15 research outputs found
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
Beam normal spin asymmetry for the process
We calculate the single spin asymmetry for the
process, for an electron beam polarized normal to the scattering plane. Such
single spin asymmetries vanish in the one-photon exchange approximation, and
are directly proportional to the absorptive part of a two-photon exchange
amplitude. As the intermediate state in such two-photon exchange process is on
its mass shell, the asymmetry allows one to access for the first time the
on-shell as well as electromagnetic
transitions. We present the general formalism to describe the beam normal spin asymmetry, and provide a numerical estimate of its
value using the nucleon, , , and
intermediate states. We compare our results with the first data from the
Qweak@JLab experiment and give predictions for the A4@MAMI experiment.Comment: 18 pages, 9 figure
Light-by-light scattering sum rules constraining meson transition form factors
Relating the forward light-by-light scattering to energy weighted integrals
of the \gamma* \gamma -fusion cross sections, with one real photon (\gamma) and
one virtual photon (\gamma*), we find two new exact super-convergence
relations. They complement the known super-convergence relation based on the
extension of the GDH sum rule to the light-light system. We also find a set of
sum rules for the low-energy photon-photon interaction. All of the new
relations are verified here exactly at leading order in scalar and spinor QED.
The super-convergence relations, applied to the \gamma* \gamma -production of
mesons, lead to intricate relations between the \gamma \gamma -decay widths or
the \gamma* \gamma -transition form factors for (pseudo-) scalar, axial-vector
and tensor mesons. We discuss the phenomenological implications of these
results for mesons in both the light-quark sector and the charm-quark sector.Comment: 32 pages, 7 figure