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Polarimetry with Zero Degree Calorimeter at STAR

STAR introduced the Zero Degree Calorimeter (ZDC) in 2009 for polarized proton collisions at √s = 500 GeV. ZDC, a Cherenkov-light sampling calorimeter, detects forward neutral particles (

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Polarimetry with Zero Degree Calorimeter at STAR

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  1. Introduction In 2009 STAR has commissioned Zero Degree Calorimeter (ZDC) as a local polarimeter for polarized proton collisions at √s = 500 GeV Local Polarimetry with Zero Degree Calorimeter at STAROleksandr Grebenyuk, LBNLSTAR Collaboration ZDC is a Cherenkov-light sampling calorimeter that detects forward neutral particles (<2 mrad) Upgraded with Shower Maximum Detector (SMD) to provide transverse position of hadronic showers STAR also uses Beam-Beam Counters (BBC) for polarimetry at √s = 200 GeV but BBC has insufficient analyzing power at higher energies Method Subtract pedestals and match gains of vertical and horizontal SMD slats Good hit = combination of highest vertical and highest horizontal slat in the event in each of the two ZDCs Yellow: ++--++--… Blue: +-+--+-+… Calculate single spin asymmetry εphys of good hits Square-root formula is insensitive to small variations in luminosity and acceptance between spin states Transverse component of the beam polarization leads to azimuthal modulation of asymmetry Results Transverse Longitudinal running STAR Preliminary Two independent analyses see consistent analyzing powers of >8% for both beams Analyzed multiple trigger conditions and fill patterns At 200 GeV analyzing power is about 4% Launched the online monitoring so results are available very quickly Observe a 10—20% transverse component in longitudinally polarized beams 2009 RHIC & AGS Annual Users’ Meeting, BNL

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