Unraveling Proton Spin: Insights from the HERMES Experiment

1. The HERMES experiment Gerard van der Steenhoven, 19 September 2004

2. Search the carriers of proton spin • Three possible sources: • quarks: • valence quarks • sea quarks • gluons • orbital momentum • Mathematically: ½ = ½ Sq + DG + Lq EMC (85): q ~ 10% ~ 10% ? ?

3. The experimental strategy • Polarization of the sea quarks • Polarization of the gluons • Orbital angular momentum • Transversity: “switch off the gluons”

4. Outline of the lecture • The origin of proton spin? • Polarization of quarks • Gluon contributions • New developments • Generalized parton distributions → DVCS → Lq,g • Transverse spin → switch off the gluons • Other surprises • The HERMES pentaquark signal • Parton energy loss in nuclei • Outlook

5. How to probe the quark polarization? Polarized deep inelastic electron scattering Measure yield asymmetry: Parallel electron & quark spins Anti-parallel electron & quark spins In the Quark-Parton Model: Spin-dependent Structure Function

6. Why HERMES? • Original purpose (~1990): • Measure inclusive spin structure functions g1(x) for proton & neutron • Measure polarization of u-, d- and sea-quarks separately: qu,d,sea(x) • What came out sofar (~2004)? • Precise data on g1n,p(x), qu,d,sea(x) • First measurements of G(x), DVCS, transversity, parton energy loss,… gluon Quark-antiquark pair → design a reliable multi-purpose detector system !

7. ~1.5m The HERMES experiment EM Calorimeter TRD RICH Magnet Target area e+BEAM Beam Loss Monitor Lambda Wheels

8. The HERMES spectrometer 27.6 GeV e+/e- 0.02 < x < 0.6, 1.0 < Q2 < 15 GeV2 p/p ~ 1-2%,  < 0.6 mrad

9. Data taking since 1996 1996-2000 2002 - 2004

10. Spin-dependent structure functions • The function g1(x): • Evaluate the integrals: • 1999 result: From hyperon decays Total spin carried by quarks

11. Q2 dependence of F2(x) and g1(x) 2 2 + + →Gluons contribute to the nucleon spin !

12. QCD analysis of world data (’03) • Next-to-Leading-Orderanalysis of -data Excellent data forx > 0.01

13. Polarized Parton Densities • First moments: • input scale • pol. singlet density: • pol. gluon density: There must be other sources of angular momentum in the proton

14. Flavour decomposition of spin • Semi-inclusive deep inelastic scattering • Hadron tags flavour of struck quark • Derive purity of tag from unpolarized data Key issue: role of sea quarks in nucleon spin

15. Aerogel p K P C4F10 p K Particle identification • Dual radiator RICH: Detection efficiencies

16. Flavour decomposition: results Polarized Parton Distribution Functions ! Hadron asymmetries (measured) Known quantities (from other data) • The method: • Conclusion: qsea  0

17. Flavour symmetry breaking Unpolarized data:Polarized data: Strong breaking of flavour symmetry No significant breaking of flavour symmetry.

18. Gluon polarization • Photoproduction of high pT–hadron pairs → • Contributing diagrams: • Corresponding asymmetries:

19. Data and plans for G/G • Asymmetry for high-pThadron pairs production: • New high-precision data → ±0.18±0.03

20. Generalized Parton Distributions t p0, r0L, g ... • Consider exclusive processes: • Deeply virtual Compton scatt. • Exclusive vector meson prod. • Collins et al. proved factorization theorem (1997): x+x x-x N N’ Distribution amplitude (meson) final state Hard scattering coefficient (QCD) Generalized Parton Distribution (GPD) (NASTY: x = xBj for quarks and x = -xBj for antiquarks → x  [-1,1])

21. GPDs give access toOrbital Angular Momentum of Quarks The remarkable properties of GPDs • Integration over x gives Proton Form Factors: Dirac Axial vector Pauli Pseudoscalar • The forward limit: • Second moment (X. Ji, PRL 1997):

22. Applying the GPD framework • GPDs enter description of different processes: • Take Fourier transform of leading GPD: AsJq = ½q + Lqinformation onJqgives data onLq. Spatial distribution of quarks in the perpendicular direction

