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The Spin-Structure of the Nucleon from pp scattering at PHENIX

The Spin-Structure of the Nucleon from pp scattering at PHENIX. Frank Ellinghaus University of Colorado November 2007 SLAC, Stanford, USA. 10 -14 m. 10 -10 m. Nucleon Structure: Early Scattering experiments. Nucleons (protons and neutrons) make up the nucleus:.

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The Spin-Structure of the Nucleon from pp scattering at PHENIX

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  1. The Spin-Structure of the Nucleon from pp scattering at PHENIX Frank Ellinghaus University of Colorado November 2007 SLAC, Stanford, USA

  2. 10-14m 10-10m Nucleon Structure: Early Scattering experiments Nucleons (protons and neutrons) make up the nucleus: Atomic nucleus: Geiger, Marsden, Rutherford 1909 -> Scattering of a-particles off a gold foil Size of nucleon? Point-like? Structure? • Used a nucleus (Helium-4) to discover the nucleus! • Improve: • Use elementary probe, interact only electromagnetically -> electron • Resolve smaller distances -> use higher energies! Frank Ellinghaus, University of Colorado

  3. scattered electron electron recoiling proton proton Electron-Nucleon Scattering q : Momentum transfer Form factor F(q) : difference to scattering from point charge Form factor is fourier transform of charge density distribution Frank Ellinghaus, University of Colorado

  4. Structure of the nucleon in the ‘50s Electron beam at Stanford linear accelerator (Hofstadter, 1955) With increasing resolution (“high” electron energy), there is deviation from the point charge prediction! What’s inside? Frank Ellinghaus, University of Colorado

  5. scattered electron electron photon proton debris proton Deep-Inelastic Scattering (DIS) Crank up the electron energy once more: Inclusive DIS • Detect only scattered lepton • 2 degrees of freedom • E’,   xB,Q2 • xB: Fraction of the (fast moving) nucleon momentum carried by quark • Q2=-q2 Cross section for inclusive DIS: F2 parametrizes the unknown nucleon structure Frank Ellinghaus, University of Colorado

  6. The structure function F2 Experiments started in the late 1960s at SLAC: (Friedman, Kendall, Taylor et al.) Electron beam: 7-17 GeV Q2: measure of spatial resolution; scale ~1/Q for Q2 = 1 GeV2, l ~ 0.2 fm F2 does not depend on the resolution (Q2) Nucleon is made of point-like particles (partons) Frank Ellinghaus, University of Colorado

  7. “Parton Model”: F2 depends on the probability of hitting a quark with momentum fraction xB F2 depends (weakly) on Q2? At higher resolution ( higher Q2) we “see” the gluons PDFs fitted using F2 at Q2=4 U=u+cu, D=d+sd The structure Function F2 : A closer look q(xB) momentum distribution of quarks in the nucleon (-> Parton Distribution Function, PDF) Frank Ellinghaus, University of Colorado

  8. 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

  9. Spin of the nucleon is 1/2(and quark spin is ½) Quark spins Gluon spins Orbital angular momenta of quarks and gluons What about the Spin of the Nucleon? • NO, not that easy! • It’s a composite object made out of quarks • (spin ½) and gluons (spin 1) • How do their spins add up to the nucleon spin? Frank Ellinghaus, University of Colorado

  10. e-p Spins aligned e-p Spins antialigned How can we measure the nucleon spin structure? • Polarize electrons and nucleons; Experiments started in mid 1970s at SLAC (E-80, E-130, V.W. Hughes, C.Y. Prescott et al.) • Electron polarization transfers to virtual photons • Compare DIS cross sections with aligned and antialigned ep spins ~ g1 (proton) > 0 -> Larger cross section for anti-aligned ep Spins -> Higher probability for aligned quark-proton Spins G. Baum et al, PRL 51, 1983 Frank Ellinghaus, University of Colorado

