The quest for the proton spin with the polarized proton collider RHIC

1. The quest for the proton spin with the polarized proton collider RHIC Kensuke Okada RIKEN BNL Research Center April 19, 2012 K.Okada

2. Outline • What is “proton spin”? • The spin program at RHIC • PHENIX experiment • Results • G • q • Transverse spin • Next steps • Summary K.Okada

3. What is the origin of the matter? 10-7 cm • Water molecule • Atom • Nucleus • Proton • Quark • Quark is an elementary particle (so far) • Quark doesn’t exist by stand-alone in the world. 10-8 cm 10-12 cm 10-13 cm 0 (<10-18 cm) K.Okada

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5. Friends of proton (hadron) • Quarks form a hadron. • Proton, neutron, and other short-lived particles. u quark u quark u quark u quark d quark d quark u quark d quark Neutral  particle neutron Proton K.Okada

6. How to see inside (Scattering)  particle • Rutherford scattering, 1911 • Shot  particle to gold atoms (actually it is He atom) Gold atom 1. Cloudlike structure or 2. Positive charge core structure K.Okada

7. Inside the proton u quark ex) Electron d quark u quark The proton may break. proton K.Okada

8. Proton is more complicated From electron scattering experiments, proton u quark u quark d quark u quark Gluon d quark u quark Anti-quark With high resolution K.Okada

9. “Spin” of proton • Some say it’s like a rotation. (the intrinsic angular momentum) • Size: 1/2 (with a unit of h=h/2) • It is 2 directions (+/-) along the axis It is already used. （ex. MRI） K.Okada

10. What builds spin ½? Contribution of quark spin to the proton spin Is only ~25%. • （In late 1980’s）e+p, +p scattering in SLAC/CERN/DESY/JLAB u quark Spin crisis •  Spin puzzle d quark u quark u quark d quark u quark proton spin? K.Okada

11. How about gluons? (RHIC) Proton spin＝ 　（quark spin）＋（gluon spin）＋（angular momentum） ~25% Direct p+p scattering proton  or  gluon photon  or  proton K.Okada

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13. Realization of polarized p+p collider BRAHMS(p) Absolute Polarimeter (H jet) RHIC pC Polarimeters Siberian Snakes PHENIX (p) STAR (p) Spin Rotators (longitudinal polarization) Spin Rotators (longitudinal polarization) Solenoid Partial Siberian Snake LINAC BOOSTER Helical Partial Siberian Snake Pol. H- Source AGS 200 MeV Polarimeter AGS Polarimeters • Keep the original polarization • Polarimeter K.Okada

14. Relativistic Heavy Ion Collider? • 1991RHICconstruction • 1993Spin program approval • 1995 RIKEN-BNL agreement. Start the special magnet construction, polarimeter development. • 2002First polarized proton proton collision (s=200GeV) • 2009Start s=500GeV spin program I joinedPHENIX in 2001 K.Okada

15. s=200 s=500 K.Okada

16. Summary of the introduction • Proton is made of quarks, anti-quarks, and gluons. • Contributions of quarks and anti-quarks are small to the proton spin. • RHIC is a strong tool to the proton spin puzzle. K.Okada

17. G, quark flavor dependence, transverse spin (A new tool) RHIC spin program (3 topics) K.Okada

18. 1．Gluon spin polarization proton pion quark q(x1) Dq(z) gluon σ proton g(x2) from DIS from pQCD from Anti-parallel Parallel Large gluon polarization  Large double spin asymmetry K.Okada

19. 2．Quark polarization (flavor dependent) via W boson production • With RHIC top energy (s=500GeV), we can use very heavy W bosons as a tool. K.Okada

20. Quark flavor u • W+: from up and anti-down quark（ud） • W-: from anti-up and down quark （ud） • The charge determines the quark pair. Proton d Proton K.Okada

21. Quark polarization • “Weak interaction” has the maximum parity violation. It means the quark spin direction is fixed. W+ Yes d u W+ No K.Okada

22. Proton spin dependence proton • The single spin asymmetry (AL) of the W+ production is sensitive to the anti-down quark polarization. • Others rely on the knowledge of fragmentation. Anti-down quark d d W+ W+ u u electron + g* d proton K.Okada

