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Hermes at HERA

Online measurement of beam polarization with two Compton polarimeters. Hermes at HERA. Beam Energy: 27.5 GeV Electrons and positrons Beam current ~50mA start of fill ~10mA end of fill Polarized (<P>~53%) P~45% now Beam helicity reversable Can be set at each expt. The HERMES Experiment.

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Hermes at HERA

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  1. Online measurement of beam polarization with two Compton polarimeters. Hermes at HERA • Beam Energy: 27.5 GeV • Electrons and positrons • Beam current • ~50mA start of fill • ~10mA end of fill • Polarized (<P>~53%) • P~45% now • Beam helicity reversable • Can be set at each expt. J Stewart

  2. The HERMES Experiment Target • Fixed target experiment. • Polarized internal gas target. • Magnetic spectrometer for momentum measurement. • Relatively large acceptance. • Excellent particle identification. J Stewart

  3. The HERMES Polarized Target • Longitudinal polarized H: <P>= 0.85 ± 0.03 r=7.6 x 1013 nucl./cm2 • Transverse polarized H: <P>= 0.78 ± 0.04 r=1.1 x 1014 nucl./cm2 • Long. Polarized D: r=2.1 x 1014 nucl./cm2 • <Pz+>=+0.85 ± 0.03 <Pz->= -0.84 ± 0.03 <Pzz+>= +0.89 ± 0.03 <Pzz->= -1.66 ± 0.05 • Unpolarized gases used: • H2,D2,He,N2,Ne,Xe J Stewart

  4. The HERMES Spectrometer Particle Identification: TRD, Preshower, Calorimeter 1997: Threshold Cherenkov 1998: RICH + Muon-ID J Stewart

  5. hadron/positron separation combining signals from: TRD, calorimeter, preshower, RICH Aerogel; n=1.03 p C4F10; n=1.0014 K Particle Identification hadron separation Dual radiator RICH forp,K, p J Stewart

  6. Semi-Inclusive Deep Inelastic Scattering The cross section can be expressed as a convolution of a distribution function and a fragmentation function. J Stewart

  7. Virtual Photon Asymmetry and DIS • Virtual photon can only couple to quarks of opposite helicity • Select quark helicity by changing target polarization direction • Different targets give sensitivity to different quark flavors J Stewart

  8. Cross Section in Deep Inelastic Scattering Purely electromagnetic → Calculable in QED b1,b2,b3,b4 for spin 1 nucleon “Tensor structure functions” Momentum distribution Unpolarized Structure Functions Polarized Structure Functions Helicity distribution J Stewart

  9. Structure Functions and Measured Asymmetries Momentum distribution of the Quarks Helicity distribution of the Quarks Measurable Asymmetries With D,d,R,g,x,h being kinematic factors Virtual Photon Asymmetries J Stewart

  10. World Data on Proton Deuteron Data shown at measured <Q2>:0.02-58 GeV2 J Stewart

  11. smearing within acceptance radiative effects detector smearing • systematic correlations between bins fully unfolded • resulting (small) statistical correlations known Model-independent unfolding • detector smearing • QED radiative effects • kinematic migration inside acceptance for each spin state • j=0 bin: kinematic migration into the acceptance J Stewart

  12. World Data on • Very precise proton data • The most precise deuteron data • The most precise neutron data • 0.021-0.9 measured range: J Stewart

  13. The Structure Function b1(x,Q2) and J Stewart

  14. The structure function b1(x,Q2) • 3.2 M DIS events • <Pzz+>= +0.89 ± 0.03 • <Pzz->= -1.66 ± 0.05 • First measurement of and • at small x • In measured range (0.002-0.85) • Qualitative agreement with coherent double-scattering models hep-ex/0506018 J Stewart

  15. Linear System in Quark Polarizations Correlation between detected hadron and the struck quark allows flavor separation Inclusive DIS →DS Semi-inclusive→ J Stewart

  16. is an all sea object and The Measured Hadron Asymmetries PROTON DEUTERIUM J Stewart

  17. Polarized Quark Densities • Polarized parallel to the proton • Polarized anti-parallel to the proton • Good agreement with LO-QCD fit • No indication for • 0.028 ± 0.033 ± 0.009 In the measured range A. Airapetian et al, Phys. Rev D 71 (2005) 012003 J Stewart

  18. Polarized Sea • Unpolarized data on sea shows the Gottfried sum rule is broken • Reanalyze polarized data: • Polarized data favor a symmetric sea ,but large uncertainties J Stewart

