R&D for SuperKEKB Electron Beam Polarization Upgrade

这是一份用极化电子升级日本SuperKEKB正负电子对撞机所需的加速器研发提案，开启了一个新的发现计划。目标是在2020年末采用极化光束的物理学。升级需要一个极化电子源，一个自旋旋转器系统和一个康普顿偏振器。这一要求是为了进行束流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器的概念设计。这一两极分化升级包括在KEK 2022-2026年路线图和Belle II SnowBulk提交的文件中。SuperKEKB的10.58GeV质心能量碰撞是世界上亮度最高的。位于相互作用点(IP)的Belle II探测器旨在以前所未有的精度和灵敏度测量弱相互作用参数并搜索新的物理，如暗扇区粒子。2020年6月，SuperKEKB提供了2.4x10^34/cm^2s^1的世界纪录瞬时光度。百丽二号将收集数据到2030年初的S。升级SuperKEKB开启了一个独特的计划，可以分别测量多种口味的弱中性电流相互作用强度，其精度达到前所未有的水平。对b夸克、c夸克和µ子的Z玻色子耦合的测量将大大提高现有的世界平均精度，并将使魅力美普适性的精度提高100倍。包括电子和tau测量，极化将给出关键标准模型参数sin^2theta_W的最精确测量，从单个实验中，探测其运行，并提供对新物理的独特敏感性。80%的横向极化电子将被注入高能环(HER)。然后在IP处旋转自旋以进行纵向碰撞，然后使用螺线管场和偶极场将自旋旋转回垂直方向。康普顿偏振仪将以~1%的精度实时监测纵向偏振。每密耳的偏振度将在百丽二号使用陶氏偏振不对称性进行测量。最初的自旋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。我们要求一名博士后和两名研究生，并为旋转旋转器组合功能磁体的工程概念设计提供资金。HQP将致力于加速器物理方面的工作，以制定该项目的自旋旋转器以及康普顿偏振仪部分的设计要求。自旋旋转器的工作将涉及将自旋旋转器集成到HER中所需的束流动力学研究，以最大限度地减少光度和偏振损失。为了更真实地进行束流动力学研究，需要对复合功能磁体进行工程概念设计。一旦有了完整的概念设计，该项目将继续进行该硬件的详细设计。还要求为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.