Measurements of Transverse Spin Effects with the Forward Pion Detector of STAR Larisa Nogach Institute of High Energy Physics, Protvino for the STAR collaboration Outline Motivation and previous measurements Detector layout Status of the analysis of Run 6 data and results Plans for Run 7 and beyond Summary

Motivation AN for p0 production at RHIC at √s=200 GeV hep-ph/0609238 Phenomenological models expect that p0 analyzing power decreases with increasing pT SPIN2006, Kyoto

Existing measurements at STAR PRL 92, 171801 (2004) nucl-ex/0602011 √s=200 GeV, <η> = 3.8 Asymmetry revealed at lower energies persists at √s=200 GeV Cross-section is consistent with NLO pQCD calculations SPIN2006, Kyoto

Transverse spin runs at STAR with forward calorimetry: 2001→2006 detector EEMC and FPD prototypes 6 matrices of FPD full FPD (8 matrices) East FPD West FPD++ ~15 ~30 ~45 ~60 0.15 0.25 0.1 6.8 3.8 ±3.3/±4.0 ±3.7/±4.0 -3.7/3.3 sampled FOM (P2L) in Run 6 is ~50 times larger than from all the previous STAR runs SPIN2006, Kyoto

STAR detector layout TPC: -1.0 <  < 1.0 Run 6 – FPD++ TPC: -1.0 <  < 1.0 FTPC: 2.8 <  < 3.8 BBC : 2.2 <  < 5.0 EEMC: 1 <  < 2 BEMC: -1 <  < 1 FPD++/FPD:  ~ 3.3/-3.7 FPD++: engineering test of the Forward Meson Spectrometer SPIN2006, Kyoto

Detector acceptance Strong correlation between xF and pT in the individual detectors Rapid change in number of events for either increasing pT at fixed xF or increasing xF at fixed pT Broader pT range in xF bins when combining data at <η>=3.3 and 3.7 SPIN2006, Kyoto

Details of data analysis Calibration: on hardware level offline cell-by-cell energy and time-dependent corrections −> is known at the level of 2% Event selection: SPIN2006, Kyoto

Details of data analysis <η>=3.3 <η>=3.7 Analyzing power is measured by “cross-ratio” method with two-arms (left-right) detector: Run-by-run comparison with mean is consistent with statistics, except right near the threshold SPIN2006, Kyoto

π0 AN at √s=200 GeV – xF-dependence AN at positive xF grows with increasing xF AN at negative xF is consistent with zero Run 6 data at <η>=3.7 are consistent with the existing measurements Small errors of the data points allow quantitative comparison with theory predictions Theory expects the reverse dependence on η SPIN2006, Kyoto

AN(pT) at xF > 0.4 Run3+Run5 data (hep-ex/0512013): Run6 data: Online calibration of CNI polarimeter Hint of AN decrease with increasing pT at pT~1-2 GeV/c Run6 data: more precise measurements consistent with the previous runs in the overlapping pT region complicated dependence on pT residual xF-dependence? => AN mapping in (xF,pT) plane is required SPIN2006, Kyoto

AN(pT) in xF-bins Combined data from three runs at <η>=3.3, 3.7 and 4.0 In each xF bin, <xF> does not significantly changes with pT Measured AN is not a smooth decreasing function of pT as predicted by theoretical models SPIN2006, Kyoto

Jet-like events Expect 4-5σ effect from existing Run 6 data Is the single spin asymmetry observed for p0 also present for the jet the p0 comes from? Answer discriminates between Sivers and Collins contributions hep-ex/0602012 PYTHIA simulations: - trigger on small cells; sum over entire side - vector sum of photon momenta reproduces most forward hard-scattered parton Expect 4-5σ effect from existing Run 6 data if observed π0 AN is due to Sivers effect SPIN2006, Kyoto

Forward Meson Spectrometer for Run 7 FMS will provide full azimuthal coverage for range 2.5  h  4.0 broad acceptance in xF-pT plane for inclusive g,p0,w,K0,… production in p+p and d(p)+Au broad acceptance for g-p0 and p0-p0 from forward jet pairs to probe low-x gluon density in p+p and d(p)+Au collisions spacers Run-7 FMS as seen from STAR interaction point SPIN2006, Kyoto

