Tail,zeros and space-like region behaviour of the electromagnetic pion form factor

Combining the real and imaginary parts of the pion form factor from the resonant region, given by a simple formula obtained by means of the dispersion relation method, with some special assumptions in the asymptotic region and sum rules, the time-like region form factor tail is predicted. Further, the existence of form factor zeros is investigated and on the basis of numerical results one can conclude that zeros of the pion form factor exist. By means of the unsubtracted dispersion relation, the predicted space-like region behaviour of the form factor is found to be in excellent agreement with the existing experimental data.     

Analysis of the pion formfactor in the time-like region

We perform a model independent analysis of the pion electromagnetic formfactor using the available space like and time-like data. Our aim is to obtain information on the structure in the time-like region above the πω threshold. The phase below is assumed to be known from ππ scattering. In the process of analytic continuation we use as stabilizing lever different types of bounds on the derivative of the formfactor.     

Nucleon electromagnetic form factors and polarization observables in space-like and time-like regions

We perform a global analysis of the experimental data of the electromagnetic nucleon form factors, in space-like and time-like regions. We give the expressions of the observables in annihilation processes, such as p + ¯↦ℓ⁺ + ℓ^-, ℓ = e or μ, in terms of form factors. We discuss some of the phenomenological models proposed in the literature for the space-like region, and consider their analytical continuation to the time-like region. After determining the parameters through a fit on the available data, we give predictions for the observables which will be experimentally accessible with large statistics, polarized annihilation reactions     

Two-photon-exchange contribution to proton form factors in both space-like and time-like regions

The two two-photon exchange corrections to the unpolarized cross section and polarized observable PT,PL in elastic ep scattering are discussed in a simple hadronic model.Comparing with previous results,theΔ contribution are re-analysed.And the similar corrections in e+e-→ p(p-)are also discussed.     

Two-photon-exchange contribution to proton form factors in both space-like and time-like regions

The two two-photon exchange corrections to the unpolarized cross section and polarized observable PT, PL in elastic ep scattering are discussed in a simple hadronic model. Comparing with previous results, the △ contribution are re-analysed. And the similar corrections in e^＋e^- → pp are also discussed.     

Space- and time-like electromagnetic pion form factors in light-cone pQCD

We present a combined analysis of the space- and time-like electromagnetic pion form factors in light-cone perturbative QCD with transverse momentum dependence and Sudakov suppression. Including the genuine non-perturbative “soft” QCD and the power suppressed twist-3 corrections to the standard twist-2 perturbative QCD result, the experimental form factor data available at moderate energies/momentum transfers can be explained reasonably well. To this end, the bulk of the existing discrepancy between the space- and time-like experimental data is ostensibly reconciled.     

The resonant time-like Kaon form factor

The K-form factor is analyzed in terms of a family of vector mesons whose couplings are determined through our earlier model for the pion form factor. Radiative corrections are applied. The model reproduces the data quite well. For the charge radius we obtain r_K = 0.56 F to be compared with r_K = (0.54 ± 0.14) F given by the Chou-Yang model.     

Investigation of the phase of the electromagnetic pion form factor in the asymptotic region

The phase of the electromagnetic pion form factor G(t) in the asymptotic region (t > 1 (GeV)²) is investigated by the dispersion sum-rule method. Using experimental data for G(t) at t < 1 (GeV)² we find that the mean weighted value of the asymptotic phase does not show strong deviations from the asymptotic value of the phase coming from an extrapolation of the data from the ?-resonance region with the help of the Gounaris-Sakurai formula. The contribution of the hypothetical ?-like resonances to the dispersion sum rules is roughly estimated. These estimates and the results of the calculation of the dispersion sum rules show that the existence of strong ?-like elastic resonances is forbidden in the 1–1.7 GeV $\text{t}$ range at least.     

Asymptotic behavior of nucleon electromagnetic form factors in time-like region

We study the asymptotic behavior of the ratio of Pauli and Dirac electromagnetic nucleon form factors, F₂/F₁, in time-like region, for different parametrizations built for the space-like region. We investigate how fast the ratio F₂/F₁ approaches the asymptotic limits according to the Phragmèn-Lindel?f theorem. We show that the QCD-inspired logarithmic behavior of this ratio results in very far asymptotics, experimentally unachievable. This is also confirmed by the normal component of the nucleon polarization, P_y, in e⁺ + e^-↦N + ˉ (in collisions of unpolarized leptons), which is a very interesting observable, with respect to this theorem. Finally we observe that the 1/Q parametrization of F₂/F₁ contradicts this theorem.     

Sum rules and the pion form factor in QCD

We propose a new approach to the investigation of the pion electromagnetic form factor in QCD based on the systematic use of the QCD sum rule technique. The theoretical curve obtained for F_π(Q²) is in good agreement with existing experimental data.     

Lattice QCD and the timelike pion form factor

We present a formula that allows one to calculate the pion form factor in the timelike region 2m(π) ≤ √(s) ≤ 4m(π) in lattice QCD. The form factor quantifies the contribution of two-pion states to the vacuum polarization. It must be known very accurately in order to reduce the theoretical uncertainty on the anomalous magnetic moment of the muon. At the same time, the formula constitutes a rare example where, in a restricted kinematic regime, the spectral function of a conserved current can be determined from Euclidean observables without an explicit analytic continuation.     

Generalized vector dominance and the pion form factor

We have calculated the pion form factor using the bound state Bethe-Salpeter amplitudes and the quark form factor of a relativistic quark model. We obtain a generalized vector dominance structure and an asymptotic behaviour in the space-like region F_π(Q)² −0.33 GeV²/Q².