32,637 research outputs found
Recoil Polarization Measurements of the Proton Electromagnetic Form Factor Ratio to Q^2 = 8.5 GeV^2
Among the most fundamental observables of nucleon structure, electromagnetic
form factors are a crucial benchmark for modern calculations describing the
strong interaction dynamics of the nucleon's quark constituents; indeed, recent
proton data have attracted intense theoretical interest. In this letter, we
report new measurements of the proton electromagnetic form factor ratio using
the recoil polarization method, at momentum transfers Q2=5.2, 6.7, and 8.5
GeV2. By extending the range of Q2 for which GEp is accurately determined by
more than 50%, these measurements will provide significant constraints on
models of nucleon structure in the non-perturbative regime
Applications of the Feynman-Hellmann theorem in hadron structure
The Feynman-Hellmann (FH) relation offers an alternative way of accessing
hadronic matrix elements through artificial modifications to the QCD
Lagrangian. In particular, a FH-motivated method provides a new approach to
calculations of disconnected contributions to matrix elements and high-momentum
nucleon and pion form factors. Here we present results for the total nucleon
axial charge, including a statistically significant non-negative total
disconnected quark contribution of around at an unphysically heavy pion
mass. Extending the FH relation to finite-momentum transfers, we also present
calculations of the pion and nucleon electromagnetic form factors up to
momentum transfers of around 7-8 GeV. Results for the nucleon are not able
to confirm the existence of a sign change for the ratio , but
suggest that future calculations at lighter pion masses will provide
fascinating insight into this behaviour at large momentum transfers
Polarization transfer in HeH: is the ratio modified in medium ?
Polarization observables in the HeH
reaction are calculated using accurate three- and four-nucleon bound-state wave
functions, a realistic model for the nuclear electromagnetic current operator,
and a treatment of final-state-interactions with an optical potential. In
contrast to earlier studies, no significant discrepancies are found between
theory and experiment both for the ratio of transverse to longitudinal
polarization transfers and for the induced polarization, when free-nucleon
electromagnetic form factors are used in the current operator. The present
results challenge the current interpretation of the experimental data in terms
of medium-modified form factors.Comment: 4 pages and 2 figue
Exposing strangeness: projections for kaon electromagnetic form factors
A continuum approach to the kaon and pion bound-state problems is used to
reveal their electromagnetic structure. For both systems, when used with parton
distribution amplitudes appropriate to the scale of the experiment, Standard
Model hard-scattering formulae are accurate to within 25% at momentum transfers
GeV. There are measurable differences between the
distribution of strange and normal matter within the kaons, e.g. the ratio of
their separate contributions reaches a peak value of at GeV. Its subsequent -evolution is accurately described by the hard
scattering formulae. Projections for kaon and pion form factors at timelike
momenta beyond the resonance region are also presented. These results and
projections should prove useful in planning next-generation experiments.Comment: 7 pages, 4 figure
Scaling law for the electromagnetic form factors of the proton
The violation of the scaling law for the electric and magnetic form factors
of the proton are examined within the cloudy bag model. The suppression of the
ratio of the electric and magnetic form factors is natural in the bag model.
The pion cloud plays a moderate role in understanding the recent data from
TJNAF.Comment: 8 pages, REVTeX, 2 figures include
First Measurement of the Electromagnetic Form Factor of the Neutral Kaon at a Large Momentum Transfer and the Effect of Breaking
At large momentum transfers the photon interacts with the charges and spins
of the constituent partons in a hadron. It is expected that the neutral kaon
can acquire finite electromagnetic form factors because its wave function is
affected by the order of magnitude difference between the mass of the strange
quark and that of the down quark, or flavor breaking. We report on the
first measurement of the form factor of neutral kaons at the large timelike
momentum transfer of GeV by measuring the cross section for
at GeV using CLEO-c data with an integrated
luminosity of 586 pb. We obtain
, with a 90% C.L. interval of
. This is nearly an order of magnitude smaller than
, and indicates that the
effect of breaking is small. In turn, this makes it unlikely that the
recently observed strong violation of the pQCD prediction,
, which is based on the
assumption of similar wave functions for the pions and kaons, can be attributed
to breaking alone.Comment: 4 pages, 2 figures, accepted to PL
The nucleon electromagnetic form factors from Lattice QCD
We evaluate the isovector nucleon electromagnetic form factors in quenched
and full QCD on the lattice using Wilson fermions. In the quenched theory we
use a lattice of spatial size 3 fm at beta=6.0 enabling us to reach low
momentum transfers and a lowest pion mass of about 400 MeV. In the full theory
we use a lattice of spatial size 1.9 fm at beta=5.6 and lowest pion mass of
about 380 MeV enabling comparison with the results obtained in the quenched
theory. We compare our lattice results to the isovector part of the
experimentally measured form factors.Comment: Typos corrected, one reference added, version accepted for
publication in Phys. Rev. D, 14 pages, 20 figure
Light-cone QCD predictions for elastic ed-scattering in the intermediate energy region
The contributions of helicity-flip matrix elements to the deuteron form
factors are discussed in the light-cone frame. Normalized , ,
and are obtained in a simple QCD-inspired model. We find
that plays an important role in . Our numerical results
are consistent with the data in the intermediate energy region.Comment: 9 pages, REVTeX file, 5 figure
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