R&D for SuperKEKB Electron Beam Polarization Upgrade

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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.