23. 3．A new way to test QCD (the theory of strong interaction) Transverse polarization, single spin asymmetry • It is related to the higher order pQCD effect. • Intuitively, it is a transverse motion. • It has to be related to the orbital momentum. • It is a hot topic in both experiment and theory. K.Okada

24. Experiment K.Okada

25. Experimental essence • Spin dependence of the interaction • Production rate • Interaction rate (=Luminosity) Each bunch is assigned different polarization direction. It reduces the systematics of detector time dependence. *) At minimum, relative rate is required. K.Okada

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27. PHENIX detector proton proton Central arms + Forward muon arms K.Okada

28. Interaction rate (=Luminosity) • From the fragments of nuclear interaction. forward proton center forward proton K.Okada

29. (Ex.1) Production rate measurement • Electromagnetic calorimeter • Detection of 2 photons from 0 particle decay (most abundant process) K.Okada

30. (Ex.2) Production rate measurement • Electromagnetic calorimeter + Tracking (Drift chamber/Pad chamber) • Detection of electron from W-boson decay. K.Okada

31. G, quark flavor dependence, transverse spin Results K.Okada

32. Gluon polarization is small • From the measurement of double spin asymmetry (ALL) of 0 production PRL(2004) pi0 PRL(2009)pi0 pT: Transverse momentum of 0 K.Okada

33. Gluon polarization is small • A large polarization model is excluded. PRL(2004) pi0 PRL(2009) pi0 pT: Transverse momentum of 0 K.Okada

34. A node in g? From a global analysis (DSSV group) PHENIX + STAR + SIDIS + DIS gis smaller than GRSV-std[g = 0.4 at Q2 = 1 (GeV/c)2] RHICdata are effective for g(x)in 0.05 < x < 0.2region. de Florian, Sassot, Stratmann and Vogelsang, PRL 101, 072001 (2008). Fit before RHIC RHIC sensitive x: ~a fractional momentum of proton K.Okada

35. Hint for non-zero G DIS2012 Werner K.Okada

36. W boson measurement at PHENIX K.Okada

37. Measurement of W boson decay Detect decay electron W- e-+ e W+ e+ + e Neutrino () will escape. K.Okada

38. We, 2 body decay signal Central arm We pT pT MW/2 pL ~pL*+pW/2 W->e (Central) (forward) Background: The tail of QCD interactions. The electron comes with other particles.  An isolation cut reduces those BG. K.Okada

39. W boson event candidate High energy, isolated K.Okada

40. W boson signal Run9 (2009) data L=8.6/pb e+ pT distribution of electron candidates Red histogram: with minimum track requirements Blue histogram: with an isolation cut e- Z decay is included. Main background : Photon conversion Charged hadron interaction Electron transverse momentum K.Okada

41. Convert to the total W production PHENIX acceptance （pT>30GeV, |y|<0.35） Full 4 Z boson contribution + : ~7%, -: ~30% Fraction of the PHENIX + : ~22%, -: ~15% The first measurement in p+p collisions. K.Okada

42. More recent measurements LHC RHIC STAR Collaboration (arXiv:1112.2980, Dec 2011) K.Okada

43. Ready for the asymmetry Single spin asymmetry (AL) (68%CL) e+ e- AL0 ! Horizontal axis: “y” corresponds to the polar angle of the electron. K.Okada

44. Muon decay in forward   K.Okada

45. Forward W (2011 run) + - The first result appeared in March. K.Okada

46. Transverse spin asymmetry(A new way to test QCD) Left • Observable: left-right asymmetry to the proton spin. Right Single particle production Plane formed by multi particles others K.Okada

47. Indication of quark transverse motion? • Large asymmetry even at RHIC energy. • It is not explained by the collinear framework.  Theory developments, Transverse motion (L) • More experimental data are required. Colliding partons unbalance Higher = more forward valence quark dominance K.Okada

48. Next steps K.Okada

49. Comments on the current status • G • Not large • W-boson, (anti-)quark spin flavor decomposition • The program has just begun. • Transverse spin program • Non-zero left-right asymmetry is observed. K.Okada

50. Comments on the current status • G • Not large (in a limited x region) • Need low-x where more gluons hide. • Clean probe (e.g. direct photon) needs high statistics. • W-boson, (anti-)quark spin flavor decomposition • The program has just begun. • Main focus for next years. • Transverse spin program • Non-zero left-right asymmetry is observed in high-x. • Need to decompose many processes. Fit before RHIC RHIC sensitive K.Okada