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Register a new userNew fit of time-like proton electromagnetic form factors from $e^+e^-$ colliders
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The data on the proton form factors in the time-like region from the BaBar, BESIII and CMD-3 Collaborations are examined to have coherent pieces of information on the proton structure. Oscillations in the annihilation cross section, previously observed, are determined with better precision. The moduli of the individual form factors, determined for the first time, their ratio and the angular asymmetry of the annihilation reaction $e^+e^-tobar p p$ are discussed. Fiits of the available data on the cross section, the effective form factor, and the form factor ratio, allow to propose a description of the electric and magnetic time-like form factors from the threshold up to the highest momenta.
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Simulation results for future measurements of electromagnetic proton form factors at PANDA (FAIR) within the PandaRoot software framework are reported. The statistical precision with which the proton form factors can be determined is estimated. The signal channel $bar p p to e^+ e^-$ is studied on the basis of two different but consistent procedures. The suppression of the main background channel, $textit{i.e.}$ $bar p p to pi^+ pi^-$, is studied. Furthermore, the background versus signal efficiency, statistical and systematical uncertainties on the extracted proton form factors are evaluated using two different procedures. The results are consistent with those of a previous simulation study using an older, simplified framework. However, a slightly better precision is achieved in the PandaRoot study in a large range of momentum transfer, assuming the nominal beam conditions and detector performance.
We have performed numerical simulations of the unpolarized e+e- --> p pbar 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.
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.
Electromagnetic form factors of proton and neutron, obtained from a new fit of data, are presented. The proton form factors are obtained from a simultaneous fit to the ratio $mu_p G_{Ep}/G_{Mp}$ determined from polarization transfer measurements and to $ep$ elastic cross section data. Phenomenological two-photon exchange corrections are taken into account. The present fit for proton was performed in the kinematical region $Q^2in (0,6)$ GeV$^2$. Both for protons and neutrons we use the latest available data. For all form factors the uncertainties and correlations of form factor parameters are investigated with the $chi^2$ method.
The possibility to compute nucleon electromagnetic form factors in the time-like region by analytic continuation of their space-like expressions has been explored in the framework of the Skyrme model. We have developed a procedure to solve analytically Fourier transforms of the nucleon electromagnetic current and hence to obtain form factors defined in all kinematical regions and fulfilling the first-principles requirements. The results are discussed and compared to data, both in space-like and time-like region.
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