856 research outputs found
Generic multiloop methods and application to N=4 super-Yang-Mills
We review some recent additions to the tool-chest of techniques for finding
compact integrand representations of multiloop gauge-theory amplitudes -
including non-planar contributions - applicable for N=4 super-Yang-Mills in
four and higher dimensions, as well as for theories with less supersymmetry. We
discuss a general organization of amplitudes in terms of purely cubic graphs,
review the method of maximal cuts, as well as some special D-dimensional
recursive cuts, and conclude by describing the efficient organization of
amplitudes resulting from the conjectured duality between color and kinematic
structures on constituent graphs.Comment: 42 pages, 18 figures, invited review for a special issue of Journal
of Physics A devoted to "Scattering Amplitudes in Gauge Theories", v2 minor
corrections, v3 added reference
Vector Effective Field Theories from Soft Limits
We present a bottom-up construction of vector effective field theories using
the infrared structure of scattering amplitudes. Our results employ two
distinct probes of soft kinematics: multiple soft limits and single soft limits
after dimensional reduction, applicable in four and general dimensions,
respectively. Both approaches uniquely specify the Born-Infeld (BI) model as
the only theory of vectors completely fixed by certain infrared conditions
which generalize the Adler zero for pions. These soft properties imply new
recursion relations for on-shell scattering amplitudes in BI theory and suggest
the existence of a wider class of vector effective field theories.Comment: 6 pages, 2 figure
Amplitudes at Weak Coupling as Polytopes in AdS_5
We show that one-loop scalar box functions can be interpreted as volumes of
geodesic tetrahedra embedded in a copy of AdS_5 that has dual conformal
space-time as boundary. When the tetrahedron is space-like, it lies in a
totally geodesic hyperbolic three-space inside AdS_5, with its four vertices on
the boundary. It is a classical result that the volume of such a tetrahedron is
given by the Bloch-Wigner dilogarithm and this agrees with the standard physics
formulae for such box functions. The combinations of box functions that arise
in the n-particle one-loop MHV amplitude in N=4 super Yang-Mills correspond to
the volume of a three-dimensional polytope without boundary, all of whose
vertices are attached to a null polygon (which in other formulations is
interpreted as a Wilson loop) at infinity.Comment: 16 pages, 5 figure
On different lagrangian formalisms for vector resonances within chiral perturbation theory
We study the relation of vector Proca field formalism and antisymmetric
tensor field formalism for spin-one resonances in the context of the large N_C
inspired chiral resonance Lagrangian systematically up to the order O(p6) and
give a transparent prescription for the transition from vector to antisymmetric
tensor Lagrangian and vice versa. We also discuss the possibility to describe
the spin-one resonances using an alternative "mixed" first order formalism,
which includes both types of fields simultaneously, and compare this one with
the former two. We also briefly comment on the compatibility of the above
lagrangian formalisms with the high-energy constraints for concrete VVP
correlator.Comment: 34 pages, 3 figure
Basics of Generalized Unitarity
We review generalized unitarity as a means for obtaining loop amplitudes from
on-shell tree amplitudes. The method is generally applicable to both
supersymmetric and non-supersymmetric amplitudes, including non-planar
contributions. Here we focus mainly on N=4 Yang-Mills theory, in the context of
on-shell superspaces. Given the need for regularization at loop level, we also
review a six-dimensional helicity-based superspace formalism and its
application to dimensional and massive regularizations. An important feature of
the unitarity method is that it offers a means for carrying over any identified
tree-level property of on-shell amplitudes to loop level, though sometimes in a
modified form. We illustrate this with examples of dual conformal symmetry and
a recently discovered duality between color and kinematics.Comment: 37 pages, 10 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
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
Mesons in Nuclei: eta and omega mesons
Data on the photoproduction of omega mesons on nuclei have been re-analyzed
in search for in-medium modifications. The data were taken with the Crystal
Barrel(CB)/TAPS detector system at the ELSA accelerator facility in Bonn. The
extracted omega line shape was found to be sensitive to the background
subtraction. In experiments at the tagged photon facility of the Mainz MAMI
accelerator photoproduction of mesons from light nuclear targets (deuteron and
3He) has been studied. The experiments used the combined Crystal Ball/TAPS
setup in Mainz. Measurements of eta- and pi0-photoproduction off a liquid
3He-target have been used for the search for the formation of eta-mesic 3He.
