Experimental constraint on the ρ−\rho - meson form factors in the time--like region

The annihilation reaction $e^++e^-\rightarrow \bar \rho+\rho$ is considered. The constraint on time-like $\rho$-meson form factors from the measurement done by the BaBar collaboration at $\sqrt{s}=10.5$ GeV is analyzed.Comment: 10 pages 3 figure

Two-component model for the axial form factor of the nucleon

The axial form factor of the nucleon is studied in a two-component model consisting of a three-quark intrinsic structure surrounded by a meson cloud. The experimental data in the space-like region are well reproduced with a minimal number of parameters. The results are similar to those obtained from a dipole fit up to 1 GeV$^2$, but they surprisingly show a zero between 2 and 3 GeV$^2$. Such behavior, predicted by the present model completely disagrees with the dipole parametrization. The theoretical expression for the form factor is then analytically extrapolated to the time-like region, giving the first predictions in this regime. This kinematical region is especially interesting for present and future colliders.Comment: 12 pages 2 figures Final version to appear in Phys. Rev.

Unitary and analytic model of nucleon EM structure, the puzzle with JLab proton polarization data and new insight into proton charge distribution

The Unitary and analytic model of nucleon electromagnetic structure, describing all existing nucleon form factor data, is briefly reviewed. Then in the framework of this model the problem of inconsistency of older proton electric form factor data in space-like region (obtained from $e^{-}p$ process by the Rosenbluth technique) with recent Jefferson Lab data on the ratio $G_{Ep}(t)/G_{Mp}(t)$ (measured in precise polarization $\overrightarrow{e}^{-}p\to e^{-}\overrightarrow{p}$ experiment) is suggested to be solved in favour of the latter data which, however, unlike older data cause an existence of the form factor zero, i.e. a difraction minimum in $|G_{Ep}(t)|$ around $t=-Q^{2}=-13$GeV$^{2}$. The new behaviour of $G_{Ep}(t)$ with the zero gives modified proton charge distribution with enlarged value of the mean square charge radius.Comment: 9 pages, 11 eps figures. Talk presented at the workshop on Lepton Scattering and the Structure of Hadrons and Nuclei, Erice (Italy), September 200

Two-component model for the deuteron electromagnetic structure

We suggest a simple phenomenological parametrization for all three deuteron electromagnetic form factors, and show that a good fit on the available data, with a minimal number of parameters, can be obtained. The present description of the deuteron electromagnetic structure is based on two components with different radii, one corresponding to two nucleons separated by $\simeq$2 fm, and a standard isoscalar contribution, saturated by $\omega$ and $\phi$ mesons, only.Comment: 5 pages, 6 fig. 2 table

Study of resonant processes for multi-pion production in pˉ+p→ℓ++ℓ−+nππ\bar p +p\to\ell ^++\ell^- +n_\pi \pi annihilation

In frame of a phenomenological approach based on Compton-like Feynman amplitudes, we study multi-pion production in antiproton nucleon collisions. The main interest of this reaction is related to the possibility to study the properties of the presumable $\bar N N$ atom and of its resonances. For the case of formation of a scalar or pseudoscalar resonant state, with $I^G(J^{P})=1^-(0^{\pm}),~0^+(0^-)$ numerical results are obtained. The differential cross section in an experimental set-up where the pions invariant mass is measured, is given with explicit dependence on the lepton pair and pions invariant mass.Comment: 9 pages, 4 figure

Phenomenological insight into JLab proton polarization data puzzle by deuteron impulse approximation

The non-relativistic impulse approximation of deuteron electromagnetic form factors is used to investigate the space-like region behavior of the proton electric form factor in regard of the two contradictory experimental results extracted either from Rosenbluth separation method or from recoil proton JLab polarization data.Comment: Revtex, 6 pages, 7 figure

Polarization effects in the reaction e++e−→ρ++ρ−e^++e^-\to \rho^+ +\rho^- and determination of the ρ−\rho - meson form factors in the time--like region

The electron positron annihilation reaction into four pion production has been studied, through the channel $e^++e^-\to \bar \rho+\rho$. The differential (and total) cross sections and various polarization observables for this reaction have been calculated in terms of the electromagnetic form factors of the corresponding $\gamma^*\rho\rho$ current. The elements of the spin--density matrix of the $\rho -$meson were also calculated. Numerical estimations have been done, with the help of phenomenological form factors obtained in the space--like region of the momentum transfer squared and analytically extended to the time-like region.Comment: 19 pages, 2 figures, to appear in Phys Rev

Cross section and polarization observables for the reaction e++e−→a1(1260)+πe^++e^-\to a_1(1260)+\pi

A model independent formalism for the electron positron annihilation reaction $e^++e^-\to a_1(1260)+\pi$ has been derived. The differential and total cross sections and the elements of the spin--density matrix of the $a_1$-meson were calculated in terms of the electromagnetic form factors of the corresponding $\gamma^*a_1\pi$ current. Simple models of $a_1$ form factors have been fitted to the available cross section data and they allow to give numerical predictions for the different observables.Comment: 19 pages, 6 figures Added formulas for axial meson decay in 3 pions Modified version to appear in PR

Polarization effects in e++e−→dˉ+de^+ +e^-\to \bar d+d and determination of time like deuteron form factors

Polarization effects in the reaction $e^++e^-\to \bar d+d$ have been investigated for the case of longitudinally polarized electron beam and arbitrary polarization of the produced deuteron, with the aim of a determination of the time-like complex deuteron electromagnetic form factors. General expressions of polarization observables are derived and numerical estimations have been carried out by means of various models of deuteron electromagnetic form factors, for kinematical conditions near threshold.Comment: 19 pages, 3 figure

Electromagnetic structure of vector mesons

Electromagnetic structure of the complete nonet of vector mesons (ρ0, ρ+, ρ−, ω, ϕ, K*0, K*+, K̄*0, K*−) is investigated in the framework of the Unitary and Analytic model and insufficient experimental information on it is discussed