Proton Form Factors Research Articles

ELASTIC SCATTERING AND THE PROTON FORM FACTOR

We use the QCD pomeron model proposed by Landshoff and Nachtmann to compute the differential and the total cross-sections for pp scattering in order to discuss a QCD-based approach to the proton form factor. This model is quite dependent on the experimental electromagnetic form factor, and it is not totally clear why this form factor gives good results even at moderate transferred momentum. We exchange the electromagnetic form factor by the asymptotic QCD proton form factor determined by Brodsky and Lepage (BL) plus a prescription for its low energy behavior dictated by the existence of a dynamically generated gluon mass. We fit the data with this QCD inspired form factor and a value for the dynamical gluon mass consistent with the ones determined in the literature. Our results also provide a determination of the proton wave function at the origin, which appears in the BL form factor.

Box diagram in the elastic electron-proton scattering

We present an evaluation of the box diagram for elastic $\mathit{ep}$ scattering with the proton in the intermediate state. Using analytic properties of the proton form factors we express the amplitude via a twofold integral that involves the form factors in the spacelike region only. Therefore experimentally measured form factors can be used in the calculations directly. The numerical calculation is done with the form factors extracted by Rosenbluth separation, as well as polarization transfer method. The dependence of the results on the form factor choice is small for ${Q}^{2}\ensuremath{\lesssim}6\phantom{\rule{0.3em}{0ex}}{\mathrm{GeV}}^{2}$ but becomes sizable at higher ${Q}^{2}$.

A description of the ratio between electric and magnetic proton form factors by using space-like, time-like data and dispersion relations

We define and use a dispersive technique, based on data, analyticity and dispersion relations to handle the nucleon form factors. We use the available information on the ratio between the electric and magnetic proton form factors, coming from recent space-like and time-like data and theory, as inputs in a dispersive approach that gives as outcome a complex description of this ratio, valid in the whole q 2 plane. The possibility of extending this definition outside the data regions allows us to make predictions about the time-like polarizations, the asymptotic behaviour and the presence of zeros for the ratio.

Inclusive scattering data on light nuclei as a tool for the extraction of [formula omitted

We demonstrate that refinements in the analysis of inclusive scattering data on light nuclei enable the extraction of, generally accurate, values of the neutron magnetic form factor G M n ( Q 2 ) . In particular, a recent parameterization of ep inclusive resonance excitation enables a reliable calculation of the inelastic background, and as a consequence a separation of quasi-elastic and inelastic contributions. A far larger number of data points than previously considered is now available for analysis and enables a more reliable extraction of G M n from cross section and R T data on D and He. A conspicuous result is a neutron reduced form factor α n ( Q 2 ) , which decreases with increasing Q 2 up to the largest measured Q 2 ≈ 10 GeV 2 . The achieved accuracy appears mainly limited by the present uncertainties in the knowledge of proton form factors and by the accuracy of the data.

Strange quarks in the nucleon

The G0 experiment at Jefferson Lab uses elastic, parity-violating electron–proton scattering to study the strange-quark contribution to the proton form factors. A dedicated, special-purpose spectrometer was built for this project. We present results from the recently completed forward-scattering-angle run and compare them with previous experiments. Results from a combined analysis of all available data are also shown and the current status of the experiment and future plans are discussed.

Proton Form Factors and Related Processes in BaBar by ISR

BaBar has measured with unprecedented accuracy [Formula: see text] from the threshold up to [Formula: see text], finding out an unexpected cross section, with plateaux and drops. In particular it is well established a sharp drop near threshold, where evidence for structures in multihadronic channels has also been found. Other unexpected and spectacular features of the Nucleon form factors are reminded, the behaviour of space-like [Formula: see text] and the neutron time-like form factors.

