334 research outputs found
Efficient analytic computation of higher-order QCD amplitudes
URL: http://www-spht.cea.fr/articles/t95/026/ Le calcul analytique efficace des amplitudes aux ordres supérieurs en QCDWe review techniques simplifying the analytic calculation of one-loop QCD amplitudes with many external legs, for use in next-to-leading-order corrections to multi-jet processes. Particularly useful are the constraints imposed by perturbative unitarity, collinear singularities and a supersymmetry-inspired organization of helicity amplitudes. Certain sequences of one-loop helicity amplitudes with an arbitrary number of external gluons have been obtained using these constraints
Inherited Twistor-Space Structure of Gravity Loop Amplitudes
At tree-level, gravity amplitudes are obtainable directly from gauge theory
amplitudes via the Kawai, Lewellen and Tye closed-open string relations. We
explain how the unitarity method allows us to use these relations to obtain
coefficients of box integrals appearing in one-loop N=8 supergravity amplitudes
from the recent computation of the coefficients for N=4 super-Yang-Mills
non-maximally-helicity-violating amplitudes. We argue from factorisation that
these box coefficients determine the one-loop N=8 supergravity amplitudes,
although this remains to be proven. We also show that twistor-space properties
of the N=8 supergravity amplitudes are inherited from the corresponding
properties of N=4 super-Yang-Mills theory. We give a number of examples
illustrating these ideas.Comment: 32 pages, minor typos correcte
MHV-Vertices for Gravity Amplitudes
We obtain a CSW-style formalism for calculating graviton scattering
amplitudes and prove its validity through the use of a special type of
BCFW-like parameter shift. The procedure is illustrated with explicit examples.Comment: 21 pages, minor typos corrected, proof added in section
Effective action and interaction energy of coupled quantum dots
We obtain the effective action of tunnel-coupled quantum dots, by modeling
the system as a Luttinger liquid with multiple barriers. For a double dot
system, we find that the resonance conditions for perfect conductance form a
hexagon in the plane of the two gate voltages controlling the density of
electrons in each dot. We also explicitly obtain the functional dependence of
the interaction energy and peak-splitting on the gate voltage controlling
tunneling between the dots and their charging energies. Our results are in good
agreement with recent experimental results, from which we obtain the Luttinger
interaction parameter .Comment: 5 pgs,latex,3 figs,revised version to be publshed in Phys.Rev.
One-Loop NMHV Amplitudes involving Gluinos and Scalars in N=4 Gauge Theory
We use Supersymmetric Ward Identities and quadruple cuts to generate n-pt
NMHV amplitudes involving gluinos and adjoint scalars from purely gluonic
amplitudes. We present a set of factors that can be used to generate one-loop
NMHV amplitudes involving gluinos or adjoint scalars in N=4 Super Yang-Mills
from the corresponding purely gluonic amplitude.Comment: 16 pages, JHEP versio
Quadriceps volumes are reduced in people with patellofemoral joint osteoarthritis
Objectives: This study aimed to (1) compare the volumes of vastus medialis (VM), vastus lateralis (VL), vastus intermedius and rectus femoris and the ratio of VM/VL volumes between asymptomatic controls and patellofemoral joint osteoarthritis (PFJ OA) participants; and (2) assess the relationships between cross-sectional area (CSA) and volumes of the VM and VL in individuals with and without PFJ OA. Methods: Twenty-two participants with PFJ OA and 11 controls aged ≥40 years were recruited from the community and practitioner referrals. Muscle volumes of individual quadriceps components were measured from thigh magnetic resonance (MR) images. The CSA of the VM and lateralis were measured at 10 equally distributed levels (femoral condyles to lesser femoral trochanter). Results: PFJ OA individuals had smaller normalized VM (mean difference 0.90 cm ·kg , α = 0.011), VL (1.50 cm ·kg , α = 0.012) and rectus femoris (0.71 cm ·kg , α = 0.009) volumes than controls. No differences in the VM/VL ratio were observed. The CSA at the third level (controls) and fourth level (PFJ OA) above the femoral condyles best predicted VM volume, whereas the VL volume was best predicted by the CSA at the seventh level (controls) and sixth level (PFJ OA) above the femoral condyles. Conclusion: Reduced quadriceps muscle volume was a feature of PFJ OA. Muscle volume could be predicted from CSA measurements at specific levels in PFJ OA patients and controls
On Perturbative Gravity and Gauge Theory
We review some applications of tree-level (classical) relations between
gravity and gauge theory that follow from string theory. Together with
-dimensional unitarity, these relations can be used to perturbatively
quantize gravity theories, i.e. they contain the necessary information for
obtaining loop contributions. We also review recent applications of these ideas
showing that N=1 D=11 supergravity diverges, and review arguments that N=8 D=4
supergravity is less divergent than previously thought, though it does appear
to diverge at five loops. Finally, we describe field variables for the
Einstein-Hilbert Lagrangian that help clarify the perturbative relationship
between gravity and gauge theory.Comment: Talk presented at Third Meeting on Constrained Dynamics and Quantum
Gravity, Villasimius (Sardinia, Italy) September 13-17, 1999 and at the
Workshop on Light-Cone QCD and Nonperturbative Hadron Physics, University of
Adelaide (Australia) December 13-22, 1999. Latex, 9 page
Recursive Calculation of One-Loop QCD Integral Coefficients
We present a new procedure using on-shell recursion to determine coefficients
of integral functions appearing in one-loop scattering amplitudes of gauge
theories, including QCD. With this procedure, coefficients of integrals,
including bubbles and triangles, can be determined without resorting to
integration. We give criteria for avoiding spurious singularities and boundary
terms that would invalidate the recursion. As an example where the criteria are
satisfied, we obtain all cut-constructible contributions to the one-loop
n-gluon scattering amplitude, A_n^{oneloop}(...--+++...), with split-helicity
from an N=1 chiral multiplet and from a complex scalar. Using the
supersymmetric decomposition, these are ingredients in the construction of QCD
amplitudes with the same helicities. This method requires prior knowledge of
amplitudes with sufficiently large numbers of legs as input. In many cases,
these are already known in compact forms from the unitarity method.Comment: 36 pages; v2 clarification added and typos fixed, v3 typos fixe
Grand Unification with Three Generations in Free Fermionic String Models
We examine the problem of constructing three generation free fermionic string
models with grand unified gauge groups. We attempt the construction of models, where is a grand unified group realized at level 1. This
structure allows those Higgs representations to appear which are necessary to
break the symmetry down to the standard model gauge group. For , we
find only models with an even number of generations. However, for we
find a number of 3 generation models.Comment: 22 pages, latex. References added to original versio
Electronic structure of nuclear-spin-polarization-induced quantum dots
We study a system in which electrons in a two-dimensional electron gas are
confined by a nonhomogeneous nuclear spin polarization. The system consists of
a heterostructure that has non-zero nuclei spins. We show that in this system
electrons can be confined into a dot region through a local nuclear spin
polarization. The nuclear-spin-polarization-induced quantum dot has interesting
properties indicating that electron energy levels are time-dependent because of
the nuclear spin relaxation and diffusion processes. Electron confining
potential is a solution of diffusion equation with relaxation. Experimental
investigations of the time-dependence of electron energy levels will result in
more information about nuclear spin interactions in solids
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