Time-like Proton Form Factors with Initial State Radiation Technique

Abstract

Electromagnetic form factors are fundamental quantities describing the internal structure of hadrons. They can be measured with scattering processes in the space-like region and annihilation processes in the time-like region. The two regions are connected by crossing symmetry. The measurements of the proton electromagnetic form factors in the time-like region using the initial state radiation technique are reviewed. Recent experimental studies have shown that initial state radiation processes at high luminosity electron-positron colliders can be effectively used to probe the electromagnetic structure of hadrons. The BABAR experiment at the B-factory PEP-II in Stanford and the BESIII experiment at BEPCII (an electron positron collider in the τ-charm mass region) in Beijing have measured the time-like form factors of the proton using the initial state radiation process e+e−→pp¯γ. The two kinematical regions where the photon is emitted from the initial state at small and large polar angles have been investigated. In the first case, the photon is in the region not covered by the detector acceptance and is not detected. The Born cross section and the proton effective form factor have been measured over a wide and continuous range of the the momentum transfer squared q2 from the threshold up to 42 (GeV/c)2. The ratio of electric and magnetic form factors of the proton has been also determined. In this report, the theoretical aspect and the experimental studies of the initial state radiation process e+e−→pp¯γ are described. The measurements of the Born cross section and the proton form factors obtained in these analyses near the threshold region and in the relatively large q2 region are examined. The experimental results are compared to the predictions from theory and models. Their impact on our understanding of the nucleon structure is discussed.

Theoretical and Experimental Essentials on Baryon Form Factors

“…The experimental values of the proton effective FF (time-like region) [11,12], obtained by various collaborations, are shown in Figure 3. Recently, the BESIII Collaboration studied the process e + e − → pp(γ) [13], using both procedures of the energy scan from √ s = 2.00 GeV up to √ s = 3.08 GeV and the initial state radiation.…”

Proton Electromagnetic Form Factors in the Time-like Region through the Scan Technique

“…energy of the collider is systematically varied. Within this paper, only former results, as well as future experiments employing the scan method, are reviewed, whereas an overview of results with the ISR method can be found in [15]. Further reviews about nucleon EM FFs in the TL region in general and other baryon FFs can be found in [16] and [17], respectively.…”

“…where c 0 is the same parameter as in Equation (15) and is determined to be c 0 = 5.61 +1.37 −1.33 GeV 2 from the TL-SL joint fit. [36,37,45,46], BABAR [47][48][49], CMD3 [34,35], BES [32], FENICE+DM2 [20][21][22]24,25,41], E835 [30,31], and PS170 [26][27][28] and in the spacelike region from the GEp Collaboration [58].…”

Electromagnetic Structure of the Neutron from Annihilation Reactions

“…TL EM FFs are complex functions of four momentum transfer and can be acossiated with the time evolution of the electric charge and magnetization within the nucleon [12]. Reviews of the results can be found in [13,14]. The neutron on the other hand still remains a mystery due to experimental difficulty of detecting a neutral hadron and unavailibility of an anti-neutron beam.…”