Manifestation of the JLab proton polarization data on the behaviour of strange nucleon form factors

Special eight-resonance unitary and analytic model of nucleon electromagnetic structure is used to analyze, first the classical proton form factor data obtained by the Rosenbluth technique and then also the contradicting JLab proton polarization data on the ratio $\mu_p G_{Ep}(Q^2)/G_{Mp}(Q^2)$ with the aim to investigate a manifestation of the latter on the strange nucleon form factors behaviour.Comment: Latex, 9 pages, 3 figures. Talk given at the PAVI06 International Workshop, 16-20 May, 2006, Milos, Greec

Is there model-independent evidence of the two-photon-exchange effect in the electron-proton elastic scattering cross section?

We re-analyze the data of the elastic electron proton scattering to look for model-independent evidence of the two-photon-exchange (TPE) effect. In contrast to previous analyses, TPE effect is parametrized in forms which are free of kinematical-singularity, in addition to being consistent with the constraint derived from crossing symmetry and the charge conjugation. Moreover, we fix the value of $R=G_E/G_M$ as determined from the data of the polarization transfer experiment. We find that, at high $Q^2 \geq 2 GeV^2$ values, the contribution of the TPE effect to the slope of $\sigma_R$ vs. $\epsilon$ is large and comparable with that arising from $G_{E}$. It also behaves quasi-linearly in the region of current data, namely, in the range of $0.2 < \epsilon < 0.95$. Hence the fact that the current elastic $ep$ cross section data shows little nonlinearity with respect to $\epsilon$ can not be used to exclude the presence of the TPE effect. More precise data at extreme angles will be crucial for a model-independent extraction of the TPE effect.Comment: 11 pages, 4 figure

On Nucleon Electromagnetic Form Factors: A Pre'cis

Electron scattering at large Q^2 probes a nucleon's quark core. This core's contribution to electromagnetic form factors may be calculated using Poincare' covariant Faddeev amplitudes combined with a nucleon-photon vertex that automatically fulfills a Ward-Takahashi identity for on-shell nucleons. The calculated behaviour of G_E^p(Q^2)/G_M^p(Q^2) on 2<Q^2(GeV^2)<6 agrees with that inferred from polarisation transfer data, and exhibits a zero at Q^2\approx 6.5 GeV^2. There is some evidence that F_2(Q^2)/F_1(Q^2) \propto [\ln(Q^2/\Lambda^2)]^2/Q^2 for Q^2>6 GeV^2.Comment: Contribution to the proceedings of "Baryons 04," the 10th International Conference on the Structure of Baryons, 25-29/Oct./04, Ecole Polytechnique, Palaiseau; 5 pages, 3 figure

Model independent properties of two-photon exchange in elastic electron proton scattering

We derive from first principles, as the C-invariance of the electromagnetic interaction and the crossing symmetry, the general properties of two-photon exchange in electron-proton elastic scattering. We show that the presence of this mechanism destroys the linearity of the Rosenbluth separation.Comment: 12 pages, no figures- Corrected misprints, changes in P. 7. No changes in conclusion

Generalized parton distributions from form factors

The electromagnetic nucleon form factors provide constraints on generalized quark distributions. Key results of the study presented here are a strong dependence of the average impact parameter of quarks on their longitudinal momentum fraction, a striking difference in the t dependence of u and d quark contributions to elastic form factors, and an estimate of the orbital angular momentum carried by valence quarks in the nucleon.Comment: 10 pages, 11 figures. To appear in: Procs. of the Workshop on Light-Cone QCD and Nonperturbative Hadron Physics 2005 (LC 2005), Cairns, Australia, 200

New empirical fits to the proton electromagnetic form factors

Recent measurements of the ratio of the elastic electromagnetic form factors of the proton, G_Ep/G_Mp, using the polarization transfer technique at Jefferson Lab show that this ratio decreases dramatically with increasing Q^2, in contradiction to previous measurements using the Rosenbluth separation technique. Using this new high quality data as a constraint, we have reanalyzed most of the world e-p elastic cross section data. In this paper, we present a new empirical fit to the reanalyzed data for the proton elastic magnetic form factor in the region 0 < Q^2 < 30 GeV^2. As well, we present an empirical fit to the proton electromagnetic form factor ratio, G_Ep/G_Mp, which is valid in the region 0.1 < Q^2 < 6 GeV^2

The Role of 5-quark Components on the Nucleon Form Factors

The covariant quark model is shown to allow a phenomenological description of the neutron electric form factor, G_E^n(Q^2), in the impulse approximation, provided that the wave function contains minor (~ 3 %) admixtures of the lowest sea-quark configurations. While that form factor is not very sensitive to whether the \bar q in the qqqq\bar q component is in the P-state or in the S-state, the calculated nucleon magnetic form factors are much closer to the empirical values in the case of the former configuration. In the case of the electric form factor of the proton, G_E^p(Q^2), a zero appears in the impulse approximation close to 9 GeV^2, when the \bar q is in the P-state. That configuration, which may be interpreted as a pion loop ("cloud") fluctuation, also leads to a clearly better description of the nucleon magnetic moments. When the amplitude of the sea-quark admixtures are set so as to describe the electric form factor of the neutron, the qqqq\bar q admixtures have the phenomenologically desirable feature, that the electric form factor of the proton falls at a more rapid rate with momentum transfer than the magnetic form factor.Comment: To appear in Nuclear Physics

Beam normal spin asymmetry in elastic lepton-nucleon scattering

We discuss the two-photon exchange contribution to observables which involve lepton helicity flip in elastic lepton-nucleon scattering. This contribution is accessed through the spin asymmetry for a lepton beam polarized normal to the scattering plane. We estimate this beam normal spin asymmetry at large momentum transfer using a parton model and we express the corresponding amplitude in terms of generalized parton distributions.Comment: 17 pages, 5 figure

Shapes of the Proton

A model proton wave function, constructed using Poincare invariance, and constrained by recent electromagnetic form factor data, is used to study the shape of the proton. Spin-dependent quark densities are defined as matrix elements of density operators in proton states of definite spin-polarization, and shown to have an infinite variety of non-spherical shapes. For high momentum quarks with spin parallel to that of the proton, the shape resembles that of a peanut, but for quarks with anti-parallel spin the shape is that of a bagel.Comment: 8 pages, 5 figures, to be submitted to Phys. Rev. C This corrects a few typos and explains some further connections with experiment

Physics at an Upgraded Fermilab Proton Driver

In 2004 the Fermilab Long Range Planning Committee identified a new high intensity Proton Driver as an attractive option for the future, primarily motivated by the recent exciting developments in neutrino physics. Over the last few months a physics study has developed the physics case for the Fermilab Proton Driver. The potential physics opportunities are discussed.Comment: 14 pages, 6 figures. Presented at the High Intensity Frontiers Workshop (HF05), Isola d'Elba, Italy, 28 May - 1 June, 200