26 research outputs found
The size of the proton - closing in on the radius puzzle
We analyze the recent electron-proton scattering data from Mainz using a
dispersive framework that respects the constraints from analyticity and
unitarity on the nucleon structure. We also perform a continued fraction
analysis of these data. We find a small electric proton charge radius, r_E^p =
0.84_{-0.01}^{+0.01} fm, consistent with the recent determination from muonic
hydrogen measurements and earlier dispersive analyses. We also extract the
proton magnetic radius, r_M^p = 0.86_{-0.03}^{+0.02} fm, consistent with
earlier determinations based on dispersion relations.Comment: 4 pages, 2 figures, fit improved, small modifications, section on
continued fractions modified, conclusions on the proton charge radius
unchanged, version accepted for publication in European Physical Journal
No Sommerfeld resummation factor in e+e- -> ppbar ?
The Sommerfeld rescattering formula is compared to the e+e- -> ppbar BaBar
data at threshold and above. While there is the expected Coulomb enhancement at
threshold, two unexpected outcomes have been found: |G^p (4M_p^2)|= 1, like for
a pointlike fermion, and moreover data show that the resummation factor in the
Sommerfeld formula is not needed. Other e+e- -> baryon-antibaryon cross
sections show a similar behavior near threshold.Comment: 9 pages, 6 figure
Novel evaluation of the two-pion contribution to the nucleon isovector form factors
We calculate the two-pion continuum contribution to the nucleon isovector
spectral functions drawing upon the new high statistics measurements of the
pion form factor by the CMD-2, KLOE, and SND collaborations. The general
structure of the spectral functions remains unchanged, but the magnitude
increases by about 10%. Using the updated spectral functions, we calculate the
contribution of the two-pion continuum to the nucleon isovector form factors
and radii. We compare the isovector radii with simple rho-pole models and
illustrate their strong underestimation in such approaches. Moreover, we give a
convenient parametrization of the result for use in future form factor
analyses.Comment: 9 pages, 2 eps figures, revtex4, CMD-2 and SND data included,
conclusions unchanged, version to appear in Phys. Lett.
Isospin breaking in the vector current of the nucleon
Extraction of the nucleon's strange form factors from experimental data
requires a quantitative understanding of the unavoidable contamination from
isospin violation. A number of authors have addressed this issue during the
past decade, and their work is reviewed here. The predictions from early models
are largely consistent with recent results that rely as much as possible on
input from QCD symmetries and related experimental data. The resulting bounds
on isospin violation are sufficiently precise to be of value to on-going
experimental and theoretical studies of the nucleon's strange form factors.Comment: 5 pages, 3 figures. Presented at the International Workshop "From
Parity Violation to Hadronic Structure and more...", Milos, Greece, 16-20 May
2006. Version 2 is only to update Refs. [21] and [25
Determination of two-photon exchange amplitudes from elastic electron-proton scattering data
Using the available cross section and polarization data for elastic
electron-proton scattering, we provide an extraction of the two-photon exchange
amplitudes at a common value of four-momentum transfer, around Q^2 = 2.5 GeV^2.
This analysis also predicts the e^+ p / e^- p elastic scattering cross section
ratio, which will be measured by forthcoming experiments.Comment: 4 pages, 5 figures, updated error analysi
Primakoff effect in eta-photoproduction off protons
We analyse data on forward eta-meson photoproduction off a proton target and
extract the eta to gamma gamma decay width utilizing the Primakoff effect. The
hadronic amplitude that enters into our analysis is strongly constrained
because it is fixed from a global fit to available gamma p to p eta data for
differential cross sections and polarizations. We compare our results with
present information on the two-photon eta-decay from the literature. We provide
predictions for future PrimEx experiments at Jefferson Laboratory in order to
motivate further studies.Comment: 5 pages, 6 figures, gamma-gamma*-eta form factor included, version to
appear in Eur. Phys. J. A
A new measurement of the structure functions and in virtual Compton scattering at 0.33 (GeV/c)
The cross section of the reaction has been measured at
(GeV/c). The experiment was performed using the electron beam
of the MAMI accelerator and the standard detector setup of the A1
Collaboration. The cross section is analyzed using the low-energy theorem for
virtual Compton scattering, yielding a new determination of the two structure
functions P_LL}-P_{TT}/epsilon and which are linear combinations of
the generalized polarizabilities of the proton. We find somewhat larger values
than in the previous investigation at the same . This difference, however,
is purely due to our more refined analysis of the data. The results tend to
confirm the non-trivial -evolution of the generalized polarizabilities and
call for more measurements in the low- region ( 1 (GeV/c)).Comment: 9 pages, 10 figures. EPJA version. slight revisions in the text and
figure