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Nucleon Spin Structure 30 Years of Experiment: What have we learned?

Nucleon Spin Structure 30 Years of Experiment: What have we learned?. M. Grosse Perdekamp, University of Illinois and RBRC. Overview. Scientific Motivation and Early Beginnings The Rabi School of Physics The SLAC – Bielefeld -- Tsukuba – Yale Collaboration

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Nucleon Spin Structure 30 Years of Experiment: What have we learned?

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  1. Nucleon Spin Structure30 Years of Experiment: What have we learned? M. Grosse Perdekamp, University of Illinois and RBRC

  2. Overview • Scientific Motivation and Early Beginnings • The Rabi School of Physics • The SLAC – Bielefeld -- Tsukuba – Yale Collaboration • Modern Experiments • Nucleon Helicity Structure • Quark spin ΔΣ • Gluon spin ΔG • Orbital angular momentum Lz  GPDs ! • Transverse Spin • Transverse spin in hard scattering QCD • Transversity and Collins Quark Fragmentation • The Sivers Effect Nucleon Spin Structure

  3. Scientific Motivation: Proton Structure Including Spin Degrees of Freedom Constituents: quarks = u, d, s and gluons Nucleon Spin Structure

  4. Proton Spin Structure from Inclusive Deep Inelastic Lepton-Nucleon Scattering spin Large Q2: measure photon-quark absorption cross section double spin asymmetry spin electron or muon probe proton target Extract spin dependent quark distribution functions from the spin structure function g1(x,Q2) Nucleon Spin Structure

  5. The Rabi School of Physics N. F. Ramsey, Eur. J. Phys. 11 (1990) 137 J. Rigden, Physics World, Nov. 1999 (I) Molecular beam laboratory at Columbia University with strong emphasize on the development of new experimental technology. (II) Field new, precise instrumentation to study fundamental questions of physics. Example: Precision Measurements of “Hydrogen Spin Structure” g-2 of the electron, P. Kusch Lamb shift, W. E. Lamb Dirac Theory  QED Tomonaga, Schwinger, Feynman Rabi, Nobel Prize 1944 Nobel Prize 1955 Nobel Prize 1965 Nucleon Spin Structure

  6. SLAC: Quark Structure of the Proton New instrumental method & fundamental physics ! Experiment: Deep inelastic electron nucleon scattering Friedman, Kendall, Taylor Nobel Prize 1990 Quantum Chromo Dynamics Nucleon Gell Mann Nobel Prize 1969 also Nakano, Nishijima Theory: quark structure of hadrons, QCD Nucleon Spin Structure

  7. Polarized Deep Inelastic Scattering a contribution from the Rabi School of Physics ! • Molecular beam technology as starting point • for the development of polarized electron • beams at Yale starting 1959. • Physics: • (a) Proton spin structure • (b) Test the Bjorken sum rule as • fundamental QCD prediction • Experiments E80+E130 at SLAC • Bielefeld – CUNY – SLAC – • Nagoya – Tsukuba – Yale • (Coward, Kondo, Hughes) • EMC experiment at CERN • (Gabathuler, Sloan, Hughes) Vernon W. Hughes Nucleon Spin Structure

  8. The Quark Spin Contribution ΔΣ Quark Spin Contribution to the Proton Spin. SLAC: 0.10 < xSLAC <0.7 CERN: 0.01 < xCERN <0.5 First Thesis on Nucleon Spin Structure E80/Yale, 1977: Noboru Sasao 0.1 < xSLAC < 0.7 A1(x) EMC, Phys.Lett.B206:364,1988 1338 citations in SPIRES 0.01 < xCERN < 0.5 “Proton Spin Crisis” x-Bjorken Nucleon Spin Structure

