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的理解，以及它们所经历的相互作用。自然界中有四种力：引力、电磁力、弱核力和强核力。一些基本粒子包括电子和构成质子和中子的夸克。这些粒子通过特定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，亚原子理论家的目标是将其他两种力也统一起来。在欧洲核子研究中心发现的希格斯玻色子在一定程度上验证了标准模型，但仍然有我们不知道的事情。例如，暗物质和暗能量还没有包括在标准模型中。因此，测试标准模型在亚原子物理学界具有很高的优先级。MOLLER合作将测量极化电子-电子（Moller）散射中的宇称违反不对称性。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，后者由Z 0玻色子介导。在MOLLER实验的运动学中，不对称性预计为~35 ppb。目标是测量其值，精度为0.73 ppb。这样的结果将在平均Q2为0.0056 GeV^2的情况下测量出电子的弱电荷，其分数精度为2.3%。测量将在杰斐逊实验室的A厅进行，11 GeV的纵向极化电子束将入射到1.5米的液氢靶上。在方位角的整个范围内并且跨越5 - 19 mrad的极角范围的Moller电子（从目标电子散射的束电子）将与背景分离，并且通过由一对环形磁体组件和精密准直器组成的光谱仪系统被带到目标下游约30 m处的环焦点。莫勒环将被一个石英探测器系统拦截;产生的切伦科夫光将提供散射通量的相对测量。在收集数据的同时，将监测光束的位置、角度和能量等特性，测量光束的偏振，辅助探测器将测量背景。将使用特殊运行来测量平均散射角。莫勒对电磁振幅与新的中性电流振幅的干扰很敏感，新的中性电流振幅弱至~10^-3 G_F，而这些电流来自尚未发现的高能动力学。这样的灵敏度水平不太可能与未来十年测量风味和CP保存过程的任何实验相匹配，并以与高能对撞机直接搜索互补的方式在多TeV尺度上为新物理学提供了一个独特的窗口。莫勒将对标准模型之外的物理学敏感，包括超对称性和一些“暗”粒子。在标准模型中，测量电子的弱电荷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的精度。因此，我们的结果将影响这个基本电弱参数的中心值，这是破译可能在大型强子对撞机（LHC）上观察到的标准模型之外的任何物理信号的关键输入。如果MOLLER测量到的弱混合角值与标准模型的预测不同，那么一定有一种新的、尚未发现的粒子，其质量可能为7.5 TeV或更高。

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 will 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 will 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 will be carried out in Hall A at Jefferson Laboratory, where an 11 GeV longitudinally polarized electron beam will be incident on a 1.5 m liquid hydrogen target. Moller electrons (beam electrons scattering off target electrons) in the full range of the azimuth and spanning the polar angular range 5 - 19 mrad, will be separated from background and brought to a ring focus ~ 30 m downstream of the target by a spectrometer system consisting of a pair of toroidal magnet assemblies and precision collimators. The Moller ring will be intercepted by a system of quartz detectors; the resulting Cherenkov light would provide a relative measure of the scattered flux. Simultaneously with data collection, beam properties such as position, angle and energy will be monitored, the polarization of the beam will be measured, and auxiliary detectors will measure backgrounds. Special runs will be used to measure the mean scattering angle. MOLLER 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. MOLLER will be sensitive to physics beyond the Standard Model, including supersymmetry and some "dark" particles. 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 will 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). If MOLLER measures a value of the weak mixing angle that differs from the Standard Model prediction, then there must be a new, as-yet-undiscovered particle that could have a mass of 7.5 TeV or higher.