Measurement of a Lepton-Lepton Electroweak Reaction (MOLLER): An Ultra-precise Measurement of the Weak Mixing Angle using Møller Scattering at Jefferson Laboratory

粒子和相互作用的标准模型总结了我们目前对自然界中基本粒子以及它们所经历的相互作用的理解。自然界中有四种力：引力、电磁力和弱核力和强核力。一些基本粒子包括电子和 组成质子和中子的夸克。这些粒子通过特定的力相互作用的强度取决于该力的“电荷”的大小。弱力和电磁力已经统一了，亚原子理论家的目标是把另外两种力也统一起来。欧洲核子研究中心希格斯玻色子的发现在一定程度上验证了标准模型，但仍有一些事情我们不知道。例如，暗物质和暗能量还不包括在标准模型中。因此，测试标准模型在亚原子物理界具有很高的优先级。 穆勒的合作建议测量极化电子-电子(穆勒)散射中违反宇称的不对称性。在标准模型中，这种不对称性是由于电磁力和弱力之间的干涉造成的，而弱力是由Z0玻色子介导的。在穆勒实验的运动学上，不对称性预计为百万分之三十五(Ppb)。目标是将其价值测量到0.73 ppb的精确度。这一结果将产生对电子弱电荷的测量，在平均Q2为0.0056 GeV^2时，分数精度为2.3%。 该测量对电磁幅值与新的中性电流幅值的干扰很敏感，新的中性点电流幅值弱到~10^-3G_F，这是从迄今尚未发现的高能动力学过程中得到的。这样的灵敏度水平不太可能在未来十年被任何测量味道和CP保存过程的实验所匹配，并导致在多TeV尺度上以一种补充高能对撞机直接搜索的方式获得新物理的独特窗口。在标准模型中，对电子弱电荷的测量产生了弱混合角的确定，其不确定度为+/-0.00026(STAT)+/-0.00013(SYST)，类似于高能对撞机的单一最佳确定精度。因此，我们的结果可能会潜在地影响这个基本电弱参数的中心值，这个参数是破译可能在大型强子对撞机(LHC)上观察到的标准模型以外的任何物理信号的关键输入。 这项测量将在杰斐逊实验室A厅进行，使用11GeV纵向极化电子入射到液氢靶上。光谱仪将被用来将穆勒电子(从目标电子散射的束电子)从背景中分离出来，并将它们聚焦在目标下游约30米处。散射电子的积分通量将由石英切伦科夫探测器系统测量。

The Standard Model of Particles and Interactions summarizes our current understanding of the fundamental particles in nature, and the interactions they undergo. There are four forces in nature: the gravitational force, the electromagnetic force and the weak and strong nuclear forces. Some fundamental particles include electrons and the quarks that make up protons and neutrons. The strength with which these particles interact via a particular force depends on the size of the “charge” for that force. The weak and electromagnetic forces have been unified and it is a goal of subatomic theorists to unify the other two forces as well. The discovery of the Higgs boson at CERN validates the Standard Model to a certain extent, but there are still things we do not know. Dark matter and dark energy, for example, are not yet included in the Standard Model. Therefore testing the Standard Model has a high priority within the subatomic physics community. The MOLLER collaboration proposes to measure the parity-violating asymmetry in polarized electron-electron (Moller) scattering. In the Standard Model, this asymmetry is due to the interference between the electromagnetic and the weak forces, the latter being mediated by the Z0 boson. The asymmetry is predicted to be ~35 parts per billion (ppb) at the kinematics of the MOLLER experiment. The goal is to measure its value to a precision of 0.73 ppb. The result would yield a measurement of the weak charge of the electron to a fractional accuracy of 2.3% at an average Q2 of 0.0056 GeV^2. The measurement is sensitive to the interference of the electromagnetic amplitude with new neutral current amplitudes as weak as ~10^-3 G_F from as yet undiscovered high energy dynamics. Such a level of sensitivity is unlikely to be matched by any experiment measuring a flavor- and CP-conserving process over the next decade, and results in a unique window to new physics at the multi-TeV scale in a manner complementary to direct searches at high energy colliders. In the Standard Model, the measurement of the weak charge of the electron yields a determination of the weak mixing angle with an uncertainty of +/- 0.00026 (stat) +/- 0.00013 (syst), similar to the accuracy of the single best such determination from high energy colliders. Thus, our result could potentially influence the central value of this fundamental electroweak parameter, a critical input to deciphering signals of any physics beyond the Standard Model that might be observed at the Large Hadron Collider (LHC). The measurement will be carried out in Hall A at Jefferson Laboratory, using 11 GeV longitudinally polarized electrons incident on a liquid hydrogen target. A spectrometer will be used to separate the Moller electrons (beam electrons scattering off target electrons) from background and focus them ~ 30 m downstream of the target. The integrated flux of scattered electrons will be measured by a system of quartz Cherenkov detectors.