1,847 research outputs found
Two-photon physics
It is reviewed how Compton scattering sum rules relate low-energy nucleon
structure quantities to the nucleon excitation spectrum. In particular, the GDH
sum rule and recently proposed extensions of it will be discussed. These
extensions are sometimes more calculationally robust, which may be an advantage
when estimating the chiral extrapolations of lattice QCD results, such as for
anomalous magnetic moments. Subsequently, new developments in our description
of the nucleon excitation spectrum will be discussed, in particular a recently
developed chiral effective field theory framework for the
-resonance region. Within this framework, we discuss results on
and masses, the transition and the
magnetic dipole moment.Comment: 10 pages, prepared for proceedings of Symposium on 20 Years of
Physics at the Mainz Mikrotro
Deeply virtual electroproduction of photons and mesons on the nucleon
We give predictions for the leading order amplitudes for deeply virtual
Compton scattering and hard meson electroproduction reactions at large Q^2 in
the valence region in terms of skewed quark distributions. We give first
estimates for the power corrections to these leading order amplitudes. In
particular, we outline examples of experimental opportunities to access the
skewed parton distributions at the current high-energy lepton facilities :
JLab, HERMES and COMPASS.Comment: 4 pages, 4 figures. Invited talk at XVth Particles and Nuclei
International Conference (PANIC 99), Uppsala, Sweden, 10-16 June 1999. To be
published by World Scientific (Singapore
Single meson contributions to the muon's anomalous magnetic moment
We develop the formalism to provide an improved estimate for the hadronic
light-by-light correction to the muon's anomalous magnetic moment a_{\mu}, by
considering single meson contributions beyond the leading pseudo-scalar mesons.
We incorporate available experimental input as well as constraints from
light-by-light scattering sum rules to estimate the effects of axial-vector,
scalar, and tensor mesons. We give numerical evaluations for the hadronic
light-by-light contribution of these states to a_{\mu}. The presented formalism
allows to further improve on these estimates, once new data for such meson
states will become available.Comment: 13 pages, 3 figure
Subtracted dispersion relation formalism for the two-photon exchange correction to elastic electron-proton scattering: comparison with data
We apply a subtracted dispersion relation formalism with the aim to improve
predictions for the two-photon exchange corrections to elastic electron-proton
scattering observables at finite momentum transfers. We study the formalism on
the elastic contribution, and make a detailed comparison with existing data for
unpolarized cross sections as well as polarization transfer observables.Comment: 49 pages, 26 figures, version published in Eur. Phys. J. A 51, no. 2,
24 (2015
Dispersive analysis of the process
We present a theoretical study of the processes from the threshold through the region in the
invariant mass. We adopt the Omn\`es representation in order to
account for rescattering effects in both s- and d-partial waves. For the
description of the resonance, we implement a coupled-channel
unitarity. The constructed amplitudes serve as an essential framework to
interpret the current experimental two-photon fusion program at BESIII. They
also provide an important input for the dispersive analyses of the hadronic
light-by-light scattering contribution to the muon's anomalous magnetic moment.Comment: 8 pages, 4 figures (journal version
Wide angle Compton scattering on the proton: study of power suppressed corrections
We study the wide angle Compton scattering process on a proton within the
soft collinear factorization (SCET) framework. The main purpose of this work is
to estimate the effect due to certain power suppressed corrections. We consider
all possible kinematical power corrections and also include the subleading
amplitudes describing the scattering with nucleon helicity flip. Under certain
assumptions we present a leading-order factorization formula for these
amplitudes which includes the hard- and soft-spectator contributions. We apply
the formalism and perform a phenomenological analysis of the cross section and
asymmetries in the wide angle Compton scattering on a proton. We assume that in
the relevant kinematical region where ~GeV the dominant
contribution is provided by the soft-spectator mechanism. The hard coefficient
functions of the corresponding SCET operators are taken in the leading-order
approximation. The analysis of existing cross section data shows that the
contribution of the helicity flip amplitudes to this observable is quite small
and comparable with other expected theoretical uncertainties. We also show
predictions for double polarization observables for which experimental
information exists.Comment: 15 pages, 5 figure
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