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Online Measurement of Beam Polarization Using Compton Polarimeters in HERA's HERMES Experiment

This paper discusses the online measurement of beam polarization at the HERA facility, specifically within the HERMES experiment. Operating at an energy of 27.5 GeV, the experiment utilized two Compton polarimeters for direct measurement of beam polarization, achieving up to 53% polarization levels. The polarized internal gas target, magnetic spectrometer, and sophisticated particle identification techniques were employed for deep inelastic scattering studies. Results include cross-sectional analysis and detailed investigations into quark polarization and structure functions.

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Online Measurement of Beam Polarization Using Compton Polarimeters in HERA's HERMES Experiment

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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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