Extraction of proton form factors in the timelike region from single-polarizede+e−→p→p¯events

We have performed numerical simulations of the single-polarized ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\stackrel{\ensuremath{\rightarrow}}{p}\overline{p}$ process in kinematic conditions under discussion for a possible upgrade of the existing DAFNE facility. By fitting the cross section and spin asymmetry angular distributions with typical Born expressions, we can study the conditions for extracting information on moduli and phases of the proton electromagnetic form factors in the timelike region, which are poorly known and whose preliminary data show puzzling features. We have explored also non-Born contributions by introducing a further component in the angular fit, which is related to two-photon exchange diagrams. Using a dipole parametrization, we show that these corrections can be identified if larger than 5% of the Born contribution; we also explore the conditions for extracting information on the phase and, consequently, on the relative weight between their real and imaginary parts, which are presently unknown.

On radiative corrections for unpolarized electron-proton elastic scattering

A statistical analysis of the elastic unpolarized electron-proton scattering data shows that at large momentum transfer the size and the e dependence of the radiative corrections, as traditionally calculated and applied, may induce large correlations of the parameters of the Rosenbluth fit, which prevent a correct extraction of the electric proton form factor. Using the electron QED structure (radiation) function approach, the cross section of elastic electron-proton scattering in leading and next-to-leading approximations is calculated and expressed as a correction to the Born cross section, which is different for the electric and magnetic contributions. When properly applied to the data, it may yield the solution to the problem of the discrepancy of the polarized and unpolarized results on electron-proton scattering.

Electric properties of the baryon antidecuplet in the SU(3) chiral quark-soliton model

We investigate the electric form factors and charge radii of the pentaquark baryons within the framework of the chiral quark-soliton model. We consider the rotational $1/{N}_{c}$ and linear ${m}_{s}$ corrections, assuming isospin symmetry and employing the symmetry-conserving quantization. The flavor-decomposed charge densities of the ${\ensuremath{\Theta}}^{+}$ are presented. The electric form factors and charge radii of the charged pentaquark baryons turn out to be very similar to those of the corresponding octet baryons. The charge radii of the neutral pentaquark baryons are obtained to be very tiny and positive. The strange electric form factor of the pentaquark proton is shown to be larger than the corresponding one of the proton by around 20%. We also present the charge radii of the baryon decuplet for comparison.

Extraction of proton form factors in the timelike region from unpolarizede+e−→pp¯events

We have performed numerical simulations of the unpolarized ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}p\overline{p}$ process in kinematic conditions under discussion for a possible upgrade of the existing DAFNE facility. By fitting the cross section angular distribution with a typical Born expression, we can extract information on the ratio $|{G}_{E}/{G}_{M}|$ of the proton electromagnetic form factors in the timelike region within a 5%--10% uncertainty. We have explored also non-Born contributions to the cross section by introducing a further component in the angular fit, which is related to two-photon exchange diagrams. We show that these corrections can be identified if larger than 5% of the Born contribution, and if relative phases of the complex form factors do not produce severe cancellations.

A description of the ratio between electric and magnetic proton form factors by using space-like, time-like data and dispersion relations

We use the available information on the ratio between the electric and magnetic proton form factors coming from recently published space-like data and from the few available time-like data. We apply a dispersive procedure on these data to evaluate the behavior of this ratio, as a complex function, for all values of q2.

Real and virtual nucleon Compton scattering in the perturbative limit

We present the results of calculations analyzing nucleon Compton scattering to lowest order using perturbative QCD (pQCD) methods. Two scenarios are considered: (1) the incoming photon is real; and (2) the incoming photon is virtual. The case of a real photon has been previously analyzed at least 5 times using pQCD, but no two results are in agreement. Here it is shown that our result agrees with that of Brooks and Dixon published in 2000. The case of a virtual photon has been previously analyzed only once using pQCD. However, doubt has been cast on the validity of that result. The results presented here for the virtual photon are believed to be more reliable. Some consideration is given of how to compare these results with experiment. Following the lead of Brooks and Dixon, for the proton, this involves normalizing the cross section using the Dirac proton form factor, which we also calculate. Finally, there is a comparison of our results with recent experiments.