  9. Nucleon Spin Structure: 30 Years of Experiment Quark Spin – Gluon Spin – Transverse Spin – GPDs – Lz SLACE80-E155 CERN EMC,SMC COMPASS FNAL E704 DESY HERMES JLAB Halls A, B, C RHIC BRAHMS, PHENIX, STAR 2000 ongoing 1995 2007 ongoing ongoing major experimental innovations semi inclusive + exclusive processes, luminosity DIS polarized proton beams, polarized proton collider polarized pp Nucleon Spin Structure

  10. A novel experimental method:Probing Proton Spin Structure in High Energy Polarized Proton Collisions Instrumentation High current polarized proton source High energy proton polarimetry Control of spin coherence during acceleration + storage Spin sorted luminosity measurements Physics Probes directly sensitive to color charge Utilize Parity violation in W-production Large Q2  clean pQCD interpretation RHIC pC Polarimeters Absolute Polarimeter (H jet) Siberian Snakes BRAHMS & PP2PP PHOBOS RHIC Spin Instrumentation Development 1995-2005 Siberian Snakes Spin Flipper PHENIX STAR Spin Rotators Helical Partial Snake Partial Snake Strong Snake Polarized Source US-Japanese collaboration at Brookhaven National Laboratory RIKEN Radiation Laboratory RIKEN BNL Research Center LINAC AGS BOOSTER 200 MeV Polarimeter Rf Dipole AGS pC Polarimeter Nucleon Spin Structure

  11. RHIC SPIN: Proton Structure with Quark and Gluon Probes at ultra-relativistic energies the proton represents a jet of quark and gluon probes For example, direct photon production ~ probe gluon content with quark probes The related double spin asymmetry: experimental double spin asymmetry quark photon quark gluon DIS pQCD ? Nucleon Spin Structure

  12. Nucleon Helicity Structure Quark spin ΔΣ , Δq(x) Gluon spin ΔG(x), ∫ ΔG(x)dx Orbital angular momentum Lz GPDs ?

  13. Inclusive Measurements of g1p, g1d and g1n S. Paul, X. Lu, H. Gao, INPC 2007 Proton Neutron • HERMES • ΔƩ = 0.33 ± 0.011(th) ± 0.025 (exp) ± 0.028 (evo) at 5 GeV2 • COMPASS • ΔƩ= 0.35 ± 0.03 (stat) ± 0.05 (syst) at 3 GeV2 • Bjorken sum 0.1821 ∓ 0.0019 (NNNLO) Nucleon Spin Structure

  14. Semi-Inclusive DIS: e+p  e+ h +X Quark & Anti-Quark Helicity Distributions [HERMES, PRL92(2004), PRD71(2005)] xΔu(x) Future: Precision DIS at JLAB-12 and at a possible electron – ion collider! u quarks large positive polarizationd quark have negative polarizationsea quarks (u, d, s ,s) compatible with 0in the measured x-range 0.02 < x < 0.6. How well do we know hadron fragmentation functions ?  new analysis of e+e- data, Hirai, Kumano, Nagai, Sudo hep-ph/0612009, INPC 2007 xΔd(x) xΔu(x) Possible Improvements  include e-p, p-p and e+e- in fragmentation function analysis  done! De Florian, Sassot, Stratmann hep-ph/0703242 xΔd(x) xΔs(x)  “add data” from b-factories e+e-  hadrons x Nucleon Spin Structure

  15. Possible Impact on the Knowledge of Hadron FFs from Analysis of b-Factory Data Belle MC: Charged h+/-, pions, kaons, protons Compilation of data available for the char- ged hadron FF FF • h+,- • pions • kaons • protons Belle MC FF Input for precision measurements of quark helicity distributions in SIDIS, with JLab-12 and a possible future electron- polarized proton collider. <1% of data sample work in progress precision at high z! Nucleon Spin Structure

  16. Another Alternative: W-production at RHIC Hermes – 243 pb-1 SIDIS: large x-coverage uncertainties from knowing fragmentation functions Ws in polarized p-p: limited x-coverage high Q2  theoretically clean no FF-info needed PHENIX – 800 pb-1 Nucleon Spin Structure

