451 research outputs found
Soft and Coulomb gluon resummation in squark-antisquark production at the LHC
We present results for a combined resummation of soft and Coloumb gluon
corrections in squark-antisquark production at the LHC, including the
non-trivial interference of the two effects and the contribution of
squark-antisquark bound states below threshold. The total correction to the
next-to-leading order approximation is found to be sizeable, and amounts to
5-14 % in the squark mass region 200 GeV-2 TeV. The scale dependence of the
total cross section is also reduced.Comment: 6 pages, 3 figures, to appear in the proceedings of the 9th
International Symposium on Radiative Corrections (RADCOR 2009), October 25-30
2009, Ascona, Switzerlan
Threshold expansion of massive coloured particle cross sections
Pair production of massive coloured particles in hadron collisions is
accompanied by potentially large radiative corrections related to the
suppression of soft gluon emission and enhanced Coulomb exchange near the
production threshold. We recently developed a framework to sum both series of
corrections for the partonic cross section using soft-collinear and
non-relativistic effective theory. If it can be argued that the resummed cross
section approximates the complete result over a significant kinematic range, an
improvement of the hadronic cross section results, even when the production is
not kinematically constrained to the threshold. This is discussed here for the
case of top quark production.Comment: 5 pages, to appear in: Proceedings of the 10th DESY Workshop on
Elementary Particle Theory: Loops and Legs in Quantum Field Theory 2010,
W\"orlitz, Germany, 25-30 Apr 201
A comparison of efficient methods for the computation of Born gluon amplitudes
We compare four different methods for the numerical computation of the pure
gluonic amplitudes in the Born approximation. We are in particular interested
in the efficiency of the various methods as the number n of the external
particles increases. In addition we investigate the numerical accuracy in
critical phase space regions. The methods considered are based on (i)
Berends-Giele recurrence relations, (ii) scalar diagrams, (iii) MHV vertices
and (iv) BCF recursion relations.Comment: 20 page
On-shell recursion relations for all Born QCD amplitudes
We consider on-shell recursion relations for all Born QCD amplitudes. This
includes amplitudes with several pairs of quarks and massive quarks. We give a
detailed description on how to shift the external particles in spinor space and
clarify the allowed helicities of the shifted legs. We proof that the
corresponding meromorphic functions vanish at z --> infinity. As an application
we obtain compact expressions for helicity amplitudes including a pair of
massive quarks, one negative helicity gluon and an arbitrary number of positive
helicity gluons.Comment: 30 pages, minor change
SUSY Ward identities for multi-gluon helicity amplitudes with massive quarks
We use supersymmetric Ward identities to relate multi-gluon helicity
amplitudes involving a pair of massive quarks to amplitudes with massive
scalars. This allows to use the recent results for scalar amplitudes with an
arbitrary number of gluons obtained by on-shell recursion relations to obtain
scattering amplitudes involving top quarks.Comment: 22 pages, references adde
Hadronic top-quark pair production with NNLL threshold resummation
We compute the total top-quark pair production cross section at the Tevatron
and LHC based on approximate NNLO results, and on the summation of threshold
logarithms and Coulomb enhancements to all orders with next-to-next-to-leading
logarithmic (NNLL) accuracy, including bound-state effects. We find
\sigma_{t\bar t} = 7.22^{+0.31+0.71}_{-0.47-0.55} pb at Tevatron and
\sigma_{t\bar t} = 162.6^{+7.4+15.4}_{-7.6-14.7} pb at LHC with 7 TeV c.o.m.
energy, for m_t=173.3 GeV. The implementation of joint soft and Coulomb
resummation, its ambiguities, and the present theoretical uncertainty are
discussed in detail. We further obtain new approximate results at N3LO.Comment: 55 pages, LaTeX, 13 figure
No triangles on the moduli space of maximally supersymmetric gauge theory
Maximally supersymmetric gauge theory in four dimensions has a remarkably
simple S-matrix at the origin of its moduli space at both tree and loop level.
This leads to the question what, if any, of this structure survives at the
complement of this one point. Here this question is studied in detail at one
loop for the branch of the moduli space parameterized by a vacuum expectation
value for one complex scalar. Motivated by the parallel D-brane picture of
spontaneous symmetry breaking a simple relation is demonstrated between the
Lagrangian of broken super Yang-Mills theory and that of its higher dimensional
unbroken cousin. Using this relation it is proven both through an on- as well
as an off-shell method there are no so-called triangle coefficients in the
natural basis of one-loop functions at any finite point of the moduli space for
the theory under study. The off-shell method yields in addition absence of
rational terms in a class of theories on the Coulomb branch which includes the
special case of maximal supersymmetry. The results in this article provide
direct field theory evidence for a recently proposed exact dual conformal
symmetry motivated by the AdS/CFT correspondence.Comment: 39 pages, 4 figure
Multigluon tree amplitudes with a pair of massive fermions
We consider the calculation of n-point multigluon tree amplitudes with a pair
of massive fermions in QCD. We give the explicit transformation rules of this
kind of massive fermion-pair amplitudes with respect to different reference
momenta and check the correctness of them by SUSY Ward identities. Using these
rules and onshell BCFW recursion relation, we calculate the analytic results of
several n-point multigluon amplitudes.Comment: 15page
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