  11. 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

  12. STAR RHIC @ BNL Relativistic Heavy Ion Collider also provides longitudinally and transversely polarized proton beams at s = 200 GeV, 62.4 GeV, (500 GeV, 2008+) Frank Ellinghaus, University of Colorado

  13. PHENIX Detector Frank Ellinghaus, University of Colorado

  14. The PHENIX Detector for Spin Physics Central Detector: • g/p0/h detection • Electromagnetic Calorimeter • p+/p- • Drift Chamber • Ring Imaging Cherenkov Detector Muon Arms: • J/y • Muon ID/Muon Tracker (m+m-) • p0 • Electromagnetic Calorimeter (MPC) Global Detectors: • Relative Luminosity • Beam-Beam Counter (BBC) • Zero-Degree Calorimeter (ZDC) • Local Polarimetry -ZDC Frank Ellinghaus, University of Colorado

  15. PHENIX longitudinally polarized pp Runs * Online value! Frank Ellinghaus, University of Colorado

  16. Cross section - pQCD applicability RUN5 200GeV -- p0 RUN6 62.4GeV -- p0 PRD76:051106,2007 Using a set of unpolarized PDFs and fragmentation functions the cross section can be compared to NLO pQCD calculations => pQCD seems at work, with large scale uncertainties at 62 GeV! Frank Ellinghaus, University of Colorado

  17. 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: pQCD, fragmentation fcts. from DIS Frank Ellinghaus, University of Colorado

  18. p0 ALL at 200GeV contd. Run 5: Phys.Rev.D76:051106,2007 GRSV: Glueck et al., PRD 63 (2001) DG=G(x),-G(x) excluded; consistent with zero in this model -> Frank Ellinghaus, University of Colorado

  19. Model dependence of DG • Measurement averages over certain x range • Shape of DG(x) cannot be extracted -> Value for first moment model dependent • Different ranges in x can be probed in 500 GeV (2009+) running (lower x) and • in running at 62 GeV (larger x, also larger scale uncertainties) -> Frank Ellinghaus, University of Colorado

  20. p0 ALL at s=62.4 GeV Scaling variable: Frank Ellinghaus, University of Colorado • At fixed xT, cross-section is 2 orders of magnitude higher at 62.4GeV than at 200GeV • Probe different x scales by using different xT • Significant result at high xT from small data set at 62.4 GeV (0.04 pb-1) when compared to 200 GeV data set (1.8pb-1)

  21. Fraction of pion production p+, p –, p0 and the sign of DG Especially in the region where qg scattering is dominant, the increasing contribution of d quarks leads to: • Charged pions begin firing the RICH at pT~4.7 GeV, which is used for particle ID • Charged pion result from Run 5 only, using Run 6 data stat. error will decrease by ~2.5 Frank Ellinghaus, University of Colorado

  22. PHENIX Run-05 Preliminary 200 GeV RUN5 200GeV – h • No eta fragmentation functions (FFs) in the literature! • First extraction of FFs from e+e- data and this (large range in pT) pp result ( gluon FFs) performed (method/code: de Florian, Sassot, Stratmann, PRD75, 2007) • ALL can now be compared to theory, RUN-6 result in a few days! • Parameterization of flavor separated FFs (comparison to p0 ALL might yield info on Ds) needs further data from semi-inclusive DIS. • Precision measurements from B factories very helpful too….. (precise data only at Mz -> small lever arm in s) Frank Ellinghaus, University of Colorado

  23. q g q Direct Photons at s=200 GeV Dominated by qg Compton: -> small unc. from FFs -> better access to sign of DG (Dq times DG) Run-5 Theoretically clean “Golden Channel” is luminosity hungry… At the end of the day all these (and the DIS, SIDIS) asymmetry data need to go into a “global” QCD fit in order to extract DG! Frank Ellinghaus, University of Colorado

