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

Frank Ellinghaus, University of Colorado Parton Distribution Functions (PDFs) From fits to F 2 measurements, unpolarized PDFs can be inferred –q=u,d (s,c) PDFs fitted using F 2 at Q 2 =4 U=u+c  u, D=d+s  d 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 Results from Inclusive Polarized DIS Analogous to unpolarized (F 2 ) case, g 1 can be used to fit polarized PDFs: Polarized PDFs extracted from fits to g 1 (proton, deuteron) Result: Quarks carry only about 30 % of the nucleon spin (  0.3) Gluon contribution  G not well constrained due to small range in x B,Q 2 (no polarized ep collider) …but polarized pp Collider !!! ->

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

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

Frank Ellinghaus, University of Colorado PHENIX longitudinally polarized pp Runs Year  s [GeV] Recorded LPol [%]FOM (P 4 L) 2003 (Run-3) pb nb (Run-4) pb nb (Run-5) pb nb (Run-6) pb nb (Run-6) pb nb -1 (Similar numbers for STAR. Experiments can separately choose longitudinal or transverse polarization.)

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

Frank Ellinghaus, University of Colorado First: Check unpolarized case! Using a set of unpolarized PDFs ( + fragmentation functions in case of hadron (  0 ) production) the cross section agrees with NLO pQCD calculations. PHENIX --  0 PRD76:051106,2007 STAR -- jets PRL 97, (2006)

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

Frank Ellinghaus, University of Colorado PHENIX --  0 at 200 GeV Run 5: Phys.Rev.D76:051106,2007 Run 6 preliminary GRSV: Glueck et al., PRD 63 (2001)  G = G(x), -G(x) excluded; GRSV-std excluded on 3 sigma level:  2 (std)  2 min > 9 Run 6 preliminary

Frank Ellinghaus, University of Colorado STAR –  0 and PHENIX -  STAR  0 consistent with PHENIX PHENIX  excludes  G  = G(x), -G(x) so far

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

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

Frank Ellinghaus, University of Colorado At fixed x T = 2p T /sqrt(s) cross-section is 2 orders of magnitude higher at 62.4GeV than at 200GeV Significant result at high x T from small data set at 62.4 GeV (0.04 pb-1) when compared to 200 GeV data (1.8pb-1) Accessing different x-ranges PHENIX:  0 A LL at  s=62.4 GeV Increased sensitivity to larger x STAR:  0 A LL at forward rapidity Increased sensitivity to smaller x Also: step towards gamma-jet correlation measurements to map out the x-dependence

Frank Ellinghaus, University of Colorado 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  G(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  G(x)