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CIPANP 2009

CIPANP 2009. Tenth Conference on the Intersections of Particle and Nuclear Physics. Unpolarized Azimuthal Asymmetries from the COMPASS Experiment Rainer Joosten University of Bonn on behalf of the COMPASS Collaboration. Perspective of the talk.

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CIPANP 2009

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  1. CIPANP 2009 Tenth Conference on the Intersections of Particle and Nuclear Physics Unpolarized Azimuthal Asymmetries from the COMPASS Experiment Rainer JoostenUniversity of Bonn on behalf of the COMPASS Collaboration

  2. Perspective of the talk SIDIS reactions in one photon exchange

  3. q(x) = q+(x) + q-(x) How does transverse momentum enter? • collinear QPM f1(x) g1(x) Dq(x) = q+(x) – q-(x) Helicity Integrated over kT: three quark distribution functions contribute in leading order h1(x) DTq(x) = qT(x) – qT(x) Transversity

  4. Transverse momentum in the QPM • collinear QPM • quarktransversemomentum kT Additional physics!

  5. Transverse momentum • … can be the transverse momentum kTof the quarks in the struck nucleon. It enters the PDFs (e.g. the Sivers PDF) • … can be the transverse momentumof the fragmenting quark (and the produced hadron). It enters the PFF (e.g. the Collins FF) • … can create kinematical effects (Cahn effect)

  6. Sivers Collins Full SIDIS cross-section in NLO A Bacchetta, M Diehl, K Goeke, A Metz, P Mulders, M Schlegel (06) 18 structure functions General expression: also valid for exclusive reactions and for entire phase space of SIDIS Unpolarizedtarget Cahn and Boer-Mulders longitudinallypolarizedtarget Kotzinian, NP B441 (1995) 234 Mulders and Tangermann, NP B461 (1996) 197 Boer and Mulders, PR D57 (1998) 5780 Bacchetta et al., PL B595 (2004) 309 Bacchetta et al., JHEP 0702 (2007) 093 NLO transverselypolarizedtarget

  7. Kinematical corrections: Cahn effect Collinear PM (kT= 0): Leading order QED with kT 0 R.N. Cahn PL B78 (1978) 269-273 … After fragmentation: contributes to cosh and cos2hmoments Access to <kT>

  8. up down Boer Mulders effectRelation to transverse space Side view Front view A. Bacchetta quarks        proton Convolutedwith Collins function Contributes to cosh and cos2hmoments Quark spin can be unevenly distributed in transverse space A distortion in the distribution of quark spin in transverse space can give rise to a Boer-Mulders function Burkardt, hep-ph/0510408

  9. Unpolarised Target Cross-Section pQCD expected to be important at pt > 1 GeV/c here <pt> ≈ 0.5GeV/c pQCD and beampolarisation Cahn effect, Boer-MuldersDF and pQCD Boer- MuldersCollins FF,Cahn effect and pQCD

  10. Experimental status • Azimuthal modulations in lpl’hX measured by • EMC • E665 • Large modulations up to 40% for cosf, while cos2f ~ 5% EMC E665 Fits from M. Anselmino, V. Barone, E. Boglione,U. D’Alesio, F. Murgia, A. Prokudin, A. Kotzinian, and C. Turk Since last year, new data from COMPASS and HERMES

  11. luminosity: ~5 . 1032 cm-2 s-1 beam intensity: 2.108m+/spill (4.8s/16.2s) beam momentum: 160 GeV/c LHC SPS N

  12. COMPASS Polarized Target: 6LiD (PT≈ 50%) (2002-2004) or NH3 (PT≈ 85%) (2007) solid target cells Polarization reversal: transverse: once a week longitudinal: every 8 hours • high energy beam • large angular acceptance • broad kinematical range • two stage spectrometer • Large Angle Spectrometer (SM1), • Small Angle Spectrometer (SM2) • PID • RICH in LAS MuonWall SM2 E/HCAL E/HCAL SM1 MuonWall Polarised Target RICH mbeam

  13. acceptance UnpolarisedAzimuthal Asymmetries • data sample: • - part of the 2004 data • L and T target polarisation • ONLY donstream target cell used • - each with both target configurations • to cancel possible polarisationeffects final azimuthal distribution Event selection: DIS events… • Q2>1 (GeV/c)2 • 0.1<y<0.9 • W>5 (GeV/c2) • Hadrons • 0.2< z < 0.85 • 0.1<pT<1.5 (GeV/c) the azimuthal distributions have to be corrected by the apparatus acceptance dedicated MC simulations for L and T target polarisation data the corrected azimuthal distributions are fitted:

  14. Systematic Error The systematic error is evaluated from: • compatibility of results with L and T target polarization (different experimental conditions, different MCs) • comparison of results obtained using two different MCs with different settings for each data set (LEPTO default, standard COMPASS high pt; ~extreme cases) • compatibility of results from subsamples corresponding to: • different periods • different geometrical regions for the scattered muon

  15. Results: sinfmodulation error bars: statistical errors bands: systematical errors Deuteron target

  16. Hermessinf results Phys.Lett.B648:164-170,2007 Proton target CLASsinf results Phys.Rev. D69 (2004) 112004

  17. UnpolarisedAzimuthal Asymmetries cosmodulation (Cahn + Boer-Mulders) First determination of chargedependentcosmoments error bars: statistical errors bands: systematical errors Deuteron target

  18. UnpolarisedAzimuthal Asymmetries cosmodulation comparison with theory Effect up to 40% Shape described by prediction Size NOT in agreement with predictions

  19. UnpolarisedAzimuthal Asymmetries cos2modulation (Cahn + Boer-Mulders) First determination of chargedependent cos2moments error bars: statistical errors bands: systematical errors Deuteron target

  20. UnpolarisedAzimuthal Asymmetries cos2modulation comparison with theory • pQCDcharge independent • Cahncharge independent • (ifkTu = kTd) • Boer–Mulderschargedependent Effect up to 10% Well in agreementwithpredictions Indicationfornon-vanishingBoer-Muldersfunction

  21. Summary Results on unpolarized asymmetries: • First results obtained separately for + and - hadrons • sinf modulation compatible with 0 • cosf modulation up to 20% (for large z or pT) and the overall trend is reproduced by the predictions • cos2f modulation smaller (10% at most). Overall good agreement with the predictions • There is a difference between h+ and h- asymmetries on cosf/cos2f Indication of non vanishing Boer-Mulders function !

  22. Outlook COMPASS proton target data: • In 2006 and 2007 COMPASS has taken data (longitudinal and transverse) on a polarised NH3 target. • These can be analyzed using the same procedure as for the deuteron (6LiD) data. • In 2009 COMPASS will perform a short ( ~4 weeks) DVCS test-run using an unpolarized liquid hydrogen target. • This might yield reasonable statistics to extract unpolarized asymmetries. • Effects of the heavy N-nucleus in the NH3 target can be studied. ThankYou !!

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