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L. Nogach, IHEP, Protvino for the STAR Collaboration PowerPoint Presentation
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L. Nogach, IHEP, Protvino for the STAR Collaboration

L. Nogach, IHEP, Protvino for the STAR Collaboration

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L. Nogach, IHEP, Protvino for the STAR Collaboration

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  1. STAR Measurements of transverse spin effects in the forward region with STAR detector L. Nogach, IHEP, Protvinofor the STAR Collaboration • Outline: • Motivation • STAR detector • Inclusive p0 measurements • More possibilities with the FMS • 2009 run at √s=500 GeV • Summary and outlook DSPIN-2009, September 1-5

  2. Motivation • Contrary to simple pQCD predictions, first measurements of  production in p↑p collisions found large transverse single spin asymmetries. Similar large effects have recently been found in e+e- and semi-inclusive deep-inelastic scattering. • Significant developments in theory in the past few years suggest common origins for these effects, but large transverse spin asymmetries inp↑+p →  + Xproduction are not yet fully understood. • More fundamental question: what is the underlying dynamics in kinematics where transverse spin effects are observed?

  3. Polarized pp collisions at RHIC RHIC pC Polarimeters Absolute Polarimeter (H jet) Siberian Snakes Siberian Snakes PHENIX STAR Spin Rotators (longitudinal polarization) Spin Rotators Pol. H- Source LINAC BOOSTER Helical Partial Siberian Snake AGS 200 MeV Polarimeter AGS pC Polarimeter FOM=P2L Strong AGS Snake

  4. STAR detector layout FMS TPC FPD BBC BEMC • FPD • Modular detector • Small xF-pT range compared to FMS FPD FMS FMS • 20x acceptance of previous forward detectors at STAR • Full azimuthal coverage for 2.5 < η < 4.0 • Array of 1264 Pb-glass cells

  5. Published measurements with the FPD (20022006) PRL 92, 171801 (2004) PRL 101, 222001 (2008) • AN at positive xF grows with increasing xF • xF dependence matches theoretical model expectations qualitatively • pT dependence at fixed xF is not consistent with 1/pT expectation of theoretical model calculations √s=200 GeV, <η> = 3.8 U.D’Alesio, F.Murgia Phys. Rev. D 70, 074009 (2004) arXiv:0712.4240 C.Kouvaris, J.Qiu, W.Vogelsang, F.Yuan, Phys. Rev. D 74, 114013 (2006) Asymmetry revealed at lower energies persists at √s=200 GeV

  6. Inclusive h AN measurements (2006) Asymmetry for h-mesons is larger than for p0 (similar to the E704 measurements): (AN)h = 0.361 ± 0.064 for 0.55<xF<0.75 arXiv:0905.2840

  7. Inclusive p0 AN with the FMS (2008) y x P arXiv:0901.2828 Octant subdivision of FMS for inclusive p0 spin sorting • Azimuthal dependence appears • to be as expected • AN is comparable to prior measurements with the FPD

  8. AN(pT) at |xF|>0.4 with the East FPD/FMS (2008) negative xF positive xF • AN for negative xF consistent with zero • Indication of AN for positive xF persists • up to pT ~5 GeV/c • Needs more transverse spin running arXiv:0901.2763+ A.Ogawa @CIPANP09

  9. High xF vector mesons 3 photon events to look forwp0g (BR= 8.9%) • pT(triplet)>2.5 GeV/c • E(triplet)>30 GeV • pT(photon cluster)>1.5 GeV/c • pT(p0)>1 GeV/c Background only MC Run8 FMS data Fit is Gaussian + p3  μ=0.784±0.008 GeV σ=0.087±0.009 GeV Scale=1339±135 Events Significant (10s) wp0g signal seen in the data => possibility to measure spin-1 meson AN arXiv:0906.2332

  10. First look at “jet-like” events in the FMS Eventselection: “Jet-shape” distribution of energy within jet-like objects in the FMS as a function of distance from the jet axis • >15 cells with energy > 0.4GeV in the event • (no single pions in the event) • cone radius = 0.5 (η-φ space) • “Jet-like” pT >1 GeV/c, xF > 0.2 • 2 perimeter fiducial volume cut (small/large cells) • “Jet shape” in data matches simulation well • Reconstructed mass does not match as well • High-tower trigger used in Run 8 biases jets arXiv:0901.2828

