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Constraining the polarized gluon PDF in polarized pp collisions at RHIC

Constraining the polarized gluon PDF in polarized pp collisions at RHIC. Frank Ellinghaus University of Colorado (for the PHENIX and STAR Collaborations) July 2008 ICHEP’08, Philadelphia, USA. PDFs fitted using F 2 at Q 2 =4 U=u+c  u, D=d+s  d. Parton Distribution Functions (PDFs).

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Constraining the polarized gluon PDF in polarized pp collisions at RHIC

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  1. Constraining the polarized gluon PDF in polarized pp collisions at RHIC Frank Ellinghaus University of Colorado (for the PHENIX and STAR Collaborations) July 2008 ICHEP’08, Philadelphia, USA

  2. PDFs fitted using F2 at Q2=4 U=u+cu, D=d+sd Parton Distribution Functions (PDFs) • From fits to F2 measurements, unpolarized PDFs can be inferred • q=u,d (s,c) • The total fraction of nucleon momentum carried by quarks: • Gluons carry the other half! U=u+cu, D=d+sd Frank Ellinghaus, University of Colorado

  3. Polarized PDFs extracted from fits to g1(proton, deuteron) Results from Inclusive Polarized DIS • Analogous to unpolarized (F2) case, g1 can be used to fit polarized PDFs: • Result: Quarks carry only about 30 % of the nucleon spin (0.3) • Gluon contribution G not well constrained due to small range in xB,Q2 (no polarized ep collider) …but polarized pp Collider !!! -> Frank Ellinghaus, University of Colorado

  4. pC Polarimeters STAR RHIC @ BNL Polarimeter (H jet) Spin Rotators Siberian Snakes Relativistic Heavy Ion Collider also provides longitudinally and transversely polarized proton beams at s = 200 GeV, 62.4 GeV, (500 GeV, 2009+) Frank Ellinghaus, University of Colorado

  5. PHENIX and STAR High rate capability Limited acceptance Large acceptance Azimuthal symmetry Frank Ellinghaus, University of Colorado

  6. PHENIX longitudinally polarized pp Runs (Similar numbers for STAR. Experiments can separately choose longitudinal or transverse polarization.) Frank Ellinghaus, University of Colorado

  7. Invariant mass spectrum of 2 photons in EMCal (M=135MeV) Measure Relative Luminosity R using beam-beam counters DG via direct measurement Access to polarized gluon distribution function via double helicity asymmetry in inclusive polarized pp scattering, e.g., pQCD, fragmentation fcts. from DIS Frank Ellinghaus, University of Colorado

  8. First: Check unpolarized case! PHENIX -- p0 STAR -- jets PRL 97, 252001 (2006) PRD76:051106,2007 Using a set of unpolarized PDFs ( + fragmentation functions in case of hadron (p0) production) the cross section agrees with NLO pQCD calculations. Frank Ellinghaus, University of Colorado

  9. Run 6 preliminary Run 5 (2005): PRL 100, 23 (2008) DG = G(x), -G(x) excluded; GRSV-std excluded with 99% CL STAR -- jets Run 6 preliminary Frank Ellinghaus, University of Colorado

  10. Run 6 preliminary GRSV: Glueck et al., PRD 63 (2001) DG = G(x), -G(x) excluded; GRSV-std excluded on 3 sigma level: 2(std)2min> 9 PHENIX -- p0 at 200 GeV Run 6 preliminary Run 5: Phys.Rev.D76:051106,2007 Frank Ellinghaus, University of Colorado

  11. PHENIX h excludes DG= G(x), -G(x) so far STAR – p0 and PHENIX - h STAR p0 consistent with PHENIX Frank Ellinghaus, University of Colorado

  12. Fraction of pion production p- STAR -- Run 5 p+ p+, p –, p0 and the sign of DG Especially in the region where qg scattering is dominant (pT > 5 GeV), the increasing contribution of d quarks (Dd<0) leads to: “Model independent” conclusion possible once enough data is available. Frank Ellinghaus, University of Colorado

  13. Measurement averages over certain x range • Shape of DG(x) cannot be extracted -> Value for first moment model dependent • Next step: Mapping of x-dependence via di-jets, di-hadrons and gamma-jet • Different ranges in x can be probed in: • 500 GeV (2009+, lower x) and 62 GeV running (larger x, larger scale unc.) -> • different rapidities -> First pol. PDF extraction using pp data DSSV, arXiv: 0804.0422 First “global” (DIS+SIDIS+pp) analysis! DG small in measured range (0.05 < x< 0.2). Contribution at small or large x? Frank Ellinghaus, University of Colorado

  14. Accessing different x-ranges STAR: p0 ALL at forward rapidity Increased sensitivity to smaller x • Also: step towards gamma-jet • correlation measurements to map • out the x-dependence PHENIX: p0 ALL at s=62.4 GeV Increased sensitivity to larger x Frank Ellinghaus, University of Colorado At fixed xT = 2pT/sqrt(s) cross-section is 2 orders of magnitude higher at 62.4GeV than at 200GeV Significant result at high xT from small data set at 62.4 GeV (0.04 pb-1) when compared to 200 GeV data (1.8pb-1)

  15. Summary • Measurements of double helicity asymmetries in polarized pp scattering at PHENIX and STAR provided a significant constraint on the polarized gluon PDF in a global QCD fit to “all” DIS, SIDIS and pp data • DG(x) small in measured range 0.05 < x < 0.2 • Different beam energies (500 GeV, 2009+) and rapidities will give access to an order of magnitude smaller x • Correlation measurements will provide sensitivity to shape of DG(x) Frank Ellinghaus, University of Colorado

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