Hadron form factors at BESIII

Abstract

Electromagnetic form factors provide fundamental information about the structure and dynamics of hadrons. They constitute a rigorous test of nonperturbative QCD as well as of phenomenological models. Using data samples collected with the BESIII detector at the BEPCII collider, the Born cross section of $e^+ e^- \to \bar p p$ at the center-of-mass energy between 2.0 GeV and 3.08 GeV is measured. The from factor ratio |GE/GM| and the magnetic form factor |GM| are extracted by fitting the polar angle distribution of the proton for data samples with large statistics. For BESIII data between 3.773 and 4.6 GeV, the initial state radiation (ISR) method is used to study $e^+ e^- \to \bar p p$ in tagged mode. The cross section of $e^+ e^- \to \bar p p$ and ratio |GE/GM| are obtained. For e$^+$e$^-$ $\to$ $\Lambda_c^+ \Lambda_c^-$ process, very weak energy dependence of cross section near threshold indicates that traditional theoretical prediction, which does not take into account strong interaction, needs to be modified. With the large statistics of multiple decay modes, the ratio |GE/GM| and |GM| is extracted. Results on the cross section and effective form factor of $e^+ e^- \to \bar\Lambda \Lambda $ are presented, improving the overall presicion. A new measurement on the cross section and effective form factor of $e^+ e^- \to \pi^+ \pi^-$ with the ISR technique to improve the understanding of the inconsistency between the experimental results and the theoretical prediction of the anomalolus magnetic moment of the moun are shown. Preliminary results recently obtained at the BESIII experiment for $e^+ e^- \to K^+ K^-$ are presented.