R&D for SuperKEKB Electron Beam Polarization Upgrade

这是一个加速器研发的建议，需要升级日本的SuperKEKB电子-正电子对撞机与极化电子，这开启了一个新的发现计划。其目标是在20世纪90年代末用偏振光进行物理学研究。升级需要一个极化电子源，一个自旋旋转系统和一个康普顿偏振计。这个请求是为了光束动力学和自旋跟踪研究，以及开发自旋旋转器和康普顿偏振计的概念设计。这种极化升级包含在KEK路线图2022-2026和Belle II SNOWMASS提交的文件中。SuperKEKB的10.58GeV质心能量碰撞是世界上最高的亮度。Belle II探测器位于相互作用点（IP），旨在以前所未有的精度和灵敏度测量弱相互作用参数并寻找新的物理现象，如暗粒子。2020年6月，SuperKEKB创造了2.4 × 10^34/cm^2s^1的瞬时亮度世界纪录。Belle II将收集到21世纪30年代初的数据。升级SuperKEKB与极化开启了一个独特的程序，以测量微弱的中性电流相互作用强度前所未有的精度分别为多种口味。对z -玻色子与b-夸克、c-夸克和μ子耦合的测量将在许多方面提高现有的世界平均精度，并将在粲-美普适性精度上提高100倍。包括电子和tau测量在内，极化将从单个实验中对关键标准模型参数sin^2theta_W进行最精确的测量，探测其运行情况，并为新物理学提供独特的灵敏度。80%的横向极化电子将被注入高能环（HER）。然后旋转自旋在IP处进行纵向碰撞，然后利用螺线管和偶极子场回到垂直方向。康普顿偏振计将以~1%的精度实时监测纵向偏振。每密尔偏振将在贝尔II使用tau偏振不对称测量。最初的旋转器概念已经开发出来，现在需要额外的专门人力来推进研究。我们申请一名博士后和两名研究生，并申请自旋旋转多功能磁体的工程概念设计经费。HQP将致力于加速器物理，以开发自旋转子的设计要求以及项目的康普顿偏振计部分。自旋器的工作将涉及光束动力学研究，需要将自旋器集成到HER中，使光度和偏振损失最小。为了更真实地进行束流动力学研究，需要对多功能磁体进行工程概念设计。一旦有了完整的概念设计，项目就会进行硬件的详细设计。还要求为参加项目会议和讲习班以及专用计算机资源的旅费提供经费。

This is a proposal for accelerator R&D needed to upgrade the Japanese-based SuperKEKB electron-positron collider with polarized electrons, which opens a new discovery program. The aim is to take physics with polarized beam in the late 2020's. The upgrade requires a polarized electron source, a spin rotator system, and a Compton polarimeter.  This request is for beam dynamics and spin tracking studies coupled with developing conceptual designs for the spin rotator and Compton polarimeter. This polarization upgrade is included in the KEK Roadmap 2022-2026 and Belle II SNOWMASS submission. SuperKEKB's 10.58GeV center-of-mass energy collisions are at the world's highest luminosities. The Belle II detector, located at the interaction point (IP), is designed to measure weak interaction parameters and search for new physics, such as dark sector particles, with unprecedented precision and sensitivity. In June 2020, SuperKEKB delivered a world-record instantaneous luminosity of 2.4x10^34/cm^2s^1. Belle II will collect data through to the early 2030's. Upgrading SuperKEKB with polarization opens a unique program to measure weak neutral-current interaction strengths of unprecedented precision separately for multiple flavours. Measurements of Z-boson couplings to b-quark, c-quark and muons will improve on existing world-average precisions by many factors and will see a 100-fold improvement on the charm-beauty universality precision. Including electron and tau measurements, polarization will give the most precise measurement of a key Standard Model parameter, sin^2theta_W, from a single experiment, probe its running, and provide unique sensitive to New Physics. Eighty-percent transversely polarized electrons are to be injected into the High Energy Ring(HER). The spin is then rotated for longitudinal collisions at the IP, and then back to vertical using solenoidal and dipole fields. A Compton polarimeter will monitor longitudinal polarization with ~1% precision in real-time. Per-mil polarization will be measured at Belle II using tau polarization asymmetries. Initial spin rotator concepts have been developed and now additional dedicated manpower is required to move the studies forward. We request a postdoc and two graduate students and funds for an engineering conceptual design of the spin rotator combined-function magnet. The HQP will work on accelerator physics to develop the design requirements of the spin rotators as well as the Compton polarimeter parts of the project. Spin rotator work will involve beam dynamics studies needed to integrate the spin rotators into the HER with a minimal loss of luminosity and polarization. The engineering conceptual design of the combined-function magnet is needed for more realistic beam dynamics studies. Once a full conceptual design is in hand, the project would proceed to the detailed design of this hardware. Funding for travel for project meetings and workshops and for dedicated computer resources is also requested.