Absolute polarimetry at RHIC

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 $\mathrm{\Delta }{P}_{beam}\mathrm{/}{P}_{beam}<5$. 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 $\mathit{\text{proton-proton}}$ 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 $\mathrm{\Delta }{P}_{beam}\mathrm{/}{P}_{beam}=4.2$ 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 $0.001<-t<0.032(\mathrm{G}\mathrm{e}\mathrm{V}\mathrm{/}c{)}^2$) are also discussed. We point out the current issues and expected optimum accuracy in 2006 and the future.