1. TRANSVERSE SPIN EFFECTS IN COMPASS

2. COmmon Muon and Proton Apparatus for Structure and Spectroscopy NA58
Czech Republic, Finland, France, Germany, India, Israel, Italy, Japan, Poland, Portugal, Russia
Bielefeld, Bochum, Bonn, Burdwan, Calcutta, CERN,
Dubna, Erlangen, Freiburg, Heidelberg, Helsinki, Lisbon,
Mainz, Miyazaky, Moscow, Munich, Nagoya, Prague, Protvino,
Saclay, Tel Aviv, Torino, Trieste, Warsaw
28 Institutes, ~230 physicists

3. LHC SPS luminosity: ~5 . 1032 cm-2 s-1
beam intensity: m+/spill (4.8s/16.2s)
beam momentum: GeV/c
longitudinally polarised muon beam
longitudinally or transversely polarised target
calorimetry
particle identification
LHC
SPS N

4. The Spectrometer for the Muon Programme
Trigger-hodoscopes
μ Filter
ECal & HCal
50 m
SM1
SM2
TWO STAGE
SPECTROMETER:
RICH
MWPC
0.003 < x < 0.5
10-3 < Q2 < 10 (GeV/c)2
Straws
6LiD Target
Gems
160 GeV μ
Drift chambers
Micromegas
LAT,
PID
Silicon
SciFi

5. Target Polarization reversed every week
Polarized Target
New COMPASS target magnet:
180 mrad geometrical acceptance
excellent field homogeneity
To match larger acceptance:
new microwave cavity
3 target cells: reduction of false asymmetries
Target material:
NH3
high polarisation
very long relaxation time (∼ 4000 h)
magnetic field rotation without polarisation loss
 Polarisation of NH3 in 2007:
-92%, +88%, -83%
Target Polarization reversed every week

6. 2007 Transverse data taking statistics
2007 Compass Data taking
Begin of run: 18 May 2007
End of run: November 2007
Split between transverse and longitudinal target polarization:
- m on tape for transverse (40.0 x1012)
- m on tape for longitudinal (41.5 x1012)
For the extraction of the asymmetries (this analysis) only a fraction of the full statistic available has been used : nearly 20% of the whole data sample available.

7. DIS Event Selection DIS cuts: Q2>1 (GeV/c)2 0.1<y<0.9
W>5 GeV/c2

8. Total statistic for this analysis
Hadron Selection
All hadrons
Track Length<10 X0
Energy Deposit in HCALs>Thr. ( 4 GeV HCal1 and 5 GeV Hcal2 )
Only 1 HCAL fired
pT>0.1 GeV/c
z>0.2
Positive hadrons
Negative hadrons
Total statistic for this analysis

9. Data quality checks Data taking stability is needed:
A dedicated set of quality checks have been developed and applied to fulfill this condition
Different estimators have been considered:
the detector profiles stability
the number of primary vertices per event
the number of tracks per primary vertex
beam particles per primary vertex
the number of K0 per primary vertex
the reconstructed mass of the K0 meson
stability of many kinematical variables:
( zvtx, Em’, fm’, xBj Q2,y, W, Ehad , fhadLab , qhadLab , fhadGNS , qhadGNS , pt)

10. Mean of kinematical quantities

11. Collins and Sivers angles
Azimuthal modulations
Collins and Sivers angles
C = h - S’
 S = h - S
S ’ azimuthal angle of spin vector of fragmenting quark (S’ = p - S)
h azimuthal angle of hadron momentum

12. SIDIS azimuthal asymmetries
All possible 8 azimuthal asymmetries extracted at once.
...
Sivers
Collins
6 further
modulations
M. Diehl, S. Sapeta,
Eur.Phys.J C41 (2005)
hep-ph/

13. Asymmetry Extraction Splitting middle cell into two parts
two couples of cells with opposite polarization
two independent values for the asymmetries per period
Extraction: 2D Binned Maximum Log-Likelihood Fit:
eight by eight grid in fh and fS;
in each bin of the matrix one expects Nj counts :

14. Asymmetry Extraction - II
Separation of acceptance and spin dependent modulations:
Coupling of two cells (u,d) with opposite polarization ()
and two periods (p1,p2) with opposite target polarization:
Reasonable assumption:
4 · 64 = 256 nonlinear equations
· 64 = 201 fit parameter,
Poisson distribution to account for low statistics:
Tests for systematic errors:
For false asymmetries: combination of cells with same polarization
Comparison of 5 estimators for asymmetry extraction included the one used in previous analysis (deuteron data)
For this analysis: overall systematic error is 30% and 50% of the
statistical error for Collins and Sivers respectively

15. Collins asymmetry – proton data
systematic errors ~ 0.3 sstat
at small x, the asymmetries are compatible with zero
in the valence region the asymmetries are different from zero,
of opposite sign for positive and negative hadrons, and have the same strength and sign as HERMES

