1,678 research outputs found
On the Coupling of Gravitons to Matter
Using relationships between open and closed strings, we present a
construction of tree-level scattering amplitudes for gravitons minimally
coupled to matter in terms of gauge theory partial amplitudes. In particular,
we present examples of amplitudes with gravitons coupled to vectors or to a
single fermion pair. We also present two examples with massive graviton
exchange, as would arise in the presence of large compact dimensions. The gauge
charges are represented by flavors of dynamical scalars or fermions. This also
leads to an unconventional decomposition of color and kinematics in gauge
theories.Comment: RevTex, 4 page
QCD and QED Corrections to Light-by-Light Scattering
We present the QCD and QED corrections to the fermion-loop contributions to
light-by-light scattering, gamma gamma to gamma gamma, in the ultrarelativistic
limit where the kinematic invariants are much larger than the masses of the
charged fermions.Comment: 17 pages, 3 figure files, JHEP styl
Dual conformal constraints and infrared equations from global residue theorems in N=4 SYM theory
Infrared equations and dual conformal constraints arise as consistency
conditions on loop amplitudes in N=4 super Yang-Mills theory. These conditions
are linear relations between leading singularities, which can be computed in
the Grassmannian formulation of N=4 super Yang-Mills theory proposed recently.
Examples for infrared equations have been shown to be implied by global residue
theorems in the Grassmannian picture. Both dual conformal constraints and
infrared equations are mapped explicitly to global residue theorems for
one-loop next-to-maximally-helicity-violating amplitudes. In addition, the
identity relating the BCFW and its parity-conjugated form of tree-level
amplitudes, is shown to emerge from a particular combination of global residue
theorems.Comment: 21 page
Monodromy--like Relations for Finite Loop Amplitudes
We investigate the existence of relations for finite one-loop amplitudes in
Yang-Mills theory. Using a diagrammatic formalism and a remarkable connection
between tree and loop level, we deduce sequences of amplitude relations for any
number of external legs.Comment: 24 pages, 6 figures, v2 typos corrected, reference adde
New Relations for Gauge-Theory Amplitudes
We present an identity satisfied by the kinematic factors of diagrams
describing the tree amplitudes of massless gauge theories. This identity is a
kinematic analog of the Jacobi identity for color factors. Using this we find
new relations between color-ordered partial amplitudes. We discuss applications
to multi-loop calculations via the unitarity method. In particular, we
illustrate the relations between different contributions to a two-loop
four-point QCD amplitude. We also use this identity to reorganize gravity tree
amplitudes diagram by diagram, offering new insight into the structure of the
KLT relations between gauge and gravity tree amplitudes. This can be used to
obtain novel relations similar to the KLT ones. We expect this to be helpful in
higher-loop studies of the ultraviolet properties of gravity theories.Comment: 40 pages, 7 figures, RevTex, v2 minor correction
Note on Bonus Relations for N=8 Supergravity Tree Amplitudes
We study the application of non-trivial relations between gravity tree
amplitudes, the bonus relations, to all tree-level amplitudes in N=8
supergravity. We show that the relations can be used to simplify explicit
formulae of supergravity tree amplitudes, by reducing the known form as a sum
of (n-2)! permutations obtained by solving on-shell recursion relations, to a
new form as a (n-3)!-permutation sum. We demonstrate the simplification by
explicit calculations of the next-to-maximally helicity violating (NMHV) and
next-to-next-to-maximally helicity violating (N^2MHV) amplitudes, and provide a
general pattern of bonus coefficients for all tree-level amplitudes.Comment: 21 pages, 9 figures; v2, minor changes, references adde
Three-Loop Superfiniteness of N=8 Supergravity
We construct the three-loop four-point amplitude of N=8 supergravity using
the unitarity method. The amplitude is ultraviolet finite in four dimensions.
Novel cancellations, not predicted by traditional superspace power-counting
arguments, render its degree of divergence in D dimensions to be no worse than
that of N=4 super-Yang-Mills theory -- a finite theory in four dimensions.
Similar cancellations can be identified at all loop orders in certain unitarity
cuts, suggesting that N=8 supergravity may be a perturbatively finite theory of
quantum gravity.Comment: 5 pages, 4 figures. In v2 references and minor clarifications adde
From lightcone actions to maximally supersymmetric amplitudes
In this article actions for N=4 SYM and N=8 supergravity are formulated in
terms of a chiral superfield, which contains only the physical degrees of
freedom of either theory. In these new actions, which originate from the
lightcone superspace, the supergravity cubic vertex is the square of the gauge
theory one (omitting the color structures). Amplitude calculations using the
corresponding Feynman supergraph rules are tedious, but can be simplified by
choosing a preferred superframe. Recursive calculations of all MHV amplitudes
in N=4 SYM and the four-point N=8 supergravity amplitude are shown to agree
with the known results and connections to the BCFW recursion relations are
pointed out. Finally, the new path integrals are discussed in the context of
the double-copy property relating N=4 SYM theory to N=8 supergravity.Comment: 29 pages, 2 figures, v2: title modified, published versio
Supertwistor space for 6D maximal super Yang-Mills
6D maximal super Yang-Mills on-shell amplitudes are formulated in superspace
using 6 dimensional twistors. The 3,4,5-point tree amplitudes are obtained by
supersymmetrizing their bosonic counterparts and confirmed through the BCFW
construction. In contrast to 4D this superspace is non-chiral, reflecting the
fact that one cannot differentiate MHV from in 6D. Combined
with unitarity methods, this superspace should be useful for the study of
multi-loop D dimensional maximal super Yang-Mills and gravity amplitudes.
Furthermore, the non-chiral nature gives a natural framework for an off-shell
construction. We show this by matching our result with off-shell D=4 N=4 super
Yang-Mills amplitudes, expressed in projective superspace.Comment: 6 figures 28 pages. with better sign
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
- …