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Spin Effects in Forward π 0 -Production in Polarized Proton-Proton Collisions at STAR

Spin Effects in Forward π 0 -Production in Polarized Proton-Proton Collisions at STAR. Dmitry Morozov, IHEP (Protvino) for the STAR Collaboration. Outlook. Motivation STAR/FPD overview Single Spin Asymmetry at FPD Differential cross sections for forward π 0 -Production

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Spin Effects in Forward π 0 -Production in Polarized Proton-Proton Collisions at STAR

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  1. Spin Effects in Forward π0-Production in Polarized Proton-Proton Collisions at STAR Dmitry Morozov, IHEP (Protvino) for the STAR Collaboration

  2. Outlook • Motivation • STAR/FPD overview • Single Spin Asymmetry at FPD • Differential cross sections for forward π0-Production • Separated xF and pT dependence of cross section • Conclusions XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  3. Motivation p +p→p0+Х • Sivers effect: Flavor dependent correlation between the proton spin (Sp), momentum (Pp) and transverse momentum (kT) of the unpolarized partons inside. The unpolarized parton distribution function fq(x,kT) is modified to: • Collins effect: Correlation between the quark spin (sq), momentum (pq) and transverse momentum (kT) of the pion. The fragmentation function of transversely polarized quark q takes the form: ∆qNfq – Sivers and ∆NDp/q- Collins Functions may produce azimuthal anisotropy XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  4. The STAR Collaboration 522 collaborators 51 institutions 12 countries Solenoid Tracker At RHIC XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  5. STAR detector layout • TPC: -1.0 <  < 1.0 • FTPC: 2.8 <  < 3.8 • BBC : 2.2 <  < 5.0 • EEMC:1 <  < 2 • BEMC:0 <  < 1 • FPD: || ~ 4.0 & ~3.7 XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  6. Forward Pion Detector • FPD module (7×7 matrix of Pb-Glass: 3.8 × 3.8 × 45 cm3) • 7 Pb-Glass active preshower detectors • Two 48-strip scintillator SMD • Pb plate in front ~2.5 radiation lengths XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  7. Definition: PBeam – beam polarization dσ↑(↓) – differential cross section of π0 then incoming proton has spin up(down) Two measurements: Single arm calorimeter: R – relative luminosity (by BBC) Two arms (left-right) calorimeter: No relative luminosity needed Single Spin Asymmetry Left π0, xF<0 π0, xF>0 positive AN: more p0 going left to polarized beam p  p Right XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  8. First AN Measurement at STAR Similar to result from E704 experiment (√s=20 GeV, 0.5 < pT < 2.0 GeV/c) STAR collaboration Phys. Rev. Lett. 92 (2004) 171801 Can be described by several models: Sivers: spin and k correlation in parton distribution functions (initial state) Collins: spin and k correlation in fragmentation function (final state) Qiu and Sterman (initial state) / Koike (final state): twist-3 pQCD calculations, multi-parton correlations √s=200 GeV, <η> = 3.8 XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  9. AN for Forward π0 at STAR Shown at SPIN 2004, Trieste, Italy The asymmetry is found to be zero for negative -0.6 < xF < -0.2 AN for positive xF is consistent with zero up to xF ~ 0.35, then increases with increasing xF Run2 (2002): <PBeam>(online) = 20%, integrated luminosity ~ 0.15 pb-1 Run3 (2003): <PBeam>(online) = 30%, integrated luminosity ~ 0.5 pb-1  more precise measurements XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  10. ppp0X cross sections at 200 GeV • The error bars are point-to-point systematic and statistical errors added in quadrature • The inclusive differential cross section for p0 production is consistent with NLO pQCD calculations at 3.3 < η < 4.0 • As η increases, systematics regarding the comparison with NLO pQCD calculations begin to emerge. The data at low pT are more consistent with the Kretzer set of fragmentation functions. Similar to what was observed by PHENIX. XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  11. Separated xF and pT dependence • The data is represented in the way similar toJ. Singh, et al Nucl. Phys. B140 (1978)189- ISR experiment at √s=45 GeV • Cross sections fall with pT at fixed xF with exponent (~ 6) independent of xF • Data show exponential dependence on xF with fixed pT = 2 GeV/c. The value of the fitted exponent (~5) may be sensitive to the interplay between hard and soft scattering processes Although Data accumulated in different running years • with different calorimeters • with different readout electronics • taken at different angles XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  12. Conclusions • Large spin effects have been found at forward p0 production in p p reaction at highest energy √s = 200 GeV • The single spin asymmetry for positive xF is consistent with zero up to xF~0.35, then increases with increasing xF • The asymmetry is found to be zero for negative xF • The inclusive differential cross section for forward p0 production at √s = 200 GeV is consistent with NLO pQCD calculations, in contrast to what was observed at lower √s • Mapping of the cross section in xF pT plane has begun … coming soon with analyzing power! XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  13. BACK-UP SLIDES XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  14. Systematics Measurements utilizing independent calorimeters consistent within uncertainties • Systematics: • Normalization uncertainty = 16%: • position uncertainty (dominant) • Energy dependent uncertainty = 13% - 27%: • energy calibration to 1% (dominant) • background/bin migration correction • kinematical constraints XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  15. MC & Data comparison XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  16. XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  17. Clustering analysis Fit to measured and parameterized shower shape Applying cuts: Number of photons ≥ 2 Etot > 20 GeV Fiducial volume cut = 1/2 cell from the calorimeter edge Energy sharing (<0.3 for cross sections) Gains are determined from π0 peak position for each tower Energy dependent gain correction Run/Luminosity dependent gain correction MC to data comparison Di-Photon Mass Reconstruction Pb-Glass calorimeter analysis includes: • Mass resolution ~ 20 MeV • The calibration is known at 2% level • Efficiencies is geometrically determined (dominated by the geometrical acceptance of the calorimeter) XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  18. 2 photon cluster example Try both 1g Cluster Energy 2g Cluster 2nd moment of cluster (long axis) Cluster categorization XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

  19. Luminosity vs PMT gain Gain stability (before correction) Gain stability (after correction) Time/luminosity dependent gain shift corrections XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia)

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