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Institute of High Energy Physics, Beijing 100049, People's Republic of China.
G.I. Budker Institute of Nuclear Physics SB RAS (BINP), Novosibirsk 630090, Russia.
Phys Rev Lett. 2020 Jan 31;124(4):042001. doi: 10.1103/PhysRevLett.124.042001.
The process of e^{+}e^{-}→pp[over ¯] is studied at 22 center-of-mass energy points (sqrt[s]) from 2.00 to 3.08 GeV, exploiting 688.5 pb^{-1} of data collected with the BESIII detector operating at the BEPCII collider. The Born cross section (σ_{pp[over ¯]}) of e^{+}e^{-}→pp[over ¯] is measured with the energy-scan technique and it is found to be consistent with previously published data, but with much improved accuracy. In addition, the electromagnetic form-factor ratio (|G_{E}/G_{M}|) and the value of the effective (|G_{eff}|), electric (|G_{E}|), and magnetic (|G_{M}|) form factors are measured by studying the helicity angle of the proton at 16 center-of-mass energy points. |G_{E}/G_{M}| and |G_{M}| are determined with high accuracy, providing uncertainties comparable to data in the spacelike region, and |G_{E}| is measured for the first time. We reach unprecedented accuracy, and precision results in the timelike region provide information to improve our understanding of the proton inner structure and to test theoretical models which depend on nonperturbative quantum chromodynamics.
利用在北京正负电子对撞机（BEPCII）上运行的北京谱仪III（BESIII）探测器收集的688.5 pb⁻¹数据，在2.00至3.08 GeV的22个质心能量点（√s）上研究了e⁺e⁻→pp̅过程。采用能量扫描技术测量了e⁺e⁻→pp̅的玻恩截面（σpp̅），发现其与先前发表的数据一致，但精度有了很大提高。此外，通过在16个质心能量点研究质子的螺旋角，测量了电磁形状因子比（|GE/GM|）以及有效形状因子（|Geff|）、电形状因子（|GE|）和磁形状因子（|GM|）的值。|GE/GM|和|GM|的测量精度很高，其不确定度与类空区域的数据相当，并且首次测量了|GE|。我们达到了前所未有的精度，类时区域的精确结果为增进我们对质子内部结构的理解以及检验依赖于非微扰量子色动力学的理论模型提供了信息。