Summary In Run 6, 6.8 pb-1 of data with average beam polarization ~60% has been acquired in p+p collisions at √s=200 GeV (xF,pT)-mapping of the transverse single spin asymmetry of inclusive p0 production at forward rapidity has been done • AN at positive xF grows with increasing xF • AN at negative xF is consistent with zero • Measured AN at “fixed” xF up to xF~0.5 does not decrease with increasing pT as expected by the theory • High precision of the measurements allows for a quantitative comparison with theoretical models and distinguishing between different dynamics SPIN2006, Kyoto

Outlook Analysis of the data at √s=62 GeV is underway Jet-like events: • detector response simulation has been started • calibration of the FPD++ outer matrices needs to be completed Construction of the FMS is ongoing − expected to be completed by Run 7 SPIN2006, Kyoto

Backup

Possible mechanisms Sivers effect [Phys. Rev. D 41, 83 (1990); 43, 261 (1991)]: Flavor dependent correlation between the proton spin (Sp), proton momentum (Pp) and transverse momentum (kT) of the unpolarized partons inside. The unpolarized parton distribution function fq(x,kT) is modified to: Collins effect [Nucl. Phys. B396, 161 (1993)]: Correlation between the quark spin (sq), quark momentum (pq) and transverse momentum (kT) of the pion. The fragmentation function of transversely polarized quark q takes the form: SPIN2006, Kyoto

Separating Sivers and Collins effects Sivers mechanism: asymmetry in the forward jet or γ production Collins mechanism: asymmetry in the forward jet fragmentation SP SP kT,q p p p p Sq kT,π Sensitive to proton spin – parton transverse motion correlations Sensitive to transversity Need to go beyond π0 detection to jets and direct photons SPIN2006, Kyoto

Single Spin Asymmetry No relative luminosity needed Left Right p  p Definition: dσ↑(↓) – differential cross section of p0 when incoming proton has spin up(down) Two methods of measurements: Single arm calorimeter: R – relative luminosity (by BBC) Pbeam – beam polarization Two arm (left-right) calorimeter: No relative luminosity needed π0, xF<0 π0, xF>0 Left Right p  p positive AN: more p0 going left to polarized beam SPIN2006, Kyoto

Run6 − FPD++ SPIN2006, Kyoto

Cell-by-cell calibration gains are determined from π0 peak position in 2γ invariant mass distributions sorted by high towers accuracy of the calibration is at the level of ~2% SPIN2006, Kyoto

Energy-dependent corrections π0 peak position depends on the energy – due to energy leakages and ADC granularity (dedicated MC study has been done) SPIN2006, Kyoto

Run-dependent corrections The FPD response vary with time/beam conditions: used for spin sorting SPIN2006, Kyoto

Correlation between FPD gains and BBC rates

(<xF>,<pT>) 0.25<xF<0.30 0.30<xF<0.35 pT bin <pT>, GeV/c <xF> 0.5-1.1 0.95 0.273 1.1-1.4 1.26 0.282 1.4-1.9 1.61 0.278 1.9-2.4 2.10 2.4-5.0 2.53 0.284 pT bin <pT>, GeV/c <xF> 0.5-1.2 1.06 0.321 1.2-1.7 1.47 0.322 1.7-2.2 1.87 0.330 2.2-2.7 2.41 0.323 2.7-5.0 2.85 0.35<xF<0.40 0.40<xF<0.47 0.5-1.5 1.29 0.368 1.5-1.9 1.70 0.370 1.9-2.4 2.08 0.374 2.4-3.0 2.64 0.373 3.0-5.0 3.18 0.377 0.5-1.7 1.45 0.423 1.7-2.2 1.94 0.426 2.2-2.7 2.38 0.433 2.7-3.3 2.94 0.430 3.3-5.0 3.54 0.435 0.47<xF<0.56 0.5-2.0 1.70 0.496 2.0-2.3 2.16 2.3-2.9 2.52 0.507 2.9-3.4 3.10 0.508 3.4-5.5 3.79 0.506

distributions in pT bins

Cross section − separated xF and pT dependence Similar to ISR analysis: J. Singh et al., Nucl. Phys. B140, 189 (1978) SPIN2006, Kyoto

Jet-like events Expect that jet-like events are ~15% of p0 events SPIN2006, Kyoto