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Register a new userShort Status Report Of The Nucleon Time-Like Form Factors Measurements
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Authors
Alessandro De Falco
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The measurements of the nucleon electromagnetic form factors in the time-like region are reviewed. Several unexpected features deserving new high precision, high statistics measurements are emphasised.
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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.
Analyticity of nucleon form factors allows to derive sum rules which, using space-like and time-like data as input, can give unique information about behaviors in energy regions not experimentally accessible. Taking advantage from new time-like data on proton-antiproton differential cross section and hence the possibility to separate electric and magnetic form factors also in the time-like region, we verify the consistency of the asymptotic behavior predicted by the perturbative QCD for the proton magnetic form factor.
Some recent advances in the theoretical description of the Nucleon electromagnetic form factors, both in the space- and time-like regions, will be briefly illustrated. In particular, both the present stage of the lattice calculations and updated phenomenological approaches, like the ones based on dispersion relations and on microscopical models, will be reported.
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.
The Born cross sections of the $e^{+}e^{-}toSigma^{+}bar{Sigma}^{-}$ and $e^{+}e^{-}toSigma^{-}bar{Sigma}^{+}$ processes are determined with high precision for center-of-mass energy from 2.3864 to 3.0200 GeV with the BESIII detector. Nonzero cross sections near threshold are observed. The resulting ratio of effective form factors for the $Sigma^{+}$ and $Sigma^{-}$ is consistent with 3, agreeing with the ratio of the incoherent sum of the squared charges of the $Sigma^{+}$ and $Sigma^{-}$ valence quarks, but disagreeing with various theoretical predictions. In addition, ratios of the $Sigma^{+}$ electric and magnetic form factors, $|G_{E}/G_{M}|$, are obtained at three center-of-mass energies through an analysis of the angular distributions. These measurements, which are studied for the first time in the off-resonance region, provide precision experimental input for understanding baryonic structure. The observed novel features of the $Sigma^{pm}$ form factors require a new theoretical description for the hyperons.
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