P2实验-未来低动量传递下电弱混合角的高精度测量
The P2 Experiment - A future high-precision measurement of the electroweak mixing angle at low momentum transfer
摘要：
本文介绍了未来的P2宇称违反电子散射设施在即将到来的MESA加速器在美因茨。该设施的物理方案包括间接的，高精度的搜索物理超越标准模型，测量中子分布在核物理，单自旋不对称性产生的双光子交换和一个可能的未来扩展到测量强子宇称破坏。P2实验的第一个测量目标是在Q^2 = 4.5 × 10^{-3} GeV^2的四动量转移下高精度地确定弱混合角，精度达到0.14%。精度与Z极的现有测量相当。它包括一个灵敏度测试的标准模型的质量规模为50 TeV，可扩展到70 TeV。这就需要测量电子-质子弹性散射中宇称违反的截面不对称性，总精度为0.56 10^-9（1.4%），150 \micro A极化电子束照射60 cm液体H_（1-x）2靶，允许提取与弱混合角直接相关的质子的弱电荷。在155 MeV的小束能量下，来自伽马Z盒图的贡献变得很小。不对称性的大小是电子散射中测量的最小不对称性，具有前所未有的精度目标。我们在这里报告的概念设计的P2光谱仪，其切伦科夫探测器，集成读出电子以及超薄，快速跟踪探测器。在确定弱混合角的准备工作中已经做了大量的理论工作。还介绍了粒子物理和核物理的进一步物理计划。
英文摘要：
This article describes the future P2 parity-violating electron scattering facility at the upcoming MESA accelerator in Mainz. The physics program of the facility comprises indirect, high precision search for physics beyond the Standard Model, measurement of the neutron distribution in nuclear physics, single-spin asymmetries stemming from two-photon exchange and a possible future extension to the measurement of hadronic parity violation. The first measurement of the P2 experiment aims for a high precision determination of the weak mixing angle to a precision of 0.14% at a four-momentum transfer of Q^2 = 4.5 10^{-3} GeV^2. The accuracy is comparable to existing measurements at the Z pole. It comprises a sensitive test of the standard model up to a mass scale of 50 TeV, extendable to 70 TeV. This requires a measurement of the parity violating cross section asymmetry -39.94 10^{-9} in the elastic electron-proton scattering with a total accuracy of 0.56 10^-9 (1.4 %) in 10,000 h of 150 \micro A polarized electron beam impinging on a 60 cm liquid H_2 target allowing for an extraction of the weak charge of the proton which is directly connected to the weak mixing angle. Contributions from gamma Z-box graphs become small at the small beam energy of 155 MeV. The size of the asymmetry is the smallest asymmetry ever measured in electron scattering with an unprecedented goal for the accuracy. We report here on the conceptual design of the P2 spectrometer, its Cherenkov detectors, the integrating read-out electronics as well as the ultra-thin, fast tracking detectors. There has been substantial theory work done in preparation of the determination of the weak mixing angle. The further physics program in particle and nuclear physics is described as well.
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