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Measurements of Transverse Spin Effects with the Forward Pion Detector of STAR

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

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Measurements of Transverse Spin Effects with the Forward Pion Detector of STAR

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  1. 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

  2. Motivation AN for p0production at RHIC at √s=200 GeV hep-ph/0609238 Phenomenological models expect that p0 analyzing power decreases with increasing pT SPIN2006, Kyoto

  3. 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

  4. Transverse spin runs at STAR with forward calorimetry: 2001→2006 sampled FOM (P2L) in Run 6 is ~50 times larger than from all the previous STAR runs SPIN2006, Kyoto

  5. STAR detector layout 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

  6. 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

  7. 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

  8. Details of data analysis <η>=3.7 <η>=3.3 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

  9. π0AN 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

  10. AN(pT) at xF > 0.4 Run3+Run5 data (hep-ex/0512013): • 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

  11. 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

  12. Jet-like events • Is the single spin asymmetry • observed for p0also present • for the jet thep0 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

  13. 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

  14. 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 p0production 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

  15. 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

  16. Backup

  17. 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

  18. 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

  19. 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 Left π0, xF<0 π0, xF>0 p  p Right Single Spin Asymmetry positive AN: more p0 going left to polarized beam SPIN2006, Kyoto

  20. Run6 − FPD++ SPIN2006, Kyoto

  21. 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

  22. Energy-dependent corrections π0peak position depends on the energy – due to energy leakages and ADC granularity (dedicated MC study has been done) SPIN2006, Kyoto

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

  24. Correlation between FPD gains and BBC rates

  25. 0.25<xF<0.30 0.30<xF<0.35 (<xF>,<pT>) 0.35<xF<0.40 0.40<xF<0.47 0.47<xF<0.56

  26. distributions in pT bins

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

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

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