Measurement of the e⁺e^- → anti pp cross section at BESIII using the untagged-initial state radiation technique

Electromagnetic form factors (FFs) are fundamental quantities which parametrise the electric and magnetic structure of hadrons. In the time-like region, proton FFs can be accessed through the annihilation process e⁺e^- → anti pp, assuming that the interaction takes place through the exchange of one virtual photon. Initial state radiation (ISR) is an effective tool to measure hadronic cross section at high luminosity e⁺e^- storage rings, such as the Beijing Electron-Positron Collider II (BEPC-II). This contribution reports on the analysis of e⁺e^- → anti ppγ for proton FF measurements at the Beijing Spectrometer III (BESIII/BEPC- II). Data collected at 7 beam energies between 3.773 and 4.6 GeV, with a total luminosity of 7.408 fb^-1, are analysed. The case of untagged ISR photon in e⁺ e^- → anti ppγ is presented.