Absolute polarimetry at RHIC

TL;DR

This paper details the development and performance of an absolute polarimeter at RHIC, achieving polarization measurement accuracy below 5%, crucial for reducing uncertainties in spin physics experiments.

Contribution

It introduces a novel absolute polarimeter system using a polarized hydrogen gas jet target and silicon detectors, achieving high-precision polarization measurements at RHIC.

Findings

Achieved polarization measurement accuracy of 4.2% in 2005.

Demonstrated the effectiveness of the CNI region for polarization analysis.

Discussed future improvements for even higher accuracy.

Abstract

Precise and absolute beam polarization measurements are critical for the RHIC spin physics program. Because all experimental spin-dependent results are normalized by beam polarization, the normalization uncertainty contributes directly to final physics uncertainties. We aimed to perform the beam polarization measurement to an accuracy of $\Delta P_{beam}/P_{beam} < 5%$. The absolute polarimeter consists of Polarized Atomic Hydrogen Gas Jet Target and left-right pairs of silicon strip detectors and was installed in the RHIC-ring in 2004. This system features \textit{proton-proton} elastic scattering in the Coulomb nuclear interference (CNI) region. Precise measurements of the analyzing power $A_N$ of this process has allowed us to achieve $\Delta P_{beam}/P_{beam} =4.2%$ in 2005 for the first long spin-physics run. In this report, we describe the entire set up and performance of the system. The procedure of beam polarization measurement and analysis results from 2004-2005 are described. Physics topics of $A_N$ in the CNI region (four-momentum transfer squared $0.001 < -t < 0.032 ~ ({\rm GeV}/c)^2$) are also discussed. We point out the current issues and expected optimum accuracy in 2006 and the future.