138 research outputs found
Compton Scattering by the Proton using a Large-Acceptance Arrangement
Compton scattering by the proton has been measured using the tagged-photon
facility at MAMI (Mainz) and the large-acceptance arrangement LARA. The new
data are interpreted in terms of dispersion theory based on the SAID-SM99K
parameterization of photo-meson amplitudes. It is found that two-pion exchange
in the t-channel is needed for a description of the data in the second
resonance region. The data are well represented if this channel is modeled by a
single pole with mass parameter m(sigma)=600 MeV. The asymptotic part of the
spin dependent amplitude is found to be well represented by pi-0-exchange in
the t-channel. A backward spin-polarizability of
gamma(pi)=(-37.1+-0.6(stat+syst)+-3.0(model))x10^{-4}fm^4 has been determined
from data of the first resonance region below 455 MeV. This value is in a good
agreement with predictions of dispersion relations and chiral pertubation
theory. From a subset of data between 280 and 360 MeV the resonance
pion-photoproduction amplitudes were evaluated leading to a E2/M1 multipole
ratio of the p-to-Delta radiative transition of EMR(340
MeV)=(-1.7+-0.4(stat+syst)+-0.2(model))%. It was found that this number is
dependent on the parameterization of photo-meson amplitudes. With the MAID2K
parameterization an E2/M1 multipole ratio of EMR(340
MeV)=(-2.0+-0.4(stat+syst)+-0.2(model))% is obtained
Automatic Generation of Efficient Linear Algebra Programs
The level of abstraction at which application experts reason about linear
algebra computations and the level of abstraction used by developers of
high-performance numerical linear algebra libraries do not match. The former is
conveniently captured by high-level languages and libraries such as Matlab and
Eigen, while the latter expresses the kernels included in the BLAS and LAPACK
libraries. Unfortunately, the translation from a high-level computation to an
efficient sequence of kernels is a task, far from trivial, that requires
extensive knowledge of both linear algebra and high-performance computing.
Internally, almost all high-level languages and libraries use efficient
kernels; however, the translation algorithms are too simplistic and thus lead
to a suboptimal use of said kernels, with significant performance losses. In
order to both achieve the productivity that comes with high-level languages,
and make use of the efficiency of low level kernels, we are developing Linnea,
a code generator for linear algebra problems. As input, Linnea takes a
high-level description of a linear algebra problem and produces as output an
efficient sequence of calls to high-performance kernels. In 25 application
problems, the code generated by Linnea always outperforms Matlab, Julia, Eigen
and Armadillo, with speedups up to and exceeding 10x
Low-Energy Compton Scattering of Polarized Photons on Polarized Nucleons
The general structure of the cross section of scattering with
polarized photon and/or nucleon in initial and/or final state is systematically
described and exposed through invariant amplitudes. A low-energy expansion of
the cross section up to and including terms of order is given which
involves ten structure parameters of the nucleon (dipole, quadrupole,
dispersion, and spin polarizabilities). Their physical meaning is discussed in
detail. Using fixed-t dispersion relations, predictions for these parameters
are obtained and compared with results of chiral perturbation theory. It is
emphasized that Compton scattering experiments at large angles can fix the most
uncertain of these structure parameters. Predictions for the cross section and
double-polarization asymmetries are given and the convergence of the expansion
is investigated. The feasibility of the experimental determination of some of
the struture parameters is discussed.Comment: 41 pages of text, 9 figures; minor revisions prior to publication in
Phys. Rev.
Fixed-t subtracted dispersion relations for Compton scattering off the nucleon
We present fixed- subtracted dispersion relations for Compton scattering
off the nucleon at energies 500 MeV, as a formalism to extract
the nucleon polarizabilities with a minimum of model dependence. The subtracted
dispersion integrals are mainly saturated by intermediate states in the
-channel and intermediate states
in the -channel . For the subprocess
, we construct a unitarized amplitude and find a
good description of the available data. We show results for Compton scattering
using the subtracted dispersion relations and display the sensitivity on the
scalar polarizability difference and the backward spin
polarizability , which enter directly as fit parameters in the
present formalism
Der 370/E, ein IBM-Emulator fuer Simulationsrechnungen und On-line-Anwendungen in der Teilchenphysik
TIB: RN 6945 (1985,17) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman
Angular and polarization dependence of Compton scattering from in the Δ-resonance region
Using linearly polarized tagged photons from coherent bremsstrahlung, differential cross sections and beam asymmetries for Compton scattering by have been measured at MAMI in the energy interval between 150 MeV and 500 MeV for scattering angles of θγlab=37°, 93° and 137°, thus largely increasing the available data base. Improved calculations in terms of the Δ-hole model completely fail to describe the data at large scattering angles. The same proved to be true for a schematic model, even after taking into account properties of nuclear photo-absorption in very detail
- …