment: 5 pages, 4 figures, RevTeX

### On the Numerical Evaluation of One-Loop Amplitudes: the Gluonic Case

We develop an algorithm of polynomial complexity for evaluating one-loop
amplitudes with an arbitrary number of external particles. The algorithm is
implemented in the Rocket program. Starting from particle vertices given by
Feynman rules, tree amplitudes are constructed using recursive relations. The
tree amplitudes are then used to build one-loop amplitudes using an integer
dimension on-shell cut method. As a first application we considered only three
and four gluon vertices calculating the pure gluonic one-loop amplitudes for
arbitrary external helicity or polarization states. We compare our numerical
results to analytical results in the literature, analyze the time behavior of
the algorithm and the accuracy of the results, and give explicit results for
fixed phase space points for up to twenty external gluons.Comment: 22 pages, 9 figures; v2: references added, version accepted for
publicatio

### Stability of NLO Global Analysis and Implications for Hadron Collider Physics

The phenomenology of Standard Model and New Physics at hadron colliders
depends critically on results from global QCD analysis for parton distribution
functions (PDFs). The accuracy of the standard next-to-leading-order (NLO)
global analysis, nominally a few percent, is generally well matched to the
expected experimental precision. However, serious questions have been raised
recently about the stability of the NLO analysis with respect to certain
inputs, including the choice of kinematic cuts on the data sets and the
parametrization of the gluon distribution. In this paper, we investigate this
stability issue systematically within the CTEQ framework. We find that both the
PDFs and their physical predictions are stable, well within the few percent
level. Further, we have applied the Lagrange Multiplier method to explore the
stability of the predicted cross sections for W production at the Tevatron and
the LHC, since W production is often proposed as a standard candle for these
colliders. We find the NLO predictions on sigma_W to be stable well within
their previously-estimated uncertainty ranges.Comment: 24 pages, 11 figures. Minor changes in response to JHEP referee
repor

### Generalized unitarity at work: first NLO QCD results for hadronic W+3jet production

We compute the leading color, next-to-leading order QCD corrections to the
dominant partonic channels for the production of a W boson in association with
three jets at the Tevatron and the LHC. This is the first application of
generalized unitarity for realistic one-loop calculations. The method performs
well in this non-trivial test and offers great promise for the future.Comment: 20 pages, 4 figure

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