16. Compass proton data
comparison with M. Anselmino et al. predictions

17. Collins Final on Deuteron

18. Sivers asymmetry – proton data
systematic errors ~ 0.5 sstat
the measured symmetries are small, compatible with zero

19. Sivers Final on Deuteron from COMPASS

20. Sivers asymmetry– proton data
comparison with the most recent predictions from M. Anselmino et al.
arXiv:

21. Results: Sivers asymmetry
comparison with predictions from
S.Arnold, A.V.Efremov, K.Goeke, M.Schlegel and P.Schweitzer, arXiv:

22. Transverse L polarization

23. Data Selection Secondary vertex downstream of primary vertex.
PT > 23 MeV/c to exclude e+e− pairs
Proton and pion momenta > 1 GeV/c
Q2 > 1 (GeV/c)2
0.1 < y < 0.9
Use of RICH (2007 data)
Λ decay distance DΛ > 7 σD
Collinearity < 10 mrad

24. Results with proton target
~60% higher statistics with respect deuteron data (after)
Systematic errors have been estimated to be smaller than statistical errors from false polarization.
No dependence on x.

25. (Old) Results on Deuteron
Analysis for deuteron data (and no RICH ID)
Only statistical errors are shown (systematic effects not larger than the statistical errors).
Small tendency for Λ, but not significant for deuteron target.

26. Summary
First preliminary results of COMPASS proton transverse run:
Collins Asymmetry:
different from zero, comparable to HERMES
agreement with predictions of Anselmino et al
Sivers Asymmetry:
small and compatible with zero within present statistical errors
Accessing Collins from L:

27. Thank You

28. The 3rd Twist-2 structure function
three quark distribution functions (DF) are necessary to describe the structure of the nucleon at LO
q(x)
f1q (x)
unpolarised DF
quark with momentum xP in a nucleon
well known – unpolarised DIS
vector charge
Dq(x)
g1q(x)
helicity DF
quark with spin parallel to the nucleon spin in a longitudinally polarised nucleon
known – polarised DIS
 axial charge
DTq(x) = q↑↑(x) - q↑↓(x)
h1q(x),
transversity DF
quark with spin parallel to the nucleon spin in a transversely polarised nucleon
 tensor charge
largely unknown
ALL 3 OF EQUAL IMPORTANCE

29. Misura di ΔTq(x) Chiral-odd: requires another chiral-odd partner
Inclusive DIS
ppl+l-X
lpl’hX
impossible
direct
measurement
ΣΔTq(x) ·ΔTq(x)
convolution with
spin dependent
fragment. func.
lpl’h1h2X
lpl’ΛX
SIDIS (e.g. COMPASS and HERMES )
Hard scattering (e.g. RHIC)
Drell-Yan
Single Spin Asym (e.g. p↑p→πX)
Hard scattering (e.g. GSI)

30. Bakker, Leader, Trueman, PRD 70 (04)
Transversity DF
q=uv, dv, qsea
quark with spin parallel to the nucleon spin in a transversely polarised nucleon
DTq(x) = q↑↑(x) - q↑↓(x)
h1q(x),
dq(x),
dTq(x)
Properties:
probes the relativistic nature of quark dynamics
no contribution from the gluons  simple Q2 evolution
Positivity: Soffer bound……………..
first moments: tensor charge……….
sum rule for transverse spin in Parton Model framework…………
it is related to GPD’s
is chiral-odd: decouples from inclusive DIS
Soffer, PRL 74 (1995)
Bakker, Leader, Trueman, PRD 70 (04)

31. SIVERS Mechanism
The Sivers DF is probably the most famous between TMDs…
gives a measure of the correlation between the transverse momentum and the transverse spin
Requires final/initial state interactions of the struck quark with the spectator system and the interference between different helicity Fock states to survive time-reversal invariance
Time-reversal invariance implies:
…to be checked
In SIDIS:

32. Global Fits

33. Global Fit

34. First Extraction of DTq
HERMES, COMPASSBELLE

35. Global Analysis
Stefano Melis DIS

36. Sivers PDF

37. 2007 Transverse data taking statistics
2007 Compass Data taking
Begin of run: 18 May 2007
End of run: November 2007
Split between transverse and longitudinal target polarization:
- m on tape for transverse (40.0 x1012)
- m on tape for longitudinal (41.5 x1012)
For the extraction of the asymmetries (this analysis) only a fraction of the full statistic available has been used : nearly 20% of the whole data sample available.
Data taking
Period
Target
Polarization
Target Polarization
Week 25
- + -
Week 30
+ - +
Week 41
Week 26
Week 31
Week 42a
Week 27
Week 39
Week 28
Week 40
Week 43