Local Polarimetry for Proton Beams with the STAR Beam Beam Counters

STAR collected data in proton-proton collisions at √ s = 200 GeV with transverse and longitudinal beam polarizations during the 2003 running period at RHIC. We present preliminary results on single transverse spin asymmetries AN in the production of forward charged hadrons detected with the Beam-Beam Counters (BBC). The asymmetries AN measured at pseudorapidities 3.9 < |η| < 5.0 are found to be of the order of 10 while asymmetries measured at smaller pseudorapidities 3.4 < |η| < 3.9 are found to be consistent with zero. The BBC and its associated scaler system for fast data recording provides the first local polarimeter at STAR. The setup has been sucessfully used to tune the spin rotator magnets and to verify longitudinal polarization at STAR. STAR is one of two large experimental facilities at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. One of the goals of the STAR physics program is to study the internal spin structure of the proton using collisions of longitudinally polarized protons at √ s = 200 − 500 GeV. The stable beam polarization direction in RHIC is transverse, and thus needs to be rotated at the STAR Interaction Region (IR) to become longitudinal[1]. The magnitude of the beam polarization is measured at a different IR by the RHIC Coulomb-Nuclear Interference (CNI) pC polarimeter[1]. During the first (transversely) polarized proton running period in 2002 STAR has measured single transverse spin cross-section asymmetries AN = (σ↓ − σ↑)/(σ↓ + σ↑) at √ s = 200 GeV in neutral pion[2] and charged hadron[3] production in the forward region. Additional sets of magnets were installed before and after the STAR IR before the second proton running period at RHIC in 2003 to obtain longitudinal polarization. Local polarimetry had been developed to measure the radial and vertical polarization components. We discuss the instrumentation and results below.