859 research outputs found
Precise predictions for Higgs production in models with color-octet scalars
We describe an effective-theory computation of the next-to-next-to-leading
order (NNLO) QCD corrections to the gluon-fusion production of a Higgs boson in
models with massive color-octet scalars in the (8,1)_0 representation.
Numerical results are presented for both the Tevatron and the LHC. The
estimated theoretical uncertainty is greatly reduced by the inclusion of the
NNLO corrections. Color-octet scalars can increase the Standard Model rate by
more than a factor of two in allowed regions of parameter space.Comment: 6 pages, 5 figures, to appear in the proceedings of the "10th DESY
Workshop on Elementary Particle Theory: Loops and Legs in Quantum Field
Theory", Woerlitz, Germany, April 25-30, 201
Numerical evaluation of loop integrals
We present a new method for the numerical evaluation of arbitrary loop
integrals in dimensional regularization. We first derive Mellin-Barnes integral
representations and apply an algorithmic technique, based on the Cauchy
theorem, to extract the divergent parts in the epsilon->0 limit. We then
perform an epsilon-expansion and evaluate the integral coefficients of the
expansion numerically. The method yields stable results in physical kinematic
regions avoiding intricate analytic continuations. It can also be applied to
evaluate both scalar and tensor integrals without employing reduction methods.
We demonstrate our method with specific examples of infrared divergent
integrals with many kinematic scales, such as two-loop and three-loop box
integrals and tensor integrals of rank six for the one-loop hexagon topology
Loop lessons from Wilson loops in N=4 supersymmetric Yang-Mills theory
N=4 supersymmetric Yang-Mills theory exhibits a rather surprising duality of
Wilson-loop vacuum expectation values and scattering amplitudes. In this paper,
we investigate this correspondence at the diagram level. We find that one-loop
triangles, one-loop boxes, and two-loop diagonal boxes can be cast as simple
one- and two- parametric integrals over a single propagator in configuration
space. We observe that the two-loop Wilson-loop "hard-diagram" corresponds to a
four-loop hexagon Feynman diagram. Guided by the diagrammatic correspondence of
the configuration-space propagator and loop Feynman diagrams, we derive Feynman
parameterizations of complicated planar and non-planar Feynman diagrams which
simplify their evaluation. For illustration, we compute numerically a four-loop
hexagon scalar Feynman diagram.Comment: 20 pages, many figures. Two references added. Published versio
The fully differential hadronic production of a Higgs boson via bottom quark fusion at NNLO
The fully differential computation of the hadronic production cross section
of a Higgs boson via bottom quarks is presented at NNLO in QCD. Several
differential distributions with their corresponding scale uncertainties are
presented for the 8 TeV LHC. This is the first application of the method of
non-linear mappings for NNLO differential calculations at hadron colliders.Comment: 27 pages, 13 figures, 1 lego plo
Hepta-Cuts of Two-Loop Scattering Amplitudes
We present a method for the computation of hepta-cuts of two loop scattering
amplitudes. Four dimensional unitarity cuts are used to factorise the integrand
onto the product of six tree-level amplitudes evaluated at complex momentum
values. Using Gram matrix constraints we derive a general parameterisation of
the integrand which can be computed using polynomial fitting techniques. The
resulting expression is further reduced to master integrals using conventional
integration by parts methods. We consider both planar and non-planar topologies
for 2 to 2 scattering processes and apply the method to compute hepta-cut
contributions to gluon-gluon scattering in Yang-Mills theory with adjoint
fermions and scalars.Comment: 37 pages, 6 figures. version 2 : minor updates, published versio
Three-jet cross sections in hadron-hadron collisions at next-to-leading order
We present a new QCD event generator for hadron collider which can calculate
one-, two- and three-jet cross sections at next-to-leading order accuracy. In
this letter we study the transverse energy spectrum of three-jet hadronic
events using the kT algorithm. We show that the next-to-leading order
correction significantly reduces the renormalization and factorization scale
dependence of the three-jet cross section.Comment: 4 pages, 4 figures, REVTEX
High-precision QCD at hadron colliders: electroweak gauge boson rapidity distributions at NNLO
We compute the rapidity distributions of W and Z bosons produced at the
Tevatron and the LHC through next-to-next-to leading order in QCD. Our results
demonstrate remarkable stability with respect to variations of the
factorization and renormalization scales for all values of rapidity accessible
in current and future experiments. These processes are therefore
``gold-plated'': current theoretical knowledge yields QCD predictions accurate
to better than one percent. These results strengthen the proposal to use W and
Z production to determine parton-parton luminosities and constrain parton
distribution functions at the LHC. For example, LHC data should easily be able
to distinguish the central parton distribution fit obtained by MRST from that
obtained by Alekhin.Comment: 47 pages, 17 figures. Minor typos, 1 reference correcte
Hidden Beauty in Multiloop Amplitudes
Planar L-loop maximally helicity violating amplitudes in N = 4 supersymmetric
Yang-Mills theory are believed to possess the remarkable property of satisfying
iteration relations in L. We propose a simple new method for studying the
iteration relations for four-particle amplitudes which involves the use of
certain linear differential operators and eliminates the need to fully evaluate
any loop integrals. We carry out this procedure in explicit detail for the
two-loop amplitude and argue that this method can be used to prove the
iteration relations to all loops up to polynomials in logarithms.Comment: 21 pages, harvmac; v2: minor change
Threshold Corrections in Precision LHC Physics: QED otimes QCD
With an eye toward LHC processes in which theoretical precisions of 1 percent
are desired, we introduce the theory of the simultaneous YFS resummation of QED
and QCD to compute the size of the expected resummed soft radiative threshold
effects in precision studies of heavy particle production at the LHC. Our
results show that both QED and QCD soft threshold effects must be controlled to
be on the conservative side to achieve such precision goals.Comment: 4 pages, no figures; presented by B.F.L. Ward in DPF200
Two-Loop Helicity Amplitudes for Quark-Gluon Scattering in QCD and Gluino-Gluon Scattering in Supersymmetric Yang-Mills Theory
We present the two-loop QCD helicity amplitudes for quark-gluon scattering,
and for quark-antiquark annihilation into two gluons. These amplitudes are
relevant for next-to-next-to-leading order corrections to (polarized) jet
production at hadron colliders. We give the results in the `t Hooft-Veltman and
four-dimensional helicity (FDH) variants of dimensional regularization. The
transition rules for converting the amplitudes between the different variants
are much more intricate than for the previously discussed case of gluon-gluon
scattering. Summing our two-loop expressions over helicities and colors, and
converting to conventional dimensional regularization, gives results in
complete agreement with those of Anastasiou, Glover, Oleari and Tejeda-Yeomans.
We describe the amplitudes for 2 to 2 scattering in pure N=1 supersymmetric
Yang-Mills theory, obtained from the QCD amplitudes by modifying the color
representation and multiplicities, and verify supersymmetry Ward identities in
the FDH scheme.Comment: 77 pages. v2: corrected errors in eqs. (3.7) and (3.8) for one-loop
assembly; remaining results unaffecte
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