1. 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

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

3. 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

4. Transverse momentum in the QPM
collinear QPM
quark transverse momentum kT
Additional physics!

5. 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)

6. 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

7. Kinematical corrections: Cahn effect
Collinear PM (kT = 0):
Leading order QED with kT  0
R.N. Cahn PL B78 (1978) …
After fragmentation:
contributes to cos h and cos 2 h moments
Access to <kT>

8. 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/

9. 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

10. Experimental status Azimuthal modulations in lpl’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

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

13. 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:

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: sinf modulation
error bars: statistical errors bands: systematical errors
Deuteron
target

16. Hermes sinf results CLAS sinf results
Phys.Lett.B648: ,2007
Proton
target
CLAS sinf results
Phys.Rev. D69 (2004)

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

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

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

20. 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

21. 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 !

22. 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 !!