[Paper Review] Polarisation and Polarimetry at HERA

B. Sobloher|arXiv (Cornell University)|Jan 18, 2012

Particle Accelerators and Free-Electron Lasers4 citations

TL;DR

This paper details the implementation and performance of three polarimeters—TPOL, LPOL, and a Fabry–Perot cavity-based system—at HERA, enabling precise measurement of longitudinal and transverse lepton beam polarisation. The cavity polarimeter achieved a systematic uncertainty of 0.9%, marking a sub-percent precision milestone in high-energy polarimetry at HERA.

ABSTRACT

Longitudinal polarisation of the lepton beam is a key ingredient to the success of the world's unique e\pmp ring collider HERA. This article aims at providing a brief introduction to the physics motivation for deep-inelastic scattering of polarised electrons or positrons off protons, the basic mechanisms to establish lepton polarisation in the high-energy storage ring and to describe briefly the three different polarimeters, which measured both the transverse and the longitudinal polarisation.

Motivation & Objective

• To enable high-precision measurement of longitudinal and transverse polarisation in the HERA electron/positron storage ring.
• To address the challenge of maintaining high beam polarisation in the presence of depolarising resonances and beam errors.
• To develop and validate multiple independent polarimetry techniques for cross-checking and improving systematic uncertainty.
• To achieve sub-percent systematic uncertainty in longitudinal polarisation measurement using a high-finesse Fabry–Perot cavity.
• To support precision physics programs in deep-inelastic scattering, including nucleon spin structure and electroweak parameters.

Proposed method

• Utilised radiative polarisation via synchrotron radiation to build up beam polarisation in the storage ring, based on the Sokolov–Ternov effect.
• Employed TPOL to measure transverse polarisation using Compton scattering asymmetry in vertical polarisation.
• Used LPOL to detect energy asymmetry from longitudinally polarised electrons in Compton scattering for polarisation measurement.
• Implemented a Fabry–Perot cavity with a 3×10⁴ finesse to enhance laser photon density, enabling continuous, high-statistics Compton scattering.
• Calibrated the cavity polarimeter using known Compton and bremsstrahlung edge positions and performed detailed simulations of detector response and background contributions.
• Conducted a global fit of energy spectra for two laser helicity states to extract longitudinal polarisation with high statistical precision.

Experimental results

Research questions

• RQ1 How can longitudinal polarisation be efficiently generated and maintained in a high-energy electron/positron storage ring like HERA?
• RQ2 What are the dominant systematic uncertainties in Compton-based polarimetry, and how can they be minimized?
• RQ3 Can a Fabry–Perot cavity system achieve sub-percent systematic uncertainty in beam polarisation measurement?
• RQ4 How do the three independent polarimetry systems (TPOL, LPOL, cavity) compare in performance and consistency over time?
• RQ5 What is the impact of beam and detector instabilities on polarisation measurement accuracy?

Key findings

• The Fabry–Perot cavity polarimeter achieved a systematic uncertainty of 0.9% on the longitudinal polarisation measurement, approaching sub-percent precision.
• Over 500 hours of efficient data were collected with the cavity polarimeter during the final phase of HERA operation, demonstrating stable and high-rate performance.
• The cavity system amplified the laser power by a factor of ~5000, reaching ~3 kW effective power through constructive interference in a high-finesse optical cavity.
• Systematic uncertainties from detector response modelling, synchrotron radiation peak description, and laser polarisation stability were found to be the dominant contributors, each at ~0.3%.
• The combined polarisation measurement uncertainty was estimated at 3.4% due to discrepancies between TPOL and LPOL, with an additional 3% systematic uncertainty assigned to account for unexplained time-dependent variations.
• The cavity polarimeter's performance validated the feasibility of high-precision, continuous polarimetry using resonant photon enhancement and energy-spectrum fitting.