Two Photon Exchange Effects In Ep Elastic Scattering And E~+e~-→π~+π~-

The proton and the pion are the the most elemental bound states due to the strong interaction.The study and analysis of proton and pion internal structure is one of the hottest topic in hadron physics.Among all the quantities used in these studies,electromagnetic form factor is one of the most important and basic non-perturbative quantities which is characterized by the internal structure of particles.In the past twenty years,great progress has been made in works on the topic of proton and pion electromagnetic form factors,both experimental and theoretical,while lots of new problems have arisen.One of them is about the two-photon exchange(TPE)effect.There is a discrepancy in the ratio R of proton electromagnetic form factors extracted from two types of experiment,that is,an unpolarized one and a polarized one in which one-photon exchange(OPE)have been considered.To throw light on this problem,physicists have taken into account TPE and have studied this effect in various ways.Yet,all these works have solved only some part of the discrepancy.The remaining discrepancy calls for further careful studies on TPE effects.The central propose of the thesis is to study the TPE effects in lepton-proton scattering and in e~+e~-→π~+π~-,together with how TPE effects will act in the extraction of electromagnetic form factors.This thesis mainly consists of two works:1.Beyond the normal two-photon exchange effect,the meson exchange effect(through ways that two photons couple with a meson)has been considered.Detailed study shows that the meson-exchange process may result in a correction to the reduced unpolarized cross section and the polarized observables,and this correction is a function of only the electromagnetic form factors and an unknown universal parameter g at fixed Q~2.When con-sidering the usual TPE contributions together with the correction from meson exchange effect,the ratioμG_E/G_Rcan be explained consistently in both unpolarized and polarized experiments.2.Within the e~+e~-→π~+π~-process at high center-of-mass energy,we employ a pQCD method to study the TPE effects,making use of both twist-2 and twist-3 distribution amplitudes(DAs).The result is quite different with the OPE case since TPE effects bring about an angular asymmetry in the differential cross section of this process.Further experimental verification of this asymmetry at high center-of-mass energy will be a precise test of TPE.