  11. Forward p0-p0 azimuthal correlations • Possible back-to-back di-jet/di-hadron Sivers measurement • Possible near-side hadron correlation for Collins fragmentation function/ • Interference fragmentation function + Transversity • Low-x / gluon saturation study – accessing lowest xBjgluon

  12. Benchmark for Drell-Yan: First look at J/ψ → e+ + e- in the FMS Reconstructed 2-cluster invariant mass • Fit with Gaussian + Offset • Gaussian fit parameters: • μ = 3.080 ± 0.020 GeV/c2 • σ = 0.082 ± 0.026 GeV/c2 • χ2/d.o.f. = 20.83/26 • Significance from the fit • 4.5 σ High-xF, high mass dilepton pairs are difficult to reconstruct • Cuts applied: • E_pair > 60.0 GeV • zγγ < 0.7 • Isolation radius: 0.4 η-φ • pT_cluster > 1.0 GeV/c First high-xFJ/ψmeasurement at √s > 62 GeV arXiv:0907.4396

  13. 2009 – first physics run at √s=500 GeV Luminosity: 56x56 bunches with 1.8x1011 intensity; 10 pb-1 integrated Polarization: ~30% (longitudinal) First look at poevent reconstruction in the FPD: • using matrix+preshower (no SMD data) • 20 GeV < Etotal < 80 GeV • fixed vertex (z=0), no minbias condition • Ng=2 Forward Pion Detector module schematic: Shower Maximum Detector lead converter 7x7 matrix of lead glass cells preshower (7 Pb-glass cells) fit by Gaussian+”gamma” function

  14. 2009 data at √s=500 GeV FPD measures energy up to ~200 GeV ═> SMD information is required to reconstruct pions above ~60 GeV Example of 2-photon event when two clusters significantly overlap in the matrix, but are clearly separated in the SMD

  15. Forward Hadron Calorimeter • Proposed to be staged at a minimal Dz from the FMS symmetrically of DX magnet • two matrices of 9x12 Pb-scintillator detectors • real jets physics with • FMS+FHC • polarization transfer coefficients • through measurement of •  polarization • in the n0 channel first simulations of L→np0

  16. Summary • Large transverse single spin asymmetries at large h are observed up to √s=200 GeV. • High precision inclusive p0AN measurements with the FPD allow for a quantitative comparison with theoretical models. • FMS allows to look at heavier mesons, “jet-like” events and particle correlations. • Essential to go beyond inclusive production to disentangle dynamical origins.

  17. Outlook • Measurements of AN in inclusive p0 production at √s=500 GeV (Run 11?). • Extend forward calorimetry to have hadronic capability (FHC) to measure full jets and to study inclusive L production. • Extend measurements of transverse single spin asymmetries from hadron production to prompt photon and jets. • Develop RHIC experiment for a future measurement of transverse single spin asymmetries for Drell-Yan production of dilepton pairs.

  18. Backups

  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 p0, xF<0 p0, xF>0 p  p Right Single Spin Asymmetry positive AN: more p0 going left to polarized beam

  20. 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: According to the latest theoretical developments, both mechanisms contribute to  AN

  21. Separating Sivers and Collins effects Sivers mechanism:asymmetry in the forward jet or g 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 To discriminate between the two effects we need to go beyond p0 detection todirect photons orjet-like events

  22. p0 inclusivecross section Cross-section is consistent with NLO pQCD calculations PRL 97, 152302 (2006) nucl-ex/0602011

  23. Forward p0 – midrapidity azimuthal correlations • pQCD inspired “GSV cuts” (Guzey, Strikman and Vogelsang, hep-ph/0407201): • |ηTPC| < 0.9 ; 2.8 < ηFMS < 3.8 • 2.5(2.0)GeV < pTFMS • 1.5(1.0)GeV < pTTPC < pTFMS • |zγγFMS|< 0.7, 0.07 < Mγγ < 0.30 GeV • only leading particle considered, corrected for pile-up • as proposed in hep-ex/0502040 • Possible back-to-back di-jet/ di-hadron Sivers measurements • Low-x / gluon saturation study • Step-stone towards transverse spin forward photon-jet arXiv:0907.3473

  24. Future runs at RHIC