Time-like Proton Form Factors with Initial State Radiation Technique

Electromagnetic form factors are fundamental quantities describing the internal structure of hadrons. They can be measured with scattering processes in the space-like region and annihilation processes in the time-like region. The two regions are connected by crossing symmetry. The measurements of the proton electromagnetic form factors in the time-like region using the initial state radiation technique are reviewed. Recent experimental studies have shown that initial state radiation processes at high luminosity electron-positron colliders can be effectively used to probe the electromagnetic structure of hadrons. The BABAR experiment at the B-factory PEP-II in Stanford and the BESIII experiment at BEPCII (an electron positron collider in the τ-charm mass region) in Beijing have measured the time-like form factors of the proton using the initial state radiation process e+e−→pp¯γ. The two kinematical regions where the photon is emitted from the initial state at small and large polar angles have been investigated. In the first case, the photon is in the region not covered by the detector acceptance and is not detected. The Born cross section and the proton effective form factor have been measured over a wide and continuous range of the the momentum transfer squared q2 from the threshold up to 42 (GeV/c)2. The ratio of electric and magnetic form factors of the proton has been also determined. In this report, the theoretical aspect and the experimental studies of the initial state radiation process e+e−→pp¯γ are described. The measurements of the Born cross section and the proton form factors obtained in these analyses near the threshold region and in the relatively large q2 region are examined. The experimental results are compared to the predictions from theory and models. Their impact on our understanding of the nucleon structure is discussed.

Time-Like Proton Form Factors in Initial State Radiation Process

The measurements of the proton electromagnetic form factors in the time-like region using the initial state radiation technique are reviewed. Recent experimental studies have shown that initial state radiation processes at high luminosity electron-positron colliders can be effectively used to probe the electromagnetic structure of hadrons. The BABAR experiment at the B-factory PEP-II in Stanford and the BESIII experiment at the $\tau$-charm factory BEPC-II in Beijing have measured the time-like form factors of the proton using the initial state radiation process $e^{+}e^{-}\to pbar{p}\gamma$. The two kinematical regions where the photon is emitted from the initial state at small and large polar angles have been investigated. In the first case the photon is in the region not covered by the detector acceptance and is not detected. The Born cross section and the proton effective form factor have been measured over a wide and continuous range of the the momentum transfer squared $q^2$ from threshold up to ~42 (GeV/c)$^2$. The ratio of electric and magnetic form factors of the proton has been also determined. In this report, the theoretical aspect and the experimental studies of the initial state radiation process $e^{+}e^{-}\to p\bar{p}\gamma$ are described. The measurements of the Born cross section and the proton form factors obtained in these analyses near the threshold region and in the relatively large $q^2$ region are examined. The experimental results are compared to the predictions from theory and models. Their impact on our understanding of the nucleon structure is discussed.

Two-Photon Exchange in Elastic Electron Scattering on Hadronic Systems

In the present review, we discuss different aspects of the two-photon exchange (TPE) physics in elastic ep scattering at high Q2, as well as at low Q2. The imaginary part of the TPE amplitude gives rise to beam and target single-spin asymmetries. Different theoretical approaches to the calculation of these observables are considered. The real part of the TPE amplitude influences the unpolarized cross-section and double-spin observables and is, most likely, responsible for the discrepancy between two methods of measurements of the proton form factors. We review different methods of calculations of the TPE amplitudes in the framework of the “hadron” and “quark-gluon” approaches. We discuss the dispersion approach suitable for low and intermediate Q2, which includes elastic and inelastic intermediate hadronic states, as well as the connection of TPE with the proton radius puzzle. The present situation with direct experimental searches for the TPE amplitude in the e+p/e−p charge asymmetry is also discussed, as well as attempts to extract the TPE amplitudes from existing experimental data obtained by the Rosenbluth and double polarization techniques. The TPE physics in other processes such as elastic мp, e-nucleus, and еп scattering is also reviewed.