  17. Gluon Spin Contribution ΔG(x) from scaling violation of world g1(x,Q2): gP1(x,Q2) Hirai, Kumano, Saito Phys.Rev.D74:014015,2006 ΔG=∫ΔG(x) dx = 0.47 ∓ 1.08 , Q2=1GeV2 Nucleon Spin Structure

  18. Gluon Polarization from Photon Gluon Fusion in DIS S. Paul, X. Lu INPC 2007 “direct” measurements • golden channel: charm production • hadron production at high PT Photon-Gluon Fusion(PGF) Favors small ΔG(x≈0.1) Nucleon Spin Structure

  19. K.Aoki, R. Fatemi, B. Surrow INPC 2007 ALL also for p+ , p- , J/Y Gluon Polarization from Inclusive Hadrons and Jets in Polarized pp 2005 data Nucleon Spin Structure

  20. Gluon Polarization from Inclusive Hadrons and Jets in Polarized pp  2006: 7.5 pb-1 @ 60% polarisation projections Nucleon Spin Structure

  21. NLO QCD Analysis of DIS A1 + ALL(π0) Hirai, Kumano, Saito Phys.Rev.D74:014015,2006 DIS A1 + ALL(π0) ACC03 x DIS + pp ∫ΔG(x) dx = 0.31 ∓ 0.32 , Q2=1GeV2 Only DIS ∫ΔG(x) dx = 0.47 ∓ 1.08 , Q2=1GeV2 Nucleon Spin Structure

  22. PHENIX π0ALL vs GSA-LO and GSC-NLO ALL PHENIX-2005 GSA-LO: ΔG = ∫ΔG(x)dx = 1.7 GSA-LO and GSC-NLO courtesy Marco Stratmann and Werner Vogelsang GSC-NLO: ΔG = ∫ΔG(x)dx = 1.0 GSA-LO Large uncertainties resulting from the functional form used for ΔG(x) in the QCD analysis! GSC-NLO pT[GeV] Nucleon Spin Structure

  23. ΔG(x) A, B and C from Gehrmann Stirling Much of the first moment ΔG = ∫ΔG(x)dx might emerge from low x! Some theoretical guidance: ΔG(x) ≤x G(x) but G(X) diverges faster than x-1 ! NEED TO EXTEND MEASUREMENTS TO LOW x !! present x-range Nucleon Spin Structure

  24. Next Steps for ΔG(x) at RHIC Increase integrated luminosity by factor 10 (2008) Extend measurements to low x  Di-hadron Production extends (2008) measurements to x  0.01 NLO treatment available: Marco Stratmann -- INPC 2007 (EMC forward calorimeters available in STAR and PHENIX!) Forward detector upgrades for direct (2011) photons and heavy flavor + electron cooling reach x  0.001 Polarized Electron Ion Collider measure ΔG(x) through scaling violations Nucleon Spin Structure

  25. Generalized Parton Distributions vs Orbital Angular Momentum ? GPDs Hu, Hd, Eu, Edprovide access to total quark contribution to proton angular momentum in exclusive processes l + N  l’ + N + γ ½ = ½ (Du+Dd+Ds) + Lq + Jg Proton spin sum J q 1 1 [ ] ò x + x xdx H q( x , , 0 ) E q( x , , 0 ) = J q 2 - 1 X. Ji, Phy.Rev.Lett.78,610(1997) Nucleon Spin Structure

  26. FirstModel Dependent Constraint of Ju vs Jd E. Burtin, P. Bertin, X. Lu, INPC 2007 Nucleon Spin Structure

  27. Transverse Spin Transverse spin in hard scattering QCD Transversity and Collins Quark Fragmentation The Sivers Effect

  28. Transverse Spin Phenomena in Hard Scattering QCD QCD: Asymmetries for transverse spin are small at high energies (Kane, Pumplin, Repko, PRL 41, 1689–1692 (1978) ) Experiment (E704, Fermi National Laboratory): π+ QCD Test ! π0 π- Is QCD the correct theory of the strong interaction? Nucleon Spin Structure

