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Franco Bradamante University of Trieste and INFN Trieste

COMPASS and HERMES contributions to our understanding of the NUCLEON SPIN. Franco Bradamante University of Trieste and INFN Trieste. INTERNATIONAL SCHOOL OF NUCLEAR PHYSICS 29th COURSE - Quarks in Hadrons and Nuclei ERICE, 19 SEPTEMBER 2007. SPIN: A HISTORY FULL OF SURPRISES.

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Franco Bradamante University of Trieste and INFN Trieste

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  1. COMPASS and HERMES contributions to our understanding of the NUCLEON SPIN Franco BradamanteUniversity of Trieste and INFN Trieste INTERNATIONAL SCHOOL OF NUCLEAR PHYSICS 29th COURSE - Quarks in Hadrons and Nuclei ERICE, 19 SEPTEMBER 2007

  2. SPIN: A HISTORY FULL OF SURPRISES • Stern & Gehrlach (1921): Space quantization associated with direction • Goudschmidt & Uhlenbeck (1926): Atomic fine structure & electron spin magnetic moment • Stern (1933): Proton anomalous magnetic moment 2.79 mN • Kusch (1947): Electron anomalous magnetic moment 1.00119 mB • Prescott & Yale-SLAC Collaboration (1978): EW interference in polarized e-d DIS, parity non-conservation • European Muon Collaboration (1988): Spin Puzzle • E704, AGS pp scattering: Transverse single spin asymmetries • ….. Erice, 19 September 2007

  3. Observe scattered electron/muon [1] • Observe current jet [1]+[2] • Observe remnant jet as well [1]+[2]+[3]  inclusive  semi-inclusive  exclusive DEEP INELASTIC SCATTERING [1] [3] [2] Erice, 19 September 2007

  4. measured at CERN, HERA, SLAC q(x)from global analysis of DIS and hard scattering data (QCD fits) PDF AND STRUCTURE FUNCTIONS: q(x) Inclusive DIS: unpolarised F2(x) = 2x·F1(x)Callan-Gross in the parton model Erice, 19 September 2007

  5. THE NUCLEON STRUCTURE three distribution functions are necessary to describe the structure of the nucleon at LO: ALL OF EQUAL IMPORTANCE! Erice, 19 September 2007

  6. HELICITY DISTRIBUTIONS Dq(x) Erice, 19 September 2007

  7. beam/target helicity HELICITY PDF AND STRUCTURE FUNCTIONS Inclusive DIS: beam and target longitudinally polarizedwrt beam direction g1measured at SLAC, EMC, SMC, HERMES, COMPASS g2suppressed by a factor g20.01 at 100 GeV (SMC, SLAC) in the parton model Erice, 19 September 2007

  8. s = 1 s = - 1/2 s = - 1/2 g1 and F1 in Quark Parton Model definitions: Erice, 19 September 2007

  9. ~0.2 0.15 target ~0.8 beam ~0.8 Measurement of g1 in inclusive DIS D, h kinematical quantities A1 and A2 are the asymmetries in g*p (n) scattering Erice, 19 September 2007

  10. THE NUCLEON SPIN in polarised DIS one measures G1is related to the MATRIX ELEMENTS of the AXIAL CURRENT and therefore to the WEAK DECAY CONSTANTS of the BARYON OCTECT from F, D, and G1 one can get Du, Dd, Ds and then DS, the SINGLET MATRIX ELEMENT of the AXIAL CURRENT, i.e. the QUARK CONTRIBUTION to the NUCLEON SPIN Erice, 19 September 2007

  11. THE NUCLEON SPIN  SPIN CRISIS Erice, 19 September 2007

  12. THE SPIN PUZZLE how to reconcile the complex nucleon structure revealed at large Q2 by DIS experiments with the simplicity of the CQM which works so well with the baryon magnetic moments and which predicts Several experiments have checked the EMC result for the proton, and measured g1 for the neutron, confirming the effect SMC at CERN E142, E143, E154, E155 at SLAC HERMES at DESY COMPASS at CERN + huge number of theoretical papers (QCD analysis) DIS FINISHED SIDIS Erice, 19 September 2007

  13. THE SPIN PUZZLE: WAYS OUT • contribution from GLUON SPIN in inclusive DIS one is not really measuring Dq Dq and DG mix up in g1 (DGLAP equations) NECESSITY OF A DIRECT MEASUREMENT OF DG → SIDIS experiments (HERMES and COMPASS) • contribution from ORBITAL ANGULAR MOMENTUM of quarks and gluons Erice, 19 September 2007

