在北京谱仪BESIII上利用初态辐射事例测量核子形状因子

The internal structure of nucleon, one of the basic building blocks of the matter, is often described in theory with form factor (FF), which also provides a way to understand its dynamics. Nucleon form factors are necessary for the interpretation of many theory problems and experimantal measurements involving strong interactions. The calculation of form factor belongs to non-perturbative region in the field theory, where the free parameters in the semi-phenomenal model expression obtained using QCD theorem have to be measured experimentally. The nucleon form factor can be measured in the space-like region and the time-like region, and most of previous experimental data are in the former, while the measurements in the later are scarce and with large errors. This project will utilize the world largest data sample corresponding to an integrated luminosity of 2.9/pb at the psi(3770) peak collected with the BESIII detector to measure the production cross section of nucleon-antinucleon pair and the time-like form factor evolution versus energy below the energy scale of 3 GeV, with the potential to study strange baryons as well, using the initial state radiation (ISR) technique in electron positron collision. Based on the large BESIII data sample and with the common methods in the ISR studies also used by other Collaborations, more accurate results are expected to test the QCD phenomenal model with better precision.

核子的结构在理论上常用形状因子描述，它反映了物质基本组成单元的内部结构和动力学性质，对解释许多与强相互作用相关的理论问题和实验测量也是非常必须的。形状因子的场论计算属于非微扰问题，用QCD原理得到的半唯象模型表达式中的自由参数需要由实验测定。核子的形状因子可以用类空和类时两种过程来测量，其中类空过程的实验数据较多，而类时过程的实验数据非常有限且测量误差很大。本项目将利用北京谱仪BESIII在psi(3770)峰位已经获取的积分亮度为2.9/fb的世界上最大数据样本，采用初态辐射（ISR）的方法测量能标在3 GeV以下核子-反核子对产生截面及其类时电磁形状因子随能量的演化，研究对象还可能拓展到奇异重子。基于BESIII较大的数据样本，借鉴其它实验组的初态辐射事例研究方法和经验，预期可以得到更精确的结果，对QCD唯象模型作出较明确的检验。

核子是物质的基本组成单元，其内部结构用形状因子描述。北京谱仪BESIII可以利用能量扫描和初态辐射两种方法测量核子的形状因子。在本项目建议提出以后，BESIII不仅在高能区累计了更多的数据，而且在低能区获取了能量扫描数据，因此在项目实施过程中对研究内容进行了扩展。BESIII于2012年6月在2.2324、2.4、2.8、3.4GeV获取了约12/pb扫描数据，对于核子形状因子测量简单而直接，因此被调整为优先研究内容，其中质子形状因子测量结果已经发表在Phys. Rev. D91, 112004, 精度比BaBar实验提高30%，在误差范围内验证了电、磁形状因子相等。除了在psi(3770)的2.9/fb 数据，BESIII近年来又在4.1-4.6GeV之间若干能量点获取了主要用于XYZ及粲物理研究的更多数据，总积分亮度达到7.4/pb, 这些数据用于初态辐射研究测量质子形状因子，同时采用了标记辐射光子和非标记辐射光子两种技术，这两项独立的研究都已经完成，目前在BESIII合作组内部评审。此外，在重子Lambda近阈产生研究中发现非零截面，此工作已经通过内部评审，即将投稿。