224 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
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
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
The gamma-N -> Delta transition in chiral effective-field theory
We describe the pion electroproduction processes in the
(1232)-resonance region within the framework of chiral effective-field
theory. By studying the observables of pion electroproduction in a
next-to-leading order calculation we are able to make predictions and draw
conclusions on the properties of the electromagnetic form
factors.Comment: 9 pages, 7 figures; prepared for the proceedings of "Shapes of
Hadrons" workshop, Athens, Greece, 25-30 April 200
Electromagnetic Nucleon-to-Delta Transition in Chiral Effective-Field Theory
We perform a relativistic chiral effective-field theory calculation of the
pion electroproduction off the nucleon () in the
Delta(1232)-resonance region. After fixing the three low-energy constants,
corresponding to the magnetic (M1), electric (E2), and Coulomb (C2) couplings, our calculation provides a prediction for the
momentum-transfer and pion-mass dependence of the form
factors. The prediction for the pion-mass dependence resolves the discrepancy
between the recent lattice QCD results and the experimental value for the
"C2/M1 ratio" at low .Comment: 4 pages (RevTeX), 4 figures; a few claryifying phrases and a
reference added, typos correcte
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