supported in part by the Deutsche Forschungsgemeinschaft(DFG,German Research Foundation)(279384907-CRC 1245);the DFG(196253076-TRR 110);the National Natural Science Foundation of China(12070131001);within the framework of the Sino-German CRC 110“Symmetries and the Emergence of Structure in QCD”;supported by the Chinese Academy of Sciences(CAS)President’s International Fellowship Initiative(PIFI)(2018DM0034);the VolkswagenStiftung(93562);the EU Horizon 2020 research and innovation programme,STRONG-2020 project(824093).
The electric radius,rE,and the magnetic radius,rM,of the proton are fundamental quantities of low-energy QCD,as they are a measure of the probe-dependent size of the proton.