Spin Effects in Forward π0-Production in Polarized Proton-Proton Collisions at STAR

Spin Effects in Forward π0-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 -Production Separated xF and pT dependence of cross section Conclusions Motivation p +p→π0+Х Sivers effect: T Flavor dependent correlation between the proton spin (Sp), momentum (Pp) and transverse momentum (k ) of the unpolarized partons inside The T unpolarized parton distribution function fq(x,k ) is modified to: ⊥ S ⋅ ( P × k P p q) ⊥ ⊥ N ⊥ Collins ƒ qeffect: (x,k q ,S P ) = ƒ q(x,kq ) + Δq ƒ q(x,kq ) ⊥ T S P k Correlation between the quark spin (sq), momentum (pq) and transverse momentum (k ) of the pion The P fragmentation P qfunction of transversely polarized quark q takes the form: s q ⋅ (p q × k π⊥ ) N ⊥ ˆ π/q (z,kπ ) + ∆ Dπ/q (z,kπ ) Dπ/q (z,k ,s q ) = D p q × k π⊥ ⊥ π ⊥ ∆qNfq – Sivers and ∆NDπ/q- Collins Functions may produce azimuthal anisotropy The STAR Collaboration 522 collaborators 51 institutions 12 countries Solenoid Tracker At RHIC STAR detector layout TPC: -1.0 < η < 1.0 FTPC: 2.8 < | η| < 3.8 BBC : 2.2 < | η| < 5.0 EEMC:1 < η < BEMC:0 < η < FPD: |η| ~ 4.0 & ~3.7 Forward Pion Detector FPD module (7×7 matrix of PbGlass: 3.8 × 3.8 × 45 cm3) Pb-Glass active preshower detectors Two 48-strip scintillator SMD Pb plate in front ~2.5 radiation lengths Single Spin Asymmetry Definition: AN = PBeam ⋅ dσ − dσ dσ ↑ + dσ ↓ ↑ PBeam – beam polarization ↑(↓) dσ – differential cross section of π then incoming proton has spin up(down) Two measurements: Single arm calorimeter:  N ↑ − RN ↓   AN = ⋅  ↑ ↓  R – relativePluminosity (by BBC) Beam  N + RN  L↑ R= ↓ L Two arms (left-right) calorimeter:  N↑ ⋅N↓ − N↑ ⋅ N↓ L R R L No relative luminosity AN = ⋅  needed PBeam  N ↑ ⋅ N ↓ + N ↑ ⋅ N ↓ L R R L  Left π0, xF0 p positive AN: more π going p left to polarized beam Right     First AN Measurement at STAR STAR collaboration Phys Rev Lett 92 (2004) 171801 Similar to result from E704 experiment (√s=20 GeV, 0.5 < pT < 2.0 GeV/c) 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, = 3.8 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): (online) = 20%, integrated luminosity ~ 0.15 pb-1 Run3 (2003): (online) = 30%, integrated luminosity ~ 0.5 pb-1 → more precise measurements pp→π 0X 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 π0 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 Separated xF and pT dependence The data is represented in the way similar to J 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 = 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 Conclusions Large spin effects have been found at forward π 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 π was observed at lower √s production at √s = 200 GeV is consistent with NLO pQCD calculations, in contrast to what Mapping of the cross section in xF pT plane has begun … coming soon with analyzing power! BACK-UP SLIDES 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 MC & Data comparison Di-Photon Mass Reconstruction Pb-Glass calorimeter analysis includes: Clustering analysis Fit to measured and parameterized shower shape Applying cuts: Number of photons ≥ Etot > 20 GeV Fiducial volume cut = 1/2 cell from the calorimeter edge Energy sharing zγ γ = E1 − E2 (E + E ) Gains are determined from π peak position for each tower2 < 0.7 (