Unpolarized Azimuthal Asymmetries from the COMPASS Experiment 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 SIDIS reactions in one photon exchange
How does transverse momentum enter? collinear QPM f1(x) q(x) = q+(x) + q-(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
Transverse momentum in the QPM collinear QPM quark transverse momentum kT Additional physics!
Transverse momentum … can be the transverse momentum kT of the quarks in the struck nucleon. It enters the PDFs (e.g. the Sivers PDF) … can be the transverse momentum of the fragmenting quark (and the produced hadron). It enters the PFF (e.g. the Collins FF) … can create kinematical effects (Cahn effect)
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 Unpolarized target Cahn and Boer-Mulders longitudinally polarized target Sivers 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 Collins NLO transversely polarized target
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 cos 2 h moments Access to <kT>
Boer Mulders effect Relation to transverse space Side view Front view down up A. Bacchetta quarks proton Convoluted with Collins function Contributes to cos h and cos 2 h moments 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
here <pt> ≈ 0.5 GeV/c Unpolarised Target Cross-Section pQCD expected to be important at pt > 1 GeV/c here <pt> ≈ 0.5 GeV/c pQCD and beam polarisation Cahn effect, Boer-Mulders DF and pQCD Boer- Mulders Collins FF, Cahn effect and pQCD
Experimental status Azimuthal modulations in lpl’hX measured by EMC 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
LHC SPS luminosity: ~5 . 1032 cm-2 s-1 beam intensity: 2.108 m+/spill (4.8s/16.2s) beam momentum: 160 GeV/c LHC SPS N
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 m beam
Unpolarised Azimuthal Asymmetries acceptance 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 polarisation effects 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 final azimuthal distribution the corrected azimuthal distributions are fitted:
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
Results: sinf modulation error bars: statistical errors bands: systematical errors Deuteron target
Hermes sinf results CLAS sinf results Phys.Lett.B648:164-170,2007 Proton target CLAS sinf results Phys.Rev. D69 (2004) 112004
Unpolarised Azimuthal Asymmetries cos modulation (Cahn + Boer-Mulders) First determination of charge dependent cos moments error bars: statistical errors bands: systematical errors Deuteron target
Unpolarised Azimuthal Asymmetries cos modulation comparison with theory Effect up to 40% Shape described by prediction Size NOT in agreement with predictions
Unpolarised Azimuthal Asymmetries cos2 modulation (Cahn + Boer-Mulders) First determination of charge dependent cos2 moments error bars: statistical errors bands: systematical errors Deuteron target
Unpolarised Azimuthal Asymmetries cos2 modulation comparison with theory pQCD charge independent Cahn charge independent (if kTu = kTd ) Boer–Mulders charge dependent Effect up to 10% Well in agreement with predictions Indication for non-vanishing Boer-Mulders function
Indication of non vanishing Boer-Mulders function ! 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 !
Thank You !! 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. Thank You !!