Density distributions, form factors and reaction cross sections for exotic 11Be and 15C nuclei

The ground state proton, neutron and matter densities of exotic 11Be and 15C nuclei are studied by means of the TFSM and BCM. In TFSM, the calculations are based on using different model spaces for the core and the valence (halo) neutron. Besides single particle harmonic oscillator wave functions are employed with two different size parameters Bc and Bv. In BCM, the halo nucleus is considered as a composite projectile consisting of core and valence clusters bounded in a state of relative motion. The internal densities of the clusters are described by single particle Gaussian wave functions. Elastic electron scattering proton form factors for these exotic nuclei are analyzed via the plane wave born approximation (PWBA). As the calculations in the BCM do not distinguish between protons and neutrons, the calculations of the proton form factors are restricted only by the TFSM. The reaction cross sections for these exotic nuclei are studied by means of the Glauber model with an optical limit approximation using the ground state densities of the projectile and target, where these densities are described by single Gaussian functions. The calculated reaction cross sections at high energy are in agreement with the experimental data.

Elastic electron scattering from 17Ne and 27P exotic nuclei

The ground state proton, neutron and matter densities andcorresponding root mean square radii of unstable proton-rich 17Neand 27P exotic nuclei are studied via the framework of the twofrequencyshell model. The single particle harmonic oscillator wavefunctions are used in this model with two different oscillator sizeparameters core b and halo , b the former for the core (inner) orbitswhereas the latter for the halo (outer) orbits. Shell model calculationsfor core nucleons and for outer (halo) nucleons in exotic nuclei areperformed individually via the computer code OXBASH. Halostructure of 17Ne and 27P nuclei is confirmed. It is found that thestructure of 17Ne and 27P nuclei have 25 / 2 (1d ) and 1/ 2 2s -dominantconfigurations, respectively. Elastic electron scattering form factorsof these exotic nuclei are also studied using the plane wave Bornapproximation. Effects of the long tail behavior of the proton densitydistribution on the proton form factors of 17Ne and 27P areanalyzed. It is found that the difference between the proton formfactor of 17Ne and that of stable 20Ne (or of 27P and that of stable31P) comes from the difference in the proton density distribution ofthe last two protons (or of the last proton) in the two nuclei. It isconcluded that elastic electron scattering will be an efficient tool (inthe near future) to examine proton-halo phenomena of proton-richnuclei.

Critical analysis of recent results on electron and positron elastic scattering on proton

Recent data on the cross section ratio for electron and positron elastic scattering on protons are discussed. A deviation from unity of this ratio constitutes a model independent signature of charge-odd contributions that arise from mechanisms beyond the one-photon approximation, as the exchange of two photons. The relevance of this issue is related to that fact that the information on the proton structure from lepton elastic (and inelastic) scattering holds only within a formalism based on the one photon exchange approximation. The present analysis shows that the deviations of the data from unity and from a previously developed theoretical approach lie within the theoretical and experimental errors. The data suggest that other reasons for explanation of the discrepancy between the electromagnetic proton form factors extracted from the experiments according to the polarized and the unpolarized methods are more likely.

Connection of different parametrizations of theproton electromagnetic current operator in discrete symmetries violation case

In this paper field and canonical parameterizations of matrix elements of proton electromagnetic current operator in discrete symmetries violation case were reviewed. The main principles of field and canonical operator parameteri- zations were described. The connection between proton form-factors in field and canonical parametrization has been constructed. Physical interpretation of the proton electromagnetic current odd part formfactors in canonical parametriza- tion has been obtained. Opportunity of using obtained results for solving the actual problems of proton electromagnetic structure theory - the non-rosenbluth ep-scattering problem and the proton radius puzzle - was discussed.

Electromagnetic proton form factors in dual large-Nc QCD: An update

An updated determination is presented of the electric and magnetic form factors of the proton, in the framework of a dual-model realization of quantum chromodynamics (QCD) in the limit of an infinite number of colors. Very good agreement with data is obtained in the space-like region up to [Formula: see text]. In particular, the ratio [Formula: see text] is predicted in very good agreement with recoil polarization measurements from Jefferson Lab, up to [Formula: see text].