Highlights in the hadron electromagnetic structure

Egle Tomasi-Gustafsson,Simone Pacetti,Andrea Bianconi

doi:10.1051/epjconf/201713801003

Abstract

In frame of a general view of proton electromagnetic form factors, two recent findings related to reanalyses of data are presented. Recent experiments in the scattering and in the annihilation region provided us with more precise data and/or extending the kinematical region, allowing a deeper analysis and a common view of these fundamental quantities. We will discuss two issues: the discrepancy between the form factors extracted from unpolarized and polarized ep elastic scattering experiments, in connection with the commonly used dipole parametrization; peculiar oscillations in e + e − → pp ( γ ) annihilation cross section, that become periodical when plotted as a function of the 3-momentum of the relative motion of the final proton and antiproton, after subtraction of a smooth function.

Highlights

Electromagnetic hadron form factors (FFs) are fundamental quantities that describe the internal structure of non-pointlike particles, their charge and magnetic currents
FFs at large momentum transfer probe the quark structure of the hadrons whereas at low momentum transfer, they probe the size of the hadron
In the space-like region, the conclusion is that no real discrepancy exists between the FF extraction from polarized and unpolarized experiments, once all corrections are taken carefully into account

Summary

Introduction

Electromagnetic hadron form factors (FFs) are fundamental quantities that describe the internal structure of non-pointlike particles, their charge and magnetic currents. With large and stable polarization, together with large acceptance spectrometers and detectors, and the development of hadron polarimetry allowed in particular to apply to the Akhiezer-Rekalo recoil proton polarization method [2, 3], that requires to measure the recoil proton(neutron) polarization in the scattering of longitudinally polarized electrons These experiments, performed at MIT Bates (USA), MAMI (Germany) and JLab (USA) at large momentum transfer, brought very precise information on the ratio of the electric to magnetic FF for protons and neutrons and showed that this ratio is not constant, but decreases for increasing momentum transfer approaching zero at 9 GeV2. This pattern is tentatively associated with the interference between two sources, respectively of the quark and of the hadron size, bringing the signature of the first instants of the hadron formation from the excited vacuum

Reanalysis of Rosenbluth data

Reanalysis of existing data

Periodic oscillations in the time-like region

Findings

Conclusion