  29. Single Transverse Spin Asymmetries AN at √=62.4 GeV and 200 GeV STAR M. Chiu INPC 2007 √s=200 GeV STAR √s=62.4 PHENIX and BRAHMS AN AN xF Large single spin asymmetries persist at higher √s=62.4 and 200 GeV xF Nucleon Spin Structure

  30. Inspect Factorized Expression for Cross Section fragmentation process Proton Structure Can initial and/or final state effects generate large transverse spin asymmetries? (ALL ~10-1) Jet hard scattering reaction pQCD Proton Structure small spin dependence (aLL~10-4) fragmentation function Nucleon Spin Structure

  31. Transverse Spin in QCD: Two Solutions (I) “Transversity” quark-distributions and Collins fragmentation Correlation between proton- und quark-spin and spin dependent fragmentation AN π+ π0 Quark transverse spin distribution Collins FF π- (II) Sivers quark-distribution Correlation between proton-spin and transverse quark momentum xF Sivers distribution Nucleon Spin Structure

  32. NL- NR AN= = 0 NL+ NR q π sq sq q q π Collins Effect in the Quark- fragmentation into the Final State NR : pions to the right Collins Effect NL : pions to the left Collins Effect: Fragmentation of a transversely polarized quark q into spin-less hadron h carries an azimuthal dependence: Nucleon Spin Structure

  33. Artru Model for Collins Fragmentation A simple model to illustrate that spin-orbital angular momentum coupling can lead to left right asymmetries in spin-dependent fragmentation: String breaks and a dd-pair with spin 1 is inserted. Proton spin is pointing up! L = -1 π+ picks up L=-1 to compensate for the pair S=1 and is emitted up. u-quark absorbs photon/gluon and flips it’s Spin. Nucleon Spin Structure

  34. Measurements of Quark Transversity Distributions and Collins Fragmentation Functions (I) SIDIS New HERMES results for Collins Asymmetries Diefenthaler, DIS 2007, Lu INPC 2007 Collins Asymmetries in semi- inclusive deep inelastic scattering e+p  e + π + X ~ Transversity (x) x Collins(z) AUT sin(f+fs) Nucleon Spin Structure

  35. Measurements of Quark Transversity Distributions and Collins Fragmentation Functions (II) e+e- New Belle Collins Asymmetries Seidl, DIS 2007 Collins Asymmetries in e+e- annihilation into hadrons e++e- π++ π- + X ~ Collins(z1) x Collins (z2) PRELIMINARY j2-p e- Q j1 e+ A12 cos(f1+f2) Nucleon Spin Structure

  36. First Extraction of Quark Transversity Distributions and Collins Fragmentation Functions SIDIS + e+e- Fit includes: HERMES SIDIS + COMPASS SIDIS + Belle e+e- transversity dist. + Collins FF Anselmino, Boglione, D’Alesio, Kotzinian, Murgia, Prokudin, Turk Phys. Rev. D75:05032,2007 Nucleon Spin Structure

  37. The Sivers Effect Sivers: Correlation between the transverse spin of the proton and the transverse momentum kT of quarks and gluons in the proton (link to orbital angular momentum?) Sp Sivers function: proton D. Sivers 1990 proton Sp Observed asymmetry: Nucleon Spin Structure

  38. Sivers Asymmetries at HERMES and COMPASS • implies non-zero Lq p+/- K+/- Nucleon Spin Structure

  39. Sivers Effect and Orbital Angular Momentum M. Burkardt > Nucleon Spin Structure

  40. The Sivers Effect : Needs Final State Soft Gluon Exchange M. Burkardt Nucleon Spin Structure

  41. What have we learned from this? The Sivers effect arises from soft gluon interactions in the final state (SIDIS) or initial state (Drell Yan). Need to modify naïve concepts of factorization which reduce hard scattering to partonic processes and neglect soft gluon interactions in the initial or final state: hard scattering matrix elements are modified with gauge link integrals that account for initial and final state soft gluon exchange. A modified concept of universality has been obtained which shows how the presence of initial or final state interactions can impact transverse momentum dependent distribution; eg. the Sivers function changes sign between SIDS and Drell Yan! There may be exciting applications elsewhere, eg. other transverse momentum dependent effects or the understanding nuclear effects in hard scattering. Nucleon Spin Structure

