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Register a new userMeasurement of proton electromagnetic form factors in the time-like region using initial state radiation at BESIII
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The electromagnetic process $e^{+}e^{-}to pbar{p}$ is studied with the initial-state-radiation technique using 7.5 fb$^{-1}$ of data collected by the BESIII experiment at seven energy points from 3.773 to 4.600 GeV. The Born cross section and the effective form factor of the proton are measured from the production threshold to 3.0 GeV/$c^{2}$ using the $pbar{p}$ invariant-mass spectrum. The ratio of electric and magnetic form factors of the proton is determined from the analysis of the proton-helicity angular distribution.
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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.
Using a data set with an integrated luminosity of 2.93 fb$^{-1}$ taken at a center-of-mass energy of 3.773 GeV with the BESIII detector at the BEPCII collider, we extract the $e^+e^-rightarrow pi^+pi^-$ cross section and the pion form factor $|F_pi|^2$ in the energy range between 600 and 900 MeV. We exploit the method of initial state radiation for this measurement, yielding a systematic uncertainty of 0.9%. We calculate the contribution of the measured cross section to the leading-order hadronic vacuum polarization contribution to $(g-2)_mu$.
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.
The process of $e^+e^- rightarrow pbar{p}$ is studied at 22 center-of-mass energy points ($sqrt{s}$) from 2.00 to 3.08 GeV, exploiting 688.5~pb$^{-1}$ of data collected with the BESIII detector operating at the BEPCII collider. The Born cross section~($sigma_{pbar{p}}$) of $e^+e^- rightarrow pbar{p}$ is measured with the energy-scan technique and it is found to be consistent with previously published data, but with much improved accuracy. In addition, the electromagnetic form-factor ratio ($|G_{E}/G_{M}|$) and the value of the effective ($|G_{rm{eff}}|$), electric ($|G_E|$) and magnetic ($|G_M|$) form factors are measured by studying the helicity angle of the proton at 16 center-of-mass energy points. $|G_{E}/G_{M}|$ and $|G_M|$ are determined with high accuracy, providing uncertainties comparable to data in the space-like region, and $|G_E|$ is measured for the first time. We reach unprecedented accuracy, and precision results in the time-like region provide information to improve our understanding of the proton inner structure and to test theoretical models which depend on non-perturbative Quantum Chromodynamics.
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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