1. Collins and Sivers asymmetries on the deuteron from COMPASS data
The International Workshop on Transverse Polarisation Phenomena in Hard Processes
Collins and Sivers asymmetries on the deuteron from COMPASS data
Igor Horn, University of Bonn
on behalf of the
COMPASS Collaboration
Villa Olmo (Como), 7−10th. September 2005

2. Outline Motivation Experimental configuration Data selection
Results from 2002 beamtime
Phenomenological models
Statistical accuracy with 2003/4 data
Summary and outlook

3. Physics at COMPASS With the muon beam With hadron beams
Gluon Polarization G/G
transverse spin distribution functions Tq (x)
Flavor dependent polarized
quark helicity densities q(x)
 physics
Diffractive VM-Production
With hadron beams
Primakoff-Reactions
polarizability of  and K
glueballs and hybrids
charmed mesons and baryons
semi-leptonic decays
double-charmed baryons

4. Spin structure functions
3 distribution functions are necessary to describe the spin structure of the nucleon at LO (twist-2), all of equal importance!
Tq(x)
q(x)
q(x)
decouples from leading twist DIS because helicity of quark must flip;
not observable in inclusive DIS;

5. Convolution with fragmentation function
How to measure ΔTq(x)?
epe’X
ppl+l-X
epe’hX
Impossible in DIS
Direct
Measurement
Convolution with fragmentation function
Semi-inclusive DIS (COMPASS and HERMES)
2) Scattering of nucleons: Drell-Yan (RHIC)
3) Scattering of nucleon and antinucleon: Drell-Yan (GSI)

6. Developments in transversity
In the last ten years:
Measurements at DESY, CERN, Jlab and RHIC
Great development in the theory of transversity: transversity is not suppressed at high energies;
Remarkable role of ΔTq(x), complementary to Δq(x).
In the last couple of years:
Role of the kT dependent distribution functions clarified (Cahn and Sivers effects, …).
Key features of transversity:
Probes relativistic nature of quarks
No gluon analog for spin-1/2 nucleon
Different Q2 evolution and sum rule than Δq(x)

7. Transverse spin physics at COMPASS
Possible quark polarimeters:
Azimuthal distribution of single hadrons
Azimuthal dependence of the plane containing hadron pairs (see A.Mielech ’s talk)
Measurement of transverse polarization of spin ½ baryons, e.g.  hyperon (see A. Ferrero’s talk)

8. Azimuthal asymmetries of single hadrons
Collins effect predicts an azimuthal asymmetry in fragmentation of transversely polarized quarks with finite transverse momentum to un-polarized hadrons
2) Sivers effect predicts an azimuthal asymmetry from the un-polarized quarks, -- an azimuthal modulation of quark transverse momentum for a transversely polarized nucleon.

9. Collins and Sivers angles
Collins: AColl ~ sin C
C = h - S’ = h + S –p
Sivers: ASiv ~ sin S
 S = h - S
S -- azimuthal angle of spin vector
of initial-state quark/nucleon
S’ -- azimuthal angle of spin vector
of fragmenting quark
with S’’ = p -S (spin flip)
h -- azimuthal angle of hadron momentum

10. Collins and Sivers effects
Collins and Sivers effect distinguishable
AUTsin( - s‘ ) AUTsin( - s ) 

11. Experimental configuration
trigger-hodoscopes
DW45
SM2 dipole
Polarized Target
straws
Muon-filter2,MW2
RICH_1
HCAL1
Gem_11
SM1 dipole
ECAL2,HCAL2
MWPC Gems Scifi
Muon-filter1,MW1
Veto
straws,MWPC,Gems,SciFi
Gems,SciFi,DCs,straws
Silicon
SciFi
Micromegas,DC,SciFi
Beam: µ+/ spill (4.8s / 16.2s) Beam momentum: GeV/c
Luminosity: ~ cm-2 s-1 Beam polarisation: ~76%

12. The frozen spin Target 1 2 two 60 cm long Target Cells
with opposite polarization
Relaxation time > 2000 hrs
superconductive
Solenoid (2.5 T)
3He – 4He Dilution
refrigerator (T~50mK)
Dipole (0.5 T)
For transversity measurements the polarization was reversed once a week 1 to 2 1 2
2002,2003,2004: LiD
Achieved polarization: ~50%
Dilution factor: ~0.38

13. Kinematical variables and cuts
Primary vertex: m, m’ + hadron
To ensure that we have DIS events and avoid large radiative corrections
0.1 Y 0.9
Q2 1 (GeV/c)2
Y=0.33
Q2=2.4 (GeV/c)2
Q2 > 1 (GeVc)2
W 5 GeV/c2
W=9.4 GeV/c2
xBj0.035
2002 data

