[论文解读] The P2 Experiment - A future high-precision measurement of the electroweak mixing angle at low momentum transfer

D. Becker, Razvan Bucoveanu+43 more|arXiv (Cornell University)|Feb 13, 2018

Particle physics theoretical and experimental studies参考文献 156被引用 50

一句话总结

P2 实验在即将到来的 MESA 加速器，旨在以约 0.14% 的精度，在 Q^2 = 4.5×10^-3 GeV^2 使用奇偶对称性破坏的弹性电子-质子散射以及高流量、低能量的偏振电子束来测量质子的弱电荷和电弱混合角 sin^2 theta_W。

ABSTRACT

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.

研究动机与目标

• 在低 Q^2 下开发对质子弱电荷 Q_W(p) 的高精度测定，以测试标准模型及其 sin^2 theta_W 的运行。
• 设计并实施使用 MESA 的、能量较低但强束流的奇偶对称性破坏程序，使用 155 MeV 的偏振电子束。
• 通过控制系统误差和辐射校正，使测得的奇偶对称性 A^PV 的总体实验精度优于 1.4%。
• 表征并管理强子和辐射校正，包括 gamma-Z 框图，对以最小理论不确定性提取 sin^2 theta_W。

提出的方法

• 在厚 LH2 靶上，使用纵向偏振的 155 MeV 电子束对弹性电子-质子散射测量奇偶对称性 A^PV。
• 使用具有大方位角接受的一体化 Cherenkov 探测器来收集约 0.1 THz 的弹性事件并进行积分读出。
• 应用蒙特卡洛误差传播将观测到的 A^exp 和实验参数映射到 Delta sin^2 theta_W，包括 gamma-Z 框贡献和形状因子依赖。
• 将实验数据与理论输入（电弱辐射校正和强子不确定性）结合，获得 sin^2 theta_W 的精度约为 0.14%。
• 描述实现所需统计所必需的 MESA 加速器、偏振源、偏振测量、束流控制以及高功率 LH2 靶的关键要素。

实验结果

研究问题

• RQ1 在 Q^2 ≈ 4.5×10^-3 GeV^2 的低 Q^2 奇偶对称性破坏电子散射测量能否以亚百分比的精度确定质子弱电荷 Q_W(p)？
• RQ2 考虑现实的束流、靶、探测器和理论不确定性，在低 Q^2 时从 A^PV 获得的 sin^2 theta_W 的可实现精度是多少？
• RQ3 gamma-Z 框图和强子形状因子对 sin^2 theta_W 提取的影响有多大，以及如何加以控制？
• RQ4 为达到所需的统计灵敏度，需要哪些束流、探测器和靶的设计（包括 MESA 的能力）？
• RQ5 P2 设施的更广泛物理潜力是什么（如中子皮厚、12C 靶、超出标准模型的搜索）？

主要发现

• Aiming for sin^2 theta_W precision of 0.14% at Q^2 = 4.5×10^-3 GeV^2.
• Target parity-violating asymmetry: <A^exp> = -39.94×10^-9 with total uncertainty Δ<A^exp>_Total = 0.56×10^-9 (1.4%) after 10,000 h with 150 µA beam on a 60 cm LH2 target.
• Projected reach for new physics sensitivity corresponds to Λ ≈ 50 TeV (potentially up to 60 TeV) via 4-fermion operator scaling; competing with other SM tests such as QWeak and MOLLER.
• Error propagation includes gamma-Z box corrections with central value Box_γZ ≈ 1.07×10^-3 and ΔBox_γZ ≈ 0.18×10^-3, contributing to the overall theory uncertainty.
• A large-scale integration detector, high-power LH2 target with density fluctuations at 10 ppmm, and a new MESA accelerator are required to achieve the stated precision.
• The study also outlines additional physics opportunities (neutron skin, carbon-12 measurements, backward-angle runs) beyond the primary sin^2 theta_W determination.