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、电磁力以及弱核力和强核力。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”大小。弱力和电磁力已经统一。统一其他两种力也是亚原子理论家的目标。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，但仍然有一些东西我们不知道，例如暗物质和暗能量。 ，尚未包含在标准模型中，因此测试标准模型在亚原子物理学界具有高度优先性，将测量极化电子-电子（莫勒）散射中的宇称破坏不对称性。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扰造成的，后者由 Z0 玻色子介导，根据 MOLLER 实验的运动学，预计不对称性约为十亿分之 35 (ppb)。目标是将其值测量到 0.73 ppb 的精度。结果将得出电子弱电荷的测量值，平均 Q2 的分数精度为 2.3%。 0.0056 GeV^2 测量将在杰斐逊实验室的 A 厅进行，其中 11 GeV 纵向偏振电子束将完整入射到 1.5 m 液态氢目标上（束流电子散射出目标电子）。方位角范围和跨越极角范围 5 - 19 mrad，将与背景分离，并通过 a 带到目标下游约 30 m 的环形焦点由一对环形磁铁组件和精密准直器组成的光谱仪系统将被石英探测器系统拦截；产生的切伦科夫光将同时提供散射通量、光束特性等的相对测量。位置、角度和能量将被监测，光束的偏振将被测量，并且辅助探测器将被用来测量平均散射角。电磁振幅与尚未发现的高能动力学弱至~10^-3 G_F的新中性电流振幅的干扰不可能与任何测量风味和CP守恒的实验相匹配。未来十年的过程，并以与高能对撞机直接搜索互补的方式，为多 TeV 尺度的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物理学。在标准模型中，电子弱电荷的测量可确定弱混合角，其不确定度为 +/- 0.00026（统计）+/- 0.00013（系统），与精度类似。因此，我们的结果将影响这个基本电弱参数的中心值，这是破译可能在标准模型之外观察到的任何物理信号的关键输入。如果莫勒测得的弱混合角值与标准模型预测的不同，那么一定存在一种尚未发现的新粒子，其质量可能为 7.5 TeV 或更高。