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Challenging Nucleon Spin – A Report from the PHENIX Experiment

This report from the PHENIX Experiment investigates the origin of proton spin and explores the role of gluons and orbital angular momentum. The study utilizes a Proton-Proton collider to directly observe gluons and measure their contribution to nucleon spin.

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Challenging Nucleon Spin – A Report from the PHENIX Experiment

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  1. Challenging Nucleon Spin – A Report from the PHENIX Experiment @ XIX International Baldin Seminar on High Energy Physics Problems "Relativistic Nuclear Physics & Quantum Chromodynamics"Dubna, Russia September 30, 2008 Kiyoshi Tanida (Kyoto Univ./RIKEN) for the PHENIX Collaboration

  2. What are we aiming at? • To answer the question: “Where the proton spin comes from?” • Background: Lepton DIS • Quark spin carries only 20-30% of proton spin→ spin puzzle • What carries the rest? • Gluon? • Orbital angular momentum? RHIC spin

  3. What we can’t know from DIS • Photon mediated  sensitive to charge2 • u : d : s : g = 4 : 1 : 1 : 0 • Gluon is invisible!(c.f., indirect methods: Q2 evolution, photon-gluon fusion) • Can we see gluons directly? Yes, what we need is a Proton-Proton collider

  4. What we measure? ~ (parton pol.)2× (aLL in parton reaction)

  5. How can we access gluons? • Typical parton level diagrams （LO） • What we actually measure are not partons, but fragmented hadrons • Come from different mix of partons • Parton information （e.g., Bjorken x） is obscured

  6. Some examples • Direct photon: g + q  g + q • No fragmentation • Small contamination (e.g.`qq  gg)  golden channel • Jet, high-pT hadron production • Mix of all subprocesses • LO  highest statistics  Good measurement with lower luminosity • Heavy quarks (charm, bottom) • gg→`qq is the main process at RHIC • W： sensitive to quark flavors • e.g.,W+ comes from`du

  7. Brhams pp2pp PHENIX STAR The Relativistic Heavy Ion Collider accelerator complex at Brookhaven National Laboratory

  8. RHIC p+p accelerator complex The polarimeters are experimental devices RHIC pC “CNI” polarimeters absolute pH polarimeter BRAHMS & PP2PP PHOBOS RHIC Siberian Snakes PHENIX STAR Siberian Snakes Spin Rotators 5% Snake LINAC BOOSTER AGS pC “CNI” polarimeter Pol. Proton Source AGS Coulomb-Nuclear Interference 200 MeV polarimeter Rf Dipoles 20% Snake

  9. PHENIX Experiment Pioneering High Energy Nuclear Interaction EXperiment

  10. 13 Countries; 62 Institutions; 550 Participants* *as of March 2005

  11. The PHENIX Detector • Philosophy • high resolution & high-rate at the cost of acceptance • trigger for rare events • Central Arms • |h| < 0.35, Df ~ p • g, p0, e, p+-, ... – Identified • Momentum, Energy • Muon Arms • 1.2 < |h| < 2.4 • Momentum (MuTr)

  12. Accumulated data Longitudinal polarization Transverse polarization

  13. Result: p0 ALL • Run6 scaling error based on online polarization values. Final scaling error expected to be ~10% • Grey band is systematic uncertainty due to Relative Luminosity, and is pT independent. • New paper will be submitted next week!

  14. PRD76:051106,2007 How to extract Dg(x)? (1) • p0s come from quarks and gluons of various x Deconvolution necessary • Are we sure that we understand contribution of partons? YES! • NLO-pQCD calculationreproduces s well p0@200 GeV, h~0

  15. How to extract Dg(x)? (2) • Practical analysis • Assume functional form: e.g., Dg(x)=Cg(x)xa(1-x)b • Search optimum parameters using data, including DIS. • Ex： GRSV（M. Gluck et al., PRD 63 (2001) 094005.） • Assume DG, other parameters are determined from DIS. • Several versions for various DG（GRSV-std, max, min, ...） • Several other analyses • For the same integral, DG, Dg(x) could be very different • Our measurement mostly constrains DG[0.02,0.3] • Details are available in our new paper (to be submitted next week)

  16. DG from p0 ALL DG~0?

  17. More results charged pions p0 at 62 GeV higher sensitivity for larger x Same gg contribution in p+ & p-  sign of Dg Also: direct g, h, jet, J/y, e, m, ... no time to show them all

  18. Left π0, xF<0 π0, xF>0 p  p Right Transverse spin physics • Transversity dq: Due to Einstein’s relativity, not the same as Dq • Unexplored leading twist PDF • Seen as AN (via Collins effect) and ATT • AN： left-right asymmetry wrt transverse polarization

  19. Forward pions • Naive pQCD predicts AN ~ mq/sqrt(s) ~ 0 However, large ANobserved in forward pions. WHY?? Proposed mechanisms - Sivers - Collins - Twist 3 - ... Very hot recently

  20. SP SP kT,q p p p p Sq kT,π Possible mechanisms (ex.) Collins mechanism:Transversity (quark polarization) × jet fragmentation asymmetry Sivers mechanism:correlation between proton spin & parton kT Sq Phys Rev D41 (1990) 83; 43 (1991) 261 Nucl Phys B396 (1993) 161 gluon effect? yes no gluon effect

  21. Midrapidity hadron AN AN p0 Run-02 AN h+/- Run-05 |h| < 0.35 hep-ex/0507073 |h| < 0.35 pT (GeV/c) PRL 95, 202001 (2005) pT (GeV/c) • AN is zero within 1% contrast with forward pions • Constrains Sivers distribution function for gluons (Anselmino et al., PRD74, 094011 (2006)) • Updated p0 analysis with >10x smaller stat. error underway.

  22. h1 h2 Quark spin _   quark quark h1 h2 Collins fragmentation function h _   quark quark (courtesy A. Bacchetta) h IFF and Collins FF Interference fragmentation function J. Collins, S.Heppelmann, G. Ladinsky, Nuclear Physics B, 420 (1994) 565 J. C. Collins, Nucl. Phys. B396, (1993) 161 FF measurements are ongoing at KEK-BELLE

  23. Asymmetry result • More results ... again, no time to show them all Still need more data...

  24. Looking forward – More data! • More data • More than 65 pb-1 with 70% polarization at 200 GeV • 500 GeV data taking starts from 2009  small x region • More channels • More detectors • Central arm: VTX • Forward region: MPC, NCC, FVTX, Muon trigger • More physics • W – flavor decomposition of quark polarization • AN – a quest for mechanismAnalysis of data from Run6 & 8 are underway

  25. Summary • Gluon polarization • Current p0 data suggests Dg(x) is small for 0.02<x<0.3 • Extension toward lower x is important detector upgrades, higher energy • sign problem  forward upgrades, direct photon • Transverse spin physics • Trying to find the mechanism to produce large AN in forward region • Access transversity • More data are still to come

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