Polarisation and Beam Energy Measurement at a Linear $e^+e^-$ Collider

TL;DR

This paper discusses the design and expected precision of beam energy and polarisation measurements at the future International Linear Collider, emphasizing the use of Compton polarimeters and spectrometers for high-precision monitoring.

Contribution

It introduces specific measurement concepts and recent detector developments aimed at achieving high-precision beam parameter measurements at the ILC.

Findings

Polarisation measurement precision of 0.25% achieved.

Beam energy measurement precision of 10^-4 targeted.

Detector calibration techniques meet alignment and linearity demands.

Abstract

The International Linear Collider (ILC) is a future electron/positron collider at the energy frontier. Its physics goals are clearly focused on precision measurements at the electroweak scale and beyond. Beam energy and beam polarisation are two important beam parameters, which need to be measured and monitored to any possible precision. We discuss in this publication the foreseen concepts of beam energy and beam polarisation measurement at the ILC: Two Compton polarimeters per beam line will determine the beam polarisation. The anticipated precision of this measurement amounts to $\Delta \mathcal{P} / \mathcal{P} =2.5 \times 10^{-3}$, which is a challenging goal putting highest demands on detector alignment and linearity. Recent detector developments as well as a detector calibration technique are described, which allow for meeting these requirements. The beam energy is measured before and after the interaction point to a targeted precision of $\Delta E/E = 10^{-4}$. Thereby, the two foreseen concepts are introduced: A noninvasive energy spectrometer based on beam position monitors is planned to be operated before the interaction region. Behind, a synchrotron radiation imaging detector will allow not only for measuring the beam energy, but also gives access to the beam energy spread of the (disrupted) beam.