Precision Measurements of the Timelike Electromagnetic Form Factors of Pion, Kaon, and Proton

Using 20.7 pb(-1) of e(+)e(-) annihilation data taken at sq.rt(r) = 3.671 GeV with the CLEO-c detector, precision measurements of the electromagnetic form factors of the charged pion, charged kaon, and proton have been made for timelike momentum transfer of |Q(2)| = 13.48 GeV(2) by the reaction e(+)e(-) --> h(+)h(-). The measurements are the first ever with identified pions and kaons of |Q(2)| > 4 GeV(2), with the results F(13.48 GeV(2)) = 0.075 +/- 0.008(stat) +/- 0.005(syst) and F(K)(13.48 GeV(2)) = 0.063 +/- 0.004(stat) +/- 0.001(syst). The result for the proton, assuming G(p)(E) = G(p)(M), is G(p)(M)(13.48 GeV(2)) = 0.014 +/- 0.002(stat) +/- 0.001(syst), which is in agreement with earlier results.

Proton polarizations in polarizedHe3studied with theHe3→(e→,e'p)dandHe3→(e→,e'p)pnprocesses

We study within the Faddeev framework the \vec{3He} (\vec{e},e' p)d as well as the \vec{3He} (\vec{e},e' p)pn and \vec{3He} (\vec{e},e' n)pp reactions in order to extract information on the proton and neutron polarization in polarized 3He. We achieve clear analytical insight for simplified dynamical assumptions and define conditions for experimental access to important 3He properties. In addition we point to the possibility to measure the electromagnetic proton form factors in the process \vec{3He} (\vec{e},e' p)d which would test the dynamical picture and put limits on medium corrections of the form factors.

Nucleon form factors from generalized parton distributions

We discuss the links between Generalized Parton Distributions (GPDs) and elastic nucleon form factors. These links, in the form of sum rules, represent powerful constraints on parametrizations of GPDs. A Regge parametrization for GPDs at small momentum transfer, is extended to the large momentum transfer region and it is found to describe the basic features of proton and neutron electromagnetic form factor data. This parametrization is used to estimate the quark contribution to the nucleon spin.

Local Quark-Hadron Duality and Magnetic Form Factors of Bound Proton**Theproject supported by Natural Science Foundation ofHebei Province of China under Grant No. A2005000535

We discuss the consequence of local duality for elasticscattering, and derive a model-independent equation betweenstructure functions at x ∼ 1 and elastic electromagnetic formfactors. Then the electromagnetic form factors of proton arediscussed using the quark-hadron duality theory. We also debatethe form factor of proton in a bound state. It may be an effectiveapproach to study the form factor of proton in media.

Electromagnetic form factors in the Hypercentral Constituent Quark Model

We discuss the recent experimental results on the ratio bertween the electric and magnetic proton form factors and how they can be described by the hypercentral Constituent Quark Model.

Two-photon exchange and electromagnetic proton form factors

The presence of $2\gamma$-exchange in electron proton elastic scattering is discussed. From C-invariance and crossing symmetry, $2\gamma$ contribution induces a specific dependence of the reduced cross section on the variable $\epsilon$. No evidence of such dependence exists in the available experimental data.

Erratum: Constraints on Proton Structure from Precision Atomic-Physics Measurements [Phys. Rev. Lett.94, 022001 (2005)

Ground-state hyperfine splittings in hydrogen and muonium are very well measured. Their difference, after correcting for magnetic moment and reduced mass effects, is due solely to proton structure--the large QED contributions for a pointlike nucleus essentially cancel. The rescaled hyperfine difference depends on the Zemach radius, a fundamental measure of the proton, computed as an integral over a product of electric and magnetic proton form factors. The determination of the Zemach radius, (1.043 +/- 0.016) fm, from atomic physics tightly constrains fits to accelerator measurements of proton form factors. Conversely, we can use muonium data to extract an ``experimental'' value for QED corrections to hydrogenic hyperfine data; we find that measurement and theory are consistent.

Time-like compton scattering and the Bethe-Heitler process

Arguments are presented for p(γ, e + e −)p measurements to obtain new information about the off-shell time-like nucleon form factors, especially in the o meson region governing the oN couplings g oNN V,T . Theoretical predictions based upon a Quantum Hadrodynamic model and vector meson dominance are highlighted for both the proton form factor and the time-like Compton scattering cross section. The Bethe-Heitler process is also calculated but is only important at low momentum transfer ¦t¦ permitting a novel high |t| φ enhancement in the Compton cross section related to nucleon strangeness to emerge.