The installation of the WASA detector at COSY opened a unique possibility to
search for the 4He-eta bound state with high statistics and high acceptance. We
are conducting a search via an exclusive measurement of the excitation function
for the dd -> 3He p pi- reaction.Comment: Proceedings of New Frontiers In QCD 2010: Exotic Hadron Systems And
Dense Matter 18 Jan - 19 Mar 2010, Kyoto, Japa
Modification of the -Meson Lifetime in Nuclear Matter
The photo production of mesons on the nuclei C, Ca, Nb and Pb has
been measured using the Crystal Barrel/TAPS detector at the ELSA tagged photon
facility in Bonn. The dependence of the meson cross section on the
nuclear mass number has been compared with three different types of models, a
Glauber analysis, a BUU analysis of the Giessen theory group and a calculation
by the Valencia theory group. In all three cases, the inelastic width
is found to be at normal nuclear matter density for an
average 3-momentum of 1.1 GeV/c. In the restframe of the meson, this
inelastic width corresponds to a reduction of the lifetime by
a factor . For the first time, the momentum dependent N
cross section has been extracted from the experiment and is in the range of 70
mb.Comment: 5 pages, 4 figure
In-medium mass from the reaction
Data on the photoproduction of mesons on nuclei have been
re-analyzed in a search for in-medium modifications. The data were taken with
the Crystal Barrel(CB)/TAPS detector system at the ELSA accelerator facility in
Bonn. First results from the analysis of the data set were published by D.
Trnka et al. in Phys. Rev. Lett 94 (2005) 192303 \cite{david}, claiming a
lowering of the mass in the nuclear medium by 14 at normal nuclear
matter density. The extracted line shape was found to be sensitive to
the background subtraction. For this reason a re-analysis of the same data set
has been initiated and a new method has been developed to reduce the background
and to determine the shape and absolute magnitude of the background directly
from the data. Details of the re-analysis and of the background determination
are described. The signal on the target, extracted in the
re-analysis, does not show a deviation from the corresponding line shape on a
target, measured as reference. The earlier claim of an in-medium mass
shift is thus not confirmed. The sensitivity of the line shape to
different in-medium modification scenarios is discussed.Comment: 13 pages and 11 figures, submitted for publicatio
Photoproduction of {\omega} Mesons off the Proton
The differential cross sections and unpolarized spin-density matrix elements
for the reaction were measured using the CBELSA/TAPS
experiment for initial photon energies ranging from the reaction threshold to
2.5 GeV. These observables were measured from the radiative decay of the
meson, . The cross sections cover the full
angular range and show the full extent of the -channel forward rise. The
overall shape of the angular distributions in the differential cross sections
and unpolarized spin-density matrix elements are in fair agreement with
previous data. In addition, for the first time, a beam of linearly-polarized
tagged photons in the energy range from 1150 MeV to 1650 MeV was used to
extract polarized spin-density matrix elements.
These data were included in the Bonn-Gatchina partial wave analysis (PWA).
The dominant contribution to photoproduction near threshold was found
to be the partial wave, which is primarily due to the sub-threshold
resonance. At higher energies, pomeron-exchange was found to
dominate whereas -exchange remained small. These -channel contributions
as well as further contributions from nucleon resonances were necessary to
describe the entire dataset: the , , and partial waves
were also found to contribute significantly.Comment: 8 pages, 5 figures, accepted for publication in Physics Letters
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