  14. TRANSVERSITY DISTRIBUTION DTq(x) Erice, 19 September 2007

  15. Transverse single pion asymmetry [E704, 1991] STAR TRANSVERSITY DISTRIBUTION:DTq(x) Since many years intriguing evidence of large transverse spin effects at high energy • hyperon polarization • high pt effects in hadronic interactions • asymmetries in hadron production Erice, 19 September 2007

  16. Transversity Distribution Function DTq(x),h1q(x),dq(x), dTq(x) , q=uv, dv, qsea recently much interest ! • properties: • DTq(x)  Dq(x) • probes the relativistic nature of quark dynamics • no contribution from the gluons  simple Q2 evolution • positivity (Soffer) bound • first moments: tensor charge • sum rule for transverse spinin Parton Model framework • it is related to GPD’s • ischiral-odd: decouples from inclusive DIS Bakker, Leader, Trueman, PRD 70 (04) Erice, 19 September 2007

  17. TRANSVERSITY DISTRIBUTION:DTq(x) being chiral-odd DTq(x) does not contribute to inclusive DIS it is much harder to measure than the helicity distribution • DTq(x) can be measured only in conjunction • with another chiral-odd partner: Collins function SIDIS DY measurable in e+e- hadrons • relevance of transverse momentum dependent (TMD) PDF and FF Sivers function Erice, 19 September 2007

  18. ONGOING ACTIVITY Erice, 19 September 2007

  19. ONGOING ACTIVITY 3 lines of attack: • Direct measurement of DG • Measure Dq(x) in SIDIS, separating flavours by identifying the hadrons in the current jet • Measure transversity (the new territory) SIDIS: COMPASS, HERMES, JLAB p↑p↑ : RHIC Erice, 19 September 2007

  20. BRAHMS & PP2PP (p) PHENIX (p) STAR (p) Relativistic Heavy Ion Collider RHIC pC Polarimeters Absolute Polarimeter (H jet) Siberian Snakes Spin Rotators 2  1011 pol. protons / bunch Partial Siberian Snake LINAC BOOSTER Pol. Proton Source 500 mA, 300 ms AGS AGS Internal Polarimeter 200 MeV Polarimeter Rf Dipoles RHIC accelerates heavy ions to 100 GeV/A and polarized protons to 250 GeV Erice, 19 September 2007

  21. far future ….. • Measurement of GPD’s (Generalized Parton Distributions) and of Lz via the Ji sum rule • p↑ p↑ : GSI ONGOING ACTIVITY 3 lines of attack: • Direct measurement of DG • Measure Dq(x) in SIDIS, separating flavours by identifying the hadrons in the current jet • Measure transversity (the new territory) SIDIS: COMPASS, HERMES, JLAB p↑p↑ : RHIC Erice, 19 September 2007

  22. POLARISED SIDIS COMPASS HERMES only a glimpse Erice, 19 September 2007

  23. COMPASS LHC SPS Erice, 19 September 2007

  24. = 17 GeV COMPASS • high energy beam • large angular acceptance • broad kinematical range m+ beam: 160 GeV/c polarisation - 80% intensity 2·108 µ+/spill (4.8s/16.2s) two stages spectrometer Large Angle Spectrometer (SM1), Small Angle Spectrometer (SM2) tracking, calorimetry, PID NEW TECHNOLOGIES MuonWall SM2 E/HCAL E/HCAL SM1 MuonWall Polarised Target SciFi Silicon Micromegas GEMs Straws SDC MWPC W45 RICH mbeam Erice, 19 September 2007

  25. 3He – 4He Dilution refrigerator (T~50mK) superconductive Solenoid (2.5 T) Dipole (0.5 T) the COMPASS target system (2002-2004) solid state target operated in frozen spin mode 2002-2004: 6LiD dilution factor f = 0.38 polarization PT = 50%~20% of the timetransversely polarised two 60 cm long cells with opposite polarisation (systematics) • 2006: • PTM replaced with the large acceptance COMPASS magnet • 2 3 cells • 2007: NH3 dN/dz 4000 2000 0 Erice, 19 September 2007 -1000 0 1000 zvtx (mm)

  26. = 7 GeV HERMES 27.5 GeV e+ particle ID: lepton ID with e~98%, hadron contamination <1% RICH: p, K, p ID Erice, 19 September 2007

  27. the HERMES polarised target pure hydrogen gas target flipped at high frequency (60- 90 s) 1H→ <|Pt|> ~ 85 ±3.8 % 2H→ <|Pt|> ~ 84 ±3.5 % 1H <|Pt|> ~ 74 ±4.2 % Erice, 19 September 2007

  28. SIDIS kinematics Erice, 19 September 2007

  29. SIDIS kinematics Erice, 19 September 2007

  30. TWO CLASSES OF PHENOMENA Transverse Spin case Longitudinal Spin case + Cahn & Boer-Mulders asymmetries Erice, 19 September 2007

