Spin Transport and Polarimetry in the Beam Delivery System of the International Linear Collider

TL;DR

This paper discusses the methods and challenges of accurately measuring and controlling beam polarization in the International Linear Collider, emphasizing spin transport effects and calibration techniques to achieve permille-level precision.

Contribution

It presents a detailed analysis of spin-transport effects and calibration strategies for polarimeters to ensure high-precision polarization measurements at the collider.

Findings

Polarimeters can be cross-calibrated to 0.1% without collisions.

Collision effects significantly impact polarization measurements.

Understanding spin-transport is crucial for precise polarization control.

Abstract

Polarised electron and positron beams are key ingredients to the physics programme of future linear colliders. Due to the chiral nature of weak interactions in the Standard Model - and possibly beyond - the knowledge of the luminosity-weighted average beam polarisation at the $e^+e^-$ interaction point is of similar importance as the knowledge of the luminosity and has to be controlled to permille-level precision in order to fully exploit the physics potential. The current concept to reach this challenging goal combines measurements from Laser-Compton polarimeters before and after the interaction point with measurements at the interaction point. A key element for this enterprise is the understanding of spin-transport effects between the polarimeters and the interaction point as well as collision effects. We show that without collisions, the polarimeters can be cross-calibrated to 0.1 %, and we discuss in detail the impact of collision effects and beam parameters on the polarisation value relevant for the interpretation of the $e^+e^-$ collision data.