Abstract
The Born cross section for the process $e^+ e^- \to p \bar p $ is measured using the initial state radiation technique with an undetected photon. This analysis is based on datasets corresponding to an integrated luminosity of 7.5 fb$^{-1}$, collected with the BESIII detector at the BEPCII collider at center of mass energies between 3.773 and 4.600 GeV. The Born cross section for the process $e^+ e^- \to p \bar p $ and the proton effective form factor are determined in the $p\bar p$ invariant mass range between 2.0 and 3.8 GeV/$c^2$ divided into 30 intervals. The proton form factor ratio ($|G_E|/|G_M|$) is measured in 3 intervals of the $p\bar p$ invariant mass between 2.0 and 3.0 GeV/$c^2$.
Highlights
Electromagnetic form factors (FFs) are fundamental quantities that describe the internal structure of hadrons
Based on data samples corresponding to an integrated luminosity of 7.5 fb−1 collected with the Beijing Spectrometer III (BESIII) detector at c.m. energies between 3.773 and 4.600 GeV, the proton FFs have been measured using the initial state radiation (ISR) technique
The Born cross section of the eþe− → ppchannel and the proton effective FF have been measured in 30 Mppintervals between 2.0 and 3.8 GeV=c2
Summary
Electromagnetic form factors (FFs) are fundamental quantities that describe the internal structure of hadrons. The proton (spin 1=2) is characterized by the electric FF GE and the magnetic FF GM They are experimentally accessible through the measurements of cross sections for elastic electron-proton scattering in the spacelike region (momentum transfer squared q2 < 0) and annihilation processes eþe− ↔ ppin the timelike region (q2 > 0) [1,2]. In this paper we study the ISR process eþe− → ppγ to measure the Born cross section of the process eþe− → ppand to determine the proton FFs in the timelike region. The differential cross section of the reaction eþe− → ppγ as a function of the ISR polar angle reaches its highest values at small angles relative to the direction of the electron (or positron) beam [25]. The Born cross section for the ISR process eþe− → ppγ (Fig. 1) integrated over the photon polar angle can be written as [25].
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