23. A 3D-view of partons in the proton Form Factor Parton Density Gen. Parton Distribution A.V. Belitsky, D. Muller, NP A711 (2002) 118c

24. Experimental access to GPDs • Exclusive meson electroproduction: • Vector mesons (0): • Pseudoscalar mesons (): • Deeply virtual Compton scattering (DVCS): DVCS Bethe-Heitler

25. Key differences Experimental access to DVCS • DVCS observables: • Cross section: • Beam charge asymmetry: • Beam spin asymmetry: • Longitudinal target spin asymmetry:

26. First DVCS results Beam spin asymmetry Beam charge asymmetry

27. What is transversity? transverse quark spin, dS • Three leading order quark distributions: momentum carried by quarks longitudinal quark spin,DS • Gluons don’t contribute toh1(x), while dominant in g1(x): •  Study nucleon spin while switching off the gluons • New QCD tests: Q2evolution h1(x) & dS > DS(lattice)

28. Measuring transversity - + quark flip target flip - + • The relevant diagram: • helicity flip of quark & target • chirally odd process • Consequences: • no gluon contributions…. … & measure single-spin asymmetries:

29. Single – Spin Asymmetries • Sivers effect: AUT driven by orbital motion struck quark: measure L • Collins effect: AUT driven by fragmentation process: measure transversity

30. First data on transversity ‘Collins’: ‘Sivers’: First evidence for non-zero Collins (h1) and Sivers effects (Lq) HERMES, hep-ex/0408013

31. Parton Energy Loss • Energy loss mechanisms: • hadron-nucleon rescattering • quark-gluon propagation (QCD: LPM effect) • Relevance: • Verification novel QCD effect • Study of Quark-Gluon Plasma in relativ. heavy-ion collisions.

32. DIS on heavy nuclei • Hadron attenuation in14N, 84Kr: Data: EPJC 21 (2001) 599 Search for quark-gluon plasma Dashed: X. Wang et al. (2002) [QCD + LPM effect + tune g(x)] Solid: Accardi et al. (2003) [Nincl. Q2 rescaling effects]

33. z x y Energy loss in hot matter • 0 production in Au + Au collisions at Phenix: • Adjust energy loss to fit data (cf. cold matter)

34. New data on hadron attenuation • Cronin effect: • enhancement at highpT2 (rescatt.) • Attenuation for0: Search for quark-gluon plasma

35. Two-hadron attenuation • Evaluate: • Partonic energy loss: R2h→1 • Hadronic energy loss: R2h~ (R1h)2  0.5(Kr) - 0.8(N) Both partonic and hadronic energy loss processes are relevant

36. The HERMES pentaquark signal • Quasi-real photoproduction: e+D Q+ X • Decay mode: Q+ p K0s  p p+p- 27.6 GeV e-beam Invariant mass from identified decay particles deuteron gas target

37. + Invariant mass peak Gauss + 3rd order polynom. • Background: 3rd order polynomial • M=1528  2.6 MeV •  = 8  2 MeV(dominated by exp. resolution) • Significance: • naïve: • realistic:

38. Background below the + Gaussian + resonances + background fit • Background: MC sim-ulation + resonances • MQ = 1527  2.3 MeV •  = 9.2  2 MeV • Significance: • naïve: 6.1  • Realistic: 4.3  Mixed event background Pythia6 background additional *+ resonances (not in Pythia6)

39. nK+ Comparison of pentaquark data • Mean: 1532.52.4 MeV Average of all data: M =1532.5  2.4 MeV Includes latest from - JINR (hep-ex/0403044) - LPI (hep-ex/0404003)

40. Latest HERMES results on + • Require additionalpin + mass spectrum • Impose veto on K*and(1116) Signal/background improves from 1:3  2 :1

41. Summary • HERMES results: • Quark sea → unpolarized • Gluons → polarized // proton • First data on transversity: Quarks carry orbital momentum? • First exploration of GPDs • Partonic energy loss seen • Co-discovery pentaquarks • The future: • End of HERA operations: summer of 2007