  31. PRESENTLY UNDER INVESTIGATION • DG • g1 • Dq, Dq (Ds !!!) • Exclusive vector meson production (r,, J/Y) • polarization LONGITUDINAL SPIN Erice, 19 September 2007

  32. LONGITUDINAL SPIN CASE MEASUREMENT OF ΔG Erice, 19 September 2007

  33. MEASUREMENTS OF THEGLUON POLARIZATION FOUR LINES OF ATTACK: • Double spin asymmetry of the OPEN CHARM cross-section • Double spin asymmetry of the HIGH-pt HADRON PAIRS (Q2 > 1 GeV2) • Double spin asymmetry of the HIGH-pt HADRON PAIRS (Q2 < 1 GeV2) • Measurement of g1 of the deuteron and QCD fit of all the world data Photon-Gluon Fusion Erice, 19 September 2007

  34. N Gluon Polarization Photon-Gluon Fusion q = c“OPEN CHARM”cross section differencein charmed mesonproduction → theory well understood → experiment challenging q = u,d,s“HIGH pT HADRON PAIRS”cross section difference in 2+1 jet production in COMPASS: events with2 hadrons with high pT → experiment “easy” → theory more difficult enrich photon-gluon fusion events with high-pt cuts pt > 0.7 GeV/c pt12 + pt22 > 2.5 (GeV/c)2xF, z > 0.1 m(h1,h2) > 1.5 GeV/c2 Erice, 19 September 2007

  35. Gluon Polarization Open Charm: D’s from D*’s D* → D πs→K ππs slow pion required 2002–2004 D0 → K π D0 → K ππ0 Erice, 19 September 2007

  36. QCD- Compton PGF LO DIS Gluon Polarization High-pt Hadrons, Q2 > 1 (GeV/c)2 PGF and background Erice, 19 September 2007

  37. Gluon Polarization High-pt Hadrons, Q2 < 1 (GeV/c)2 ratios of processes resolved photons Erice, 19 September 2007

  38. Gluon Polarization COMPASS preliminary results high-pt pairs, Q2>1GeV2 : 2002–2003 high-pt pairs, Q2< 1GeV2 : 2002–2004 open charm: 2002–2004 Erice, 19 September 2007

  39. ΔG/G SMC,HERMESand COMPASS results GRSV-max strongly disfavored HERMES open charm SMC Erice, 19 September 2007

  40. A1d and QCD fit Inclusive DIS asymmetry Q2>1 (GeV/c)2 world results COMPASS PLB 647(2007)8 • good agreement with previous experiments • significantly improved statistics at low x • no tendency towards negative values at x<0.03 Erice, 19 September 2007

  41. QCD fit Erice, 19 September 2007

  42. QCD fit world data at Q02 = 3 GeV2solutions with ΔG > 0 Erice, 19 September 2007

  43. QCD fit world data at Q02 = 3 GeV2solutions with ΔG < 0 Erice, 19 September 2007

  44. QCD fit note: NLO fits, LO data Erice, 19 September 2007

  45. DS = a0 in MS from HERMES fit: similar results (theory) (exp) (evol) a0 = 0.330 ± 0.011 ± 0.025 ± 0.028 a0 = 0.33 ± 0.03(stat) ± 0.05(sys+evol) Erice, 19 September 2007

  46. estimate for ΔG from HERMES fit effect of new, final HERMES data on g1d without HERMES g1d with HERMES g1d (test done with BB code, Nucl.Phys.B636(2002)225) H.Boettcher, private communications Erice, 19 September 2007

  47. CONCLUSION from ΔG MEASUREMENTS ΔG SMALL more precise measurements will come soon COMPASS 2006 RHIC RUN6 …. interest in orbital angular momentum GPD’s Ji’s SUM RULE Erice, 19 September 2007

  48. TRANSVERSE SPIN measurement of the transversity distribution function Erice, 19 September 2007

  49. Transversity DF: how to measure it the Transversity DF is chiral-odd: observable effects are given only by the product ofTq (x)and an other chiral-odd function can be measured in SIDIS on a transversely polarised target via “quark polarimetry” all measured in COMPASS / HERMES Erice, 19 September 2007

  50. the fragmentation function of a transversely polarised quark has aspin dependent part “Collins” FF: Collins asymmetry the “quark polarimetry” relies on the Collins effect a quark moving “horizontally” and polarized “upwards” would emit the leading meson preferentially on the “left” side of the jet in SIDIS the “Collins angle” is • C = h-s’ = h+S - p h,s’,Sazimuthal angles of the hadron momentum, of the spin of the fragmenting quark and of the nucleon in the GNS Erice, 19 September 2007

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