  42. Goals for the Future Quantitative understanding of transverse spin phenomena in QCD Do Sivers and Collins mechanisms reconcile QCD with transverse spin phenomena? Precision measurements of transversity distributions and Collins fragmentation function measurements. This will complete the experimental survey of the nucleon at leading twist. Determine sum of first moments (tensor charge) which can be compared to lattice calculations. Survey Sivers and Boer Mulders effects in SIDIS and pp Fundamental understanding of factorization and universality in hard scattering. Relation to orbital angular momentum ?! Future results expected from COMPASS, RHIC, JLAB, Belle, JLAB-12-GEV, JPARC FAIR and EIC. This includes high precision measurements in e-p, e-e and p-p possibly first systematic study of factorization + universality Nucleon Spin Structure

  43. Transversity, Sivers and Boer Muldersin the Proton Wavefunction Transversity : correlation between transverse proton spin and quark spin Sivers : correlation between transverse proton spin and quark transverse momentum Boer/Mulders: correlation between transverse quark spin and quark transverse momentum Sp– Sq – coupling ? Sp- Lq– coupling ?? Sq- Lq– coupling ?? Nucleon Spin Structure

  44. Summary • Bjorken sum rule holds • Integral quark spin contributions are well known • Δq(x), Δq(x) only well known for up-quarks only • Hints that ΔG(x) is small at x~0.1. ∫ΔG(x)dxremains • largely unconstraint  RHIC luminosity, low-x • Possible route to OAM through • Exp. Observation of Sivers and Collins asymmetries • Theoretical advance in understanding TMD + concepts of factorization and universality • Plenty of work for theory + existing and future experimental tools! Nucleon Spin Structure

  45. Sivers in SIDIS and Drell Yan vs Factorization and Universality Nucleon Spin Structure

  46. Transverse Spin Drell Yan at RHIC vsπ-SiversAsymmetry in Deep Inelastic Scattering Important test at RHIC of the fundamental QCD prediction of thenon-universality of the Sivers effect! requires very high luminosity (~ 250pb-1) Nucleon Spin Structure

  47. Non-universality of Sivers Asymmetries: Unique Prediction of Gauge Theory ! Simple QED example: Drell-Yan: repulsive DIS: attractive Same inQCD: As a result: Nucleon Spin Structure

  48. Experiment SIDIS vs Drell Yan: Sivers|DIS= − Sivers|DY *** TestQCD Prediction of Non-Universality *** HERMES Sivers Results RHIC II Drell Yan Projections 0 Sivers Amplitude Markus Diefenthaler DIS Workshop Műnchen, April 2007 0 0.1 0.2 0.3 x Nucleon Spin Structure

  49. Is pQCD applicable at RHIC? • Can one extract G(x,Q2) from pp? • NLO pQCD vs RHIC data Nucleon Spin Structure

  50. Global QCD Analysis for G(x,Q2)and q(x,Q2): J. Pumplin et.al JEHP 0207:012 (2002) CTEQ6: use DGLAP Q2-evolution of quark and gluon distributions to extract q(x,Q2) and G(x,Q2) from global fit to data sets at different scales Q2. error on G(x,Q2) +/- 10% Quark and Gluon Distributions H1 + Zeus F2 CTEQ6M up-quarks CDF + D0 Jets gluon CTEQ5M1 10-410-3 10-2 10-1 0.5 x error for d(x,Q2) error for u(x,Q2) down anti-down +/- 5% +/- 5% 10-410-3 10-2 10-1 0.5 x Nucleon Spin Structure

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