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

轻子-轻子电弱反应 (MOLLER) 的测量：杰斐逊实验室使用 Mäller 散射对弱混合角进行超精确测量

• 批准号: SAPPJ-2014-00033
• 负责人: Mammei, Juliette
• 金额: $ 4.74万
• 依托单位: University of Manitoba
• 依托单位国家: 加拿大
• 项目类别: Subatomic Physics Envelope - Project
• 财政年份: 2014
• 资助国家: 加拿大
• 起止时间: 2014-01-01 至 2015-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=568037
• 关键词: Measurement; Lepton; Electroweak; Reaction; MOLLER

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.

粒子和相互作用的标准模型总结了我们当前对自然界基本粒子及其所经历的相互作用的理解。自然界中有四种力量：重力，电磁力和弱和强核力量。一些基本颗粒包括构成质子和中子的夸克。这些颗粒通过特定力相互作用的强度取决于该力的“电荷”的大小。弱和电磁力已经统一，也是亚原理理论家也统一其他两种力量的目标。在CERN中发现希格斯玻色子在一定程度上验证了标准模型，但我们仍然不知道。例如，标准模型中尚未包含暗物质和暗能量。因此，测试标准模型在亚原子物理社区中具有很高的优先级。 Moller协作将测量极化电子（Moller）散射中的平等侵略不对称性。在标准模型中，这种不对称是由于电子和弱力之间的干扰，后者是由Z0玻色子介导的。在Moller实验的运动学上，不对称性预计为约35份（PPB）。目标是将其值衡量为0.73 ppb。结果将在平均Q2为0.0056 GEV^2的平均Q2时对电子的弱电荷进行测量。该测量将在杰斐逊实验室的A H厅进行，那里11 GEV纵向极化的电子束将入射在1.5 m的液态氢靶标上。 Moller电子（散射目标电子的梁电子）在方位角的整个范围内，跨越极性角范围5-19 MRAD，将与背景分开，并通过由一对圆环磁体组件和精确的符号器组成的光谱仪系统将目标焦点分开。 Moller环将被石英检测器系统拦截。所得的Cherenkov光将提供散射通量的相对度量。同时，将监测数据收集，诸如位置，角度和能量之类的梁特性，测量梁的极化，并测量辅助探测器。特殊运行将用于测量平均散射角。 Moller对电磁放大器对新的中性电流放大器的干扰敏感，从未发现的高能量动力学较弱，高达〜10^-3 g_f。在接下来的十年中，测量风味和CP支持过程的任何实验都不太可能与这种敏感性相匹配，并以多种方式完整性地在高能胶合管上进行直接搜索，从而为新物理学提供了独特的窗口。超越标准模型，包括超对称性和一些“黑暗”颗粒对物理敏感。在标准模型中，电子的弱电荷的测量结果得出弱混合角的确定，不确定性为+/- 0.00026（STAT）+/- 0.00013（SYST），类似于从高能量围墙中确定的单一最佳这样的确定的准确性。这是我们的结果会影响该基本电子参数的核心价值，这是对超出标准模型以外的任何物理学信号的关键输入，这是在大型强子撞机（LHC）上可能观察到的。如果moller测量与标准模型预测不同的弱混合角值，则必须有一个新的，尚未发现的粒子，其质量可能为7.5 TEV或更高。