1. Measurement of Transverse Spin Effects at COMPASS
Anselm Vossen
ALU Freiburg
On Behalf of the COMPASS Collaboration

2. Outline Transversity kT dependent Quark Distribution q(x,kT) Outlook
Possible Measurements
Transversity at COMPASS
Theory predictions
kT dependent Quark Distribution q(x,kT)
Sivers Effect
Sivers Asymmetries at COMPASS
Outlook

3. Structure of the Nucleon
Collins effect m’ _
Lz-1 Lz _ + * * ? m _ + q
Helicity base:
independent amplitudes:
Hadronic Tensor N _ +

4. Measuring Transversity
Transversity base:
Probabilistic interpretation  ↑  ↓ ↑   _ _ + = _ +
Difference in densities for ↑, ↓ quarks
in ↑ nucleon
HERMES, BELLE:

5. Collins Fragmentation - Modell
K. Bruhwel, TJNAF
Collins Effekt in String Fragmentation
(X. Artru)

6. Collins Effect in SIDIS
Ph transverse momentum of the hadron
S’ transverse spin of the struck quark m’ m S’ g* FC Ph x
“Collins angle” C = h - s’ z
Fragmentation Function for a transversely polarised quark into unpolarised hadron
spin dependent part : “Collins fragmentation function”
Number of
Events:

7. Collins Asymmetries

8. Collins: Comparison with Theory
A.V. Efremov, K. Goeke and P. Schweitzer, PRD73 (2006)
DTq from Chiral Quark Soliton Model
Collins FF from Fit to HERMES Data
 unfav FF ~ - fav FF
 DTu, DTd cancel in deuterium
COMPASS

9. Transversity signal in 2 Hadron Correlations
Azimuthal Asymmetry measurement similar to Collins Asymmetry extraction
(z2P1+z1P2)/(z1+z2) =Ph S’ Ph
F2H m m’ z x g*

10. Results, z Binning

11. Results, xBj binning

12. Predictions 2H Asymmetries
Bacchetta, Radici: Phys.Rev. D74 (2006)
p+-p- production

13. Sivers Effect + } x x
M. Burkardt, PRD, 66, (2002)

14. Sivers Effect m’
“Sivers angle” m S g* FS Ph x
Modulation in FS of the hadron distribution
Correlation between the nucleon spin and quark motion
Sivers effect ≠0 ⇒ Quark angular momentum ≠ 0 z
modulation in FS of the hadron distribution

15. Sivers Asymmetries

16. Sivers: Comparison with Theory
M. Anselmino et al., PRD72 (2005)
including transverse momenta, FF from literature
13 parameter fit to HERMES and COMPASS 2002 data to extract the Sivers functions
comparison with COMPASS data ok
COMPASS

17. Conclusion/Outlook
Measurement of Transversity and Sivers effect provides new insights in soft QCD effects
Experiments have shown unexpected effects
COMPASS data on deuteron is important to extract DTd and D0Td
COMPASS measures this year with proton target
Polarization data has often been the
graveyard of fashionable theories.
If theorists had their way, they might
just ban such measurements altogether
out of self-protection.
J.D. Bjorken
St. Croix, 1987

18. COMPASS 2004 Setup high energy beam Large angular acceptance
Broad kinematical range
two stages spectrometer
Large Angle Spectrometer (SM1),
Small Angle Spectrometer (SM2)
tracking, calorimetry, PID
NEW TECHNOLOGIES
MuonWall
SM2
E/HCAL
E/HCAL
SM1
MuonWall
RICH
m beam
Polarised Target
: 6LiD (Deuteron)
dilution factor f = 0.38 polarization PT =50%

19. The RICH detector 3 m 6 m 5 m threshold momenta: pp = 2 GeV/c
C4F10
photon
detectors
threshold momenta:
pp = 2 GeV/c
pK = 9 GeV/c
pP =17 GeV/c
fully efficient for transverse data
in 2003 and 2004

20. COMPASS Transversly Polarised Target
two 60 cm long cells
with opposite polarization (systematics)
dN/dz
4000
2000
-1000
1000
zvtx (mm)
: 6LiD (Deuteron)
dilution factor f = 0.38 polarization PT = 50%
during data taking with transverse polarization
polarization reversal in the 2 cells after ~ 5 days

21. COMPASS Experiment @ CERN
COmmon Muon and Proton Apparatus for Structure and Spectroscopy
SPS
fixed target experiment at the CERN SPS
240 physicists from 28 Institutes, 11 Countries

22. h azimuthal angle of the hadron
s’ azimuthal angle of the transverse spin of the struck quark
s’ = p - s
s azimuthal angle of the transverse spin of the initial quark
“Collins angle” C = h - s’ = h + s - p

23. Predictions for 2H Signal
QCD N’06, Bacchetta, Radici: hep-ph/
ARS
ARS

24. Results, MInv Binning

25. Transverse Spin @ COMPASS
Transverse Spin Measurements provide new insights into Spin Structure and Quark dynamics inside the nucleon
COMPASS provides key measurements
Collins (Transversity)
Sivers (kT dependent unpol. PDF)
M. Anselmino et al., PRD72 (2005)
Efremov, Goeke, Schweitzer, PRD73 (2006)
COMPASS
Predictions
COMPASS
Predictions
xBj
xBj