Qweak: Weak Charge of the Proton and New Physics at Jefferson Laboratory (JLab E02-020, E05-008)

在过去的十年里，弗吉尼亚州纽波特纽斯的杰斐逊实验室一直在寻找超越基本粒子相互作用标准模型的物理学。 “Qweak”实验将测量质子的弱电荷--这是决定质子如何响应弱相互作用的基本性质。 标准模型的一个关键预测是，所有相互作用的强度，以“耦合常数”为特征，实际上是依赖于能量尺度的。 对于弱相互作用，这一性质尚未得到高精度的检验。 通过测量质子的弱电荷，我们将能够确定预期的弱相互作用与能量标度的“运行”是否实际上被实验证实。 如果不是这样，那么一定有一些新的物理过程正在发生，这将对我们理解物质的基本相互作用产生深远的影响。 我们的实验测试的关键是隔离弱相互作用的电子-质子散射的贡献，通过比较散射率时，电子束被极化在两个镜像状态，称为宇称破坏。 这意味着我们比较了当电子束自旋与它们的运动方向平行和反平行时入射到液氢（质子）靶上的散射率。 对电子自旋方向的预期依赖性非常小-只有百万分之0.2，我们正在测量这个微小的不对称性，其值在+/- 4%以内，以测试标准模型理论的预测。 经过7年的发展，Qweak实验从2010年年中开始，在杰斐逊实验室的连续电子束加速器设施中几乎全天候地采集数据。 已经获得了大量的数据集，从中将确定质子的弱电荷。 我们利用Jefferson实验室的优质极化电子束来帮助确保系统误差非常小，以便进行这些非常精确的测量。自2001年首次构想Qweak实验以来，我们的团队在其中发挥了关键作用;在NSERC的支持下，我们的贡献已经并将继续成为确保这一具有挑战性的努力取得成功的关键。

A search for physics beyond the Standard Model of fundamental particle interactions has been underway for the past decade at Jefferson Laboratory in Newport News, VA. The 'Qweak' experiment will measure the proton's weak charge -- a fundamental property that determines how a proton responds to the weak interaction. One of the Standard Model's key predictions is that the strengths of all interactions, characterized by 'coupling constants', are in fact energy scale - dependent. This property has not yet been tested to high precision for the weak interaction. By measuring the proton's weak charge, we will be able to determine whether the anticipated 'running' of the weak interaction with energy scale is in fact borne out by experiment. If it is not, some new physics processes must be taking place, which would have profound implications for our understanding of the fundamental interactions of matter. The key to our experimental test is to isolate the weak interaction contribution to electron-proton scattering by comparing the scattering rates when the electron beam is polarized in two mirror-image states, referred to as parity-violation. This means that we compare the scattering rate when the electron beam spins are aligned parallel and antiparallel to their direction of motion, incident on a liquid hydrogen (proton) target. The expected dependence on the electron spin direction is very small - only 0.2 parts per million, and we are measuring this tiny asymmetry to within +/- 4% of its value in order to test the predictions of the Standard Model theory. After 7 years of development, the Qweak experiment took data almost full time at Jefferson Laboratory's continuous electron beam accelerator facility for 2 years, beginning in mid-2010. A substantial data set has been acquired, from which the weak charge of the proton will be determined. We made use of Jefferson Lab's excellent quality polarized electron beam to help ensure that systematic errors would be very small, in order to carry out these very precise measurements. Our group has played a key role in the Qweak experiment since it was first conceived in 2001; our contributions, with NSERC support, have been and will continue to be pivotal in ensuring the success of this challenging effort.