  24. Summary longitudinal Spin Structure Quarks carry only about 30% (EMC 1988, ….) of the nucleon spin Indirect measurements via fits to inclusive DIS data (small lever arm in x, ) and direct measurements by HERMES, COMPASS, SMC (small -> large scale uncertainties), and by PHENIX, STAR: Gluon contribution small in measured range Contributions unknown: Only known quantitative way via “measurements” (DVCS etc.) of GPDs (Ji, 97) by H1, ZEUS, HERMES, JLab Qualitative: Sivers Function Frank Ellinghaus, University of Colorado

  25. Transversity and friends Transversity: The 3rd Twist-2 structure function…fundamental! unpolarised quarks and nucleons q(x): spin averaged longitudinally polarized quarks and nucleons Dq(x): helicity difference transversely polarized quarks and nucleons dq(x): helicity flip • Friends: • Collins fragmentation function-> spin-dependent fragmentation of the • transversely polarized quarks, teams up with transversity • Sivers distribution function -> transverse momentum distribution of • unpolarized quarks in transversely polarized nucleon • …… Sizeable asymmetries seen in ep (HERMES) and e+e- (Belle) and especially in pp! Frank Ellinghaus, University of Colorado

  26. Single Transverse Spin Asymmetries in pp Large Single Spin Asymmetries in pp scattering at large xF at FERMILAB E704 sustain to = 200 GeV at STAR (Sivers?, Collins?, higher twist, ….) AN at xF ~ 0 (and small pT) is consistent with zero s=19.4 GeV, pT=0.5-2.0 GeV/c s=200 GeV Frank Ellinghaus, University of Colorado

  27. PRL 95, 202001 (2005) p+p0+X at s=200 GeV/c2 AN : h+/h- AN of mid-rapidity p0 and h±at Ös=200 GeV PLB 603,173 (2004) process contribution to 0, =0, s=200 GeV • AN consistent with zero at mid-rapidity and small pT (see E704). • Mid-rapidity data at small pT sensitive to gluons, constrains magnitude of gluon Sivers function (Anselmino et al., PRD 74, 2006) • What happens if qq sets in (valence quarks) at high pT? Frank Ellinghaus, University of Colorado

  28. array of ≈ 220 PbWO4 crystals The Muon Piston Calorimeter (MPC) Large xF region for p0 can now be explored at PHENIX with recently installed forward calorimeters (Muon Piston calorimeter, MPC) APD Holder PbWO4 Crystal PreAmp Frank Ellinghaus, University of Colorado

  29. First result… MPC (south) commissioned during 200 GeV transverse running in RUN 6 (early 2006) MPC ready in time for a few days of transverse data taking at 62.4 GeV s=62.4 GeV s=19.4 GeV, pT=0.5-2.0 GeV/c Very promising result from only a few days (!) of data taking. North MPC installed recently. Frank Ellinghaus, University of Colorado

  30. Quark Sivers = 0 Gluon Sivers = Max Quark Sivers = Max Gluon Sivers = 0 AN of J/yat Ös=200 GeV • No theoretical predictions for J/ production yet • Asymmetry sensitive to sivers function in open charm production (Anselmino et al., PRD 70 2004) • What can measurements in J/ production tell us? How problematic is it that the production mechanism is not well known? Need help from theory….. Frank Ellinghaus, University of Colorado

  31. Future: Flavor separation of Dq and Dq W production: • Projections for RHIC assume that ∫Ldt ~980pb-1 can be delivered at √s=500 GeV between 2009 and 2012-> 300 pb-1 recorded. • Smearing for muons not taken into account. Requires high luminosity at 500 GeV and a muon trigger upgrade (under construction). Frank Ellinghaus, University of Colorado

  32. Summary • Using longitudinally polarized pp scattering PHENIX adds a dataset especially sensitive to the polarized gluon PDF for a global QCD fit to “all” DIS, SIDIS and pp data • Measurements of transverse single spin asymmetries at forward and mid-rapidity serve as a valuable data set in the investigation of the transverse spin puzzle • Future efforts are focused on the measurement of W at 500GeV (2009+) in order to investigate the flavor separated polarized quark PDFs Frank Ellinghaus, University of Colorado

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