14. Event selection: further cuts
Hadron selection:
energy deposit in hadron calorimeters (if present) > 5 GeV(HCAL1) or 8 GeV (HCAL2)
penetration length < 10 X0
no π / K / p separation by RICH for the 2002 data
Kinematic cuts on hadrons:
pt > 0.1 GeV/c; z > 0.25; z > 1- ∑ zi (leading hadron analysis) (zi = fraction of available energy carried by hadron)
pt > 0.1 GeV/c; z > (analysis with all hadrons)

15. Event selection 2002 statistics: Positive hadrons ~ 8.7 ·105
2002 data
z 0.25
pT0.1 GeV/c
pT0.51 GeV/c
z 0.45
2002 statistics:
Positive hadrons ~ 8.7 ·105
Negative hadrons ~7.0 ·105
Total ~ 1.6 ·106

16. Monte Carlo studies Good agreement between MC and real data!
MC events generated with Lepto 6.5.1
Data MC
Taken into account:
Trigger geometry
Tracking efficiencies
rms: rad
Good agreement between MC and real data!
all reconstructed hadrons
correctly reconstructed leading hadrons
correctly reconstructed leading hadron,
but leading hadron is not charged π
(~20% of the final sample, mainly K and p
(RICH not used in analysis) )
z=0.25

17. Extraction of Collins and Sivers asymmetries
Rate asymmetries:
j = C, S;
F muon flux
n target density
σ cross-section
aj acceptance efficiency
f dilution factor
PT target polarization
DNN depolarization factor
Asymmetries for two target cells are ( separately ) extracted,
using periods of opposite polarization, then combined:

18. Systematics Ratio of the acceptances and efficiencies for both
target cells vs. Coll, Siv does NOT change between two spin orientations
Results STABLE under the following tests:
● Splitting the target cells in two parts
● Splitting the data in high and low hadron momenta
● Using a different method to extract the raw asymmetry
● Different binning
Systematic errors are smaller than statistical errors

19. Results: x z p [ Gev/c ] A A Statistical Errors Collins Sivers T
Phys. Rev. Lett. 94, (2005)
Statistical Errors A
Collins A
Sivers x z
p [ Gev/c ] T

20. Results: x z p [ Gev/c ] A A Statistical Errors Collins Sivers T
Phys. Rev. Lett. 94, (2005)
Statistical Errors A
Collins A
Sivers x z
p [ Gev/c ] T

21. Interpretations Small asymmetries, some explanations:
Cancellation between proton and neutron;
Too small Collins mechanism.
If and large as from preliminary measurement by BELLE (hep-ex/ ), this is evidence for cancellation in isoscalar target;
Phenomenological fits of the data from HERMES and prediction for COMPASS by Vogelsang and Yuan (hep-ph/ ), Efremov et al. (hep-ph/ , Phys.Part.Nucl.35:S139-S142,2004) and Anselmino et al. (hep-ph/057181)

22. Sivers single-spin asymmetries
M. Anselmino et al. hep-ph/057181

23. Prediction of Sivers asymmetries on a proton target
M. Anselmino et al. hep-ph/057181
Asymmetry predicted to be larger with z>0.4, 0.2<pT<1 GeV/c and x>0.02

24. Sivers single-spin asymmetries on a deuteron target
W. Vogelsang and F. Yuan hep-ph/

25. Collins single-spin asymmetries on a deuteron target
W. Vogelsang and F. Yuan hep-ph/
Fit Parameters: Set I for CFF

26. Collins single-spin asymmetries on a deuteron target
W. Vogelsang and F. Yuan hep-ph/
Fit Parameters: Set II for CFF

27. Collected data Data sample with transverse spin: π
Year Number of SPS spills Number of good DIS events
~ ~1.6 ∙ 106
~ ~4 ∙ (2002)
~ ~2 ∙ (2003) π
Data sample increased in years 2003/4:
trigger system enlarged;
2004 longer run.
RICH particle identification is available for data!
E-calorimeter information is available for 2004 data K p

28. Expected statistical accuracy for Collins asymmetries
30 days

29. Summary and Outlook
Collins and Sivers single-spin asymmetries shown from 2002 data published by PRL;
The 2002 asymmetries are small and compatible with the non zero proton asymmetries measured by HERMES;
Existing phenomenological models are in a good agreement with COMPASS and HERMES data;
Making use of existing deuteron data ( ), accuracy will improve by a factor ~ 3, results are coming soon;
Data (of comparable statistics) will be collected on a transversely polarized proton target (NH3) in 2006 and will allow a flavor separation.

30. Thank you for your attention!