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Register a new userVector mesons and electromagnetic form factor of the $Lambda$ hyperon
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The measured electromagnetic form factors of $Lambda$ hyperon in the time-like region are significantly deviated from pQCD prediction. We attribute the non-vanishing cross section near threshold to be the contribution of below-threshold $phi$(2170) state, supporting its exotic structure. Above the threshold, we find significant role of a wide vector meson with the mass of around 2.34 GeV, which would be the same state present in $pbar{p}$ annihilation reactions. As a result, we give a satisfactory description of the behavior of existing data without modifying pQCD expectation.
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Using dispersion theory the low-energy electromagnetic form factors for the transition of a Sigma to a Lambda hyperon are related to the pion vector form factor. The additionally required input, i.e. the two-pion--Sigma--Lambda amplitudes are determined from relativistic next-to-leading-order (NLO) baryon chiral perturbation theory including the baryons from the octet and optionally from the decuplet. Pion rescattering is again taken into account by dispersion theory. It turns out that the inclusion of decuplet baryons is not an option but a necessity to obtain reasonable results. The electric transition form factor remains very small in the whole low-energy region. The magnetic transition form factor depends strongly on one not very well determined low-energy constant of the NLO Lagrangian. One obtains reasonable predictive power if this low-energy constant is determined from a measurement of the magnetic transition radius. Such a measurement can be performed at the future Facility for Antiproton and Ion Research (FAIR).
The near-threshold $e^+e^- to Lambdabar{Lambda}$ reaction is studied with the assumption that the production mechanism is due to a near-$Lambda bar{Lambda}$-threshold resonance. The cross section of $e^+e^- to Lambdabar{Lambda}$ reaction is parametrized in terms of the electromagnetic form factors of $Lambda$ hyperon, which are obtained within the vector meson dominance model. It is shown that the contribution to the $e^+e^- to Lambdabar{Lambda}$ reaction from a new narrow state with quantum numbers $J^{PC}=1^{--}$ is dominant for energies very close to threshold. The mass of this new state is about 2232 MeV, which is very close to the mass threshold of $Lambda bar{Lambda}$, while its width is just a few MeV. This solves the problem that all previous calculations seriously underestimate the near-threshold total cross section of the $e^+e^- to Lambdabar{Lambda}$ reaction.
By the analysis of the world data base of elastic electron scattering on the proton and the neutron (for the latter, in fact, on $^2H$ and $^3He$) important experimental insights have recently been gained into the flavor compositions of nucleon electromagnetic form factors. We report on testing the Graz Goldstone-boson-exchange relativistic constituent-quark model in comparison to the flavor contents in low-energy nucleons, as revealed from electron-scattering phenomenology. It is found that a satisfactory agreement is achieved between theory and experiment for momentum transfers up to $Q^2sim$ 4 GeV$^2$, relying on three-quark configurations only. Analogous studies have been extended to the $Delta$ and the hyperon electromagnetic form factors. For them we here show only some sample results in comparison to data from lattice quantum chromodynamics.
We investigate the response of the bound state structure of a two-boson system, within a Yukawa model with a scalar boson exchange, to the inclusion of the cross-ladder contribution to the ladder kernel of the Bethe-Salpeter equation. The equation is solved by means of the Nakanishi integral representation and light-front projection. The valence light-front wave function and the elastic electromagnetic form factor beyond the impulse approximation, with the inclusion of the two-body current, generated by the cross-ladder kernel, are computed. The valence wave function and electromagnetic form factor, considering both ladder and ladder plus cross-ladder kernels, are studied in detail. Their asymptotic forms are found to be quite independent of the inclusion of the cross-ladder kernel, for a given binding energy. The asymptotic decrease of form factor agrees with the counting rules. This analysis can be generalized to fermionic systems, with a wide application in the study of the meson structure.
The transition form factor for electrodisintegration of a two-body bound system is calculated in the Bethe-Salpeter framework. For the initial (bound) and the final (scattering) states, we use our solutions of the Bethe-Salpeter equation in Minkowski space which were first obtained recently. The gauge invariance, which manifests itself in the conservation of the transition electromagnetic current Jq = 0, is studied numerically. It results from a cancellation between the plane wave and the final state interaction contributions. This cancellation takes place only if the initial bound state BS amplitude, the final scattering state and the operator of electromagnetic current are strictly consistent with each other, that is if they are found in the same dynamical framework. A reliable result for the transition form factor can be obtained in this case only.
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