1. TRANSVERSE SPIN EFFECTS
COMPASS
measurements of
TRANSVERSE SPIN EFFECTS
Franco Bradamante
Trieste University and INFN
on behalf of the Collaboration
IWHSS 09
International Workshop on Hadron Structure and Spectroscopy
March 30–April 1, 2009, Schloss Waldthausen, Mainz

2. OUTLOOK the COMPASS experiment results from 6LiD data
unpolarised azimuthal asymmetries
Collins asymmetry
Sivers asymmetry
other TMD asymmetries
first results from NH3 data
Collins and Sivers asymmetries
conclusions
Collins and Sivers mechanisms have been firmly established
IWHSS09
F. Bradamante

3. OUTLOOK the COMPASS experiment results from 6LiD data
unpolarised azimuthal asymmetries
Collins asymmetry
Sivers asymmetry
other TMD asymmetries
first results from NH3 data
Collins and Sivers asymmetries
conclusions
Collins and Sivers mechanisms have been firmly established
IWHSS09