  19. HERMES 1996-2000 HERMES >2002 Distribution Functions Leading Twist • 3 distribution functions survive the integration over transverse quark momentum unpolarized DF Helicity DF Transversity DF Transversity basis vector charge axial charge tensor charge J Stewart

  20. Properties of the Transversity DFs • For non-relativistic quarks dq(x)=Dq(x) • dq(x) probes the relativistic nature of the quarks • Due to Angular Momentum Conservation • Different QCD evolution • No gluon component • Predominately sensitive to valence quarks • Bounds • Soffer Bound: • T-even • Chiral odd • Not measurable in inclusive DIS J Stewart

  21. Forbidden • Need chiral odd fragmentation function Measuring Transversity • Transverse quark polarization affects transverse hadron momentum • Observed asymmetry in azimuthal angle about lepton scattering plane • Need a chiral odd fragmentation function: ‘Collins FF’ J Stewart

  22. Sivers Function • Distribution function • Naïve T-ODD • Chiral even • a remnant of the quark transverse momentum can survive the photo-absorption and the fragmentation process • Can be inherited in the transverse momentum component • influence azimuthal distribution • Non-vanishing Sivers function requires quark orbital angular momentum • Cross section depends on the angle between the target spin direction and the hadron production plane J Stewart

  23. angle of hadron relative to final quark spin angle of hadron relative to initial quark spin amplitudes fit simultaneously (prevents mixing effects due to acceptance) Single target-spin asymmetry J Stewart

  24. Collins Moment • Result is consistent with the published Collins moment. • Large negative p- moment unexpected • One possibility • Additional information on the Collins fragmentation function needed to extract the transversity distribution. • Belle J Stewart

  25. Sivers Moment • p+ sivers moment > 0! • Clear sign for non-zero orbital angular momentum! • Sivers moment for p- is consistent with zero. • Unfavored frag.? • Unpolarized fragmentation functions are known • Sivers function can be extracted. • f^1T(x) DIS = - f^1T(x) DY • UNIVERSALITY J Stewart

  26. Why are Fragmentation functions important? In Semi-inclusive DIS: • Important for Dq,dq, and • Test factorization • Test universality Extract p, K, and p multiplicities: J Stewart

  27. PID with the RICH Multiplicity Extraction Unpolarized H&D data Excl. VM Corr. Experimental Multiplicities in acceptance Acceptance Radiative effects MC Born Level multiplicities J Stewart

  28. Monti Carlo: Lepto in combination with JETSET; PDF: CTEQ-6L Fragmentation parameters tuned to HERMES multiplicities in the acceptance Data: Q2>1GeV2, W2>10GeV2, z>0.2, 2GeV <p< 15GeV (p, K, and P) Excellent Agreement even at the cross section level DATA/MC <10%! p+ p- K+ K- P- P+ MC tuning J Stewart

  29. p± Multiplicities vs z • Systematic uncertainties mainly from hadron PID correction • Q2>1GeV2, W2>10GeV2 • Comparison with EMC FF, Nucl. Phys. B321 (1989) 541 • Reasonable agreement with FF from S. Kretzer J Stewart

  30. K± Multiplicity vs z • Charge separated Kaon multiplicities • Systematic uncertainty mainly from hadron PID • Low K- statistics at high z  will collect more data J Stewart

  31. Agreement with existing Frag. Fns. J Stewart

  32. Summary Longitudinally Polarized Target Data • The structure functions have been measured. • First measurement of b1. • First direct measurement of the helicity distributions Transversely Polarized Target Data • Collins: • Non-Zero asymmetries measured. • Disfavored fragmentation functions appear to be important and have opposite sign to the favored. • Sivers: • p+ Amplitude is greater than zero. • Orbital angular momentum must be non-zero! J Stewart

  33. Outlook • Data taking with transverse polarized target will continue till November. • Expect about 5M DIS events in the final data set. • New multiplicities • New millennium extraction of Dq (purity free). • New extraction using isoscalor method J Stewart

  34. Backup Slides J Stewart

  35. J Stewart

  36. Purities - - - J Stewart

  37. J Stewart

  38. J Stewart

  39. Distribution and Fragmentation Functions J Stewart

  40. Exclusive VM Contamination • Exclusive vector meson (VM) contribution estimated using Pythia-6 • Correct data set for VM contamination. • Different process than SIDIS • Evaluate ratio: • Large contamination for p at high z • Contribution for K moderate vs z • Contribution grows for small x for both p and K J Stewart

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