Antiproton-Proton Annihilation into Electron-Positron Pairs and Gamma-Ray Pairs

Nucleon electromagnetic form factors in the timelike region

“…The momentum transfer squared, q 2 , of the em FF in this case is accessible by measuring the invariant mass of the lepton pair, or, alternatively, it is calculated for a given antiproton beam momentum assuming the target protons to be at rest. First upper limits for the TL proton FF have indeed been obtained at relatively high energies at BNL [20] and CERN [21] in q 2 ranges between 5 (GeV/c) 2 and 8 (GeV/c) 2 . For historical reasons we report FF results from e + e − experiments as Figure 5: Differential cross section for the reaction pp → e + e − .…”

“…Three of those theoretical predictions are shown in Fig. (21). The work corresponding to the red dashed line, cf.…”

“…system, and k the antiproton momentum in the CM system ( see also Eq. (21). From the measurement of the differential cross section (cf.…”

“…First attempts to measure the proton FF using the inverse reaction pp → e + e − date back to the mid 1960's. Indeed, first upper limits stem from antiproton beam experiments at BNL [20] and CERN [21]. The discovery of this reaction was finally possible using an antiproton beam at PS/CERN in 1976 [22].…”

“…It is obvious from Fig. (21) that a precision measurement of the |G p E /G p M | ratio at PANDA will be very sensitive to distinguish between different hadronic models. Three of those theoretical predictions are shown in Fig.…”