44 research outputs found
Universality in W+Multijet Production
We study -boson production accompanied by multiple jets at 7 TeV at the
LHC. We study the jet-production ratio, of total cross sections for +- to
+()-jet production, and the ratio of distributions in the total
transverse hadronic jet energy . We use the ratios to
extrapolate the total cross section, and the differential distribution in
, to +6-jet production. We use the BlackHat software
library in conjunction with SHERPA to perform the computations.Comment: Merge of Moriond 2014 and Loops & Legs 2014 proceedings, 6 pages, 2
figure
High multiplicity W+jets predictions at NLO
In these proceedings we present results from a recent calculation for the
production of a W boson in conjunction with five jets at next-to-leading order
in perturbative QCD. We also use results at lower multiplicities to extrapolate
the cross section to the same process with six jets.Comment: 5 pages, Proceedings for the DIS2013 conferenc
The BlackHat Library for One-Loop Amplitudes
We present recent next-to-leading order (NLO) results in perturbative QCD
obtained using the BlackHat software library. We discuss the use of n-tuples to
separate the lengthy matrix-element computations from the analysis process. The
use of n-tuples allows many analyses to be carried out on the same phase-space
samples, and also allows experimenters to conduct their own analyses using the
original NLO computation.Comment: Talk given at ACAT 2013, Beijing, China, May 16--21, 2013; 6 pages, 2
figures; added reference
NLO vector boson production with light jets
In this contribution we present recent progress in the computation of
next-to-leading order (NLO) QCD corrections for the production of an
electroweak vector boson in association with jets at hadron colliders. We focus
on results obtained using the virtual matrix element library BLACKHAT in
conjunction with SHERPA, focusing on results relevant to understanding the
background to top production.Comment: 4+2 epsilon pages, Submitted for the proceedings of TOP2011 - 4th
International Workshop on Top Quark Physics, 25-30th September 2011, Sant
Feliu de Guixols, Spai
Scattering amplitudes with massive fermions using BCFW recursion
We study the QCD scattering amplitudes for \bar{q}q \to gg and \bar{q}q \to
ggg where q is a massive fermion. Using a particular choice of massive fermion
spinor we are able to derive very compact expressions for the partial spin
amplitudes for the 2 \to 2 process. We then investigate the corresponding 2 \to
3 amplitudes using the BCFW recursion technique. For the helicity conserving
partial amplitudes we again derive very compact expressions, but were unable to
treat the helicity-flip amplitudes recursively, except for the case where all
the gluon helicities are the same. We therefore evaluate the remaining partial
amplitudes using standard Feynman diagram techniques.Comment: 21 page
From Yang-Mills Lagrangian to MHV Diagrams
We prove the equivalence of a recently suggested MHV-formalism to the
standard Yang-Mills theory. This is achieved by a formally non-local change of
variables. In this note we present the explicit formulas while the detailed
proofs are postponed to a future publication.Comment: Latex,11 pages, minor changes, reference added, version to appear in
JHE
MHV Techniques for QED Processes
Significant progress has been made in the past year in developing new `MHV'
techniques for calculating multiparticle scattering amplitudes in Yang-Mills
gauge theories. Most of the work so far has focussed on applications to Quantum
Chromodynamics, both at tree and one-loop level. We show how such techniques
can also be applied to abelian theories such as QED, by studying the simplest
tree-level multiparticle process, e^+e^- to n \gamma. We compare explicit
results for up to n=5 photons using both the Cachazo, Svrcek and Witten `MHV
rules' and the related Britto-Cachazo-Feng `recursion relation' approaches with
those using traditional spinor techniques.Comment: 19 pages, 10 figures. References adde
From Trees to Loops and Back
We argue that generic one-loop scattering amplitudes in supersymmetric
Yang-Mills theories can be computed equivalently with MHV diagrams or with
Feynman diagrams. We first present a general proof of the covariance of
one-loop non-MHV amplitudes obtained from MHV diagrams. This proof relies only
on the local character in Minkowski space of MHV vertices and on an application
of the Feynman Tree Theorem. We then show that the discontinuities of one-loop
scattering amplitudes computed with MHV diagrams are precisely the same as
those computed with standard methods. Furthermore, we analyse collinear limits
and soft limits of generic non-MHV amplitudes in supersymmetric Yang-Mills
theories with one-loop MHV diagrams. In particular, we find a simple explicit
derivation of the universal one-loop splitting functions in supersymmetric
Yang-Mills theories to all orders in the dimensional regularisation parameter,
which is in complete agreement with known results. Finally, we present concrete
and illustrative applications of Feynman's Tree Theorem to one-loop MHV
diagrams as well as to one-loop Feynman diagrams.Comment: 52 pages, 17 figures. Some typos in Appendix A correcte
Top-mass effects in differential Higgs production through gluon fusion at order \alpha_s^4
Effects from a finite top quark mass on differential distributions in the
Higgs+jet production cross section through gluon fusion are studied at
next-to-leading order in the strong coupling, i.e. . Terms
formally subleading in are calculated, and their influence on the
transverse momentum and rapidity distribution of the Higgs boson are evaluated.
We find that, for the differential K-factor, the heavy-top limit is valid at
the 2-3% level as long as the transverse momentum of the Higgs remains below
about 150 GeV.Comment: 21 pages, 12 figure
Tree-Level Formalism
We review two novel techniques used to calculate tree-level scattering
amplitudes efficiently: MHV diagrams, and on-shell recursion relations. For the
MHV diagrams, we consider applications to tree-level amplitudes and focus in
particular on the N=4 supersymmetric formulation. We also briefly describe the
derivation of loop amplitudes using MHV diagrams. For the recursion relations,
after presenting their general proof, we discuss several applications to
massless theories with and without supersymmetry, to theories with massive
particles, and to graviton amplitudes in General Relativity. This article is an
invited review for a special issue of Journal of Physics A devoted to
"Scattering Amplitudes in Gauge Theories".Comment: 40 pages, 8 figures, invited review for a special issue of Journal of
Physics A devoted to "Scattering Amplitudes in Gauge Theories", R.
Roiban(ed), M. Spradlin(ed), A. Volovich(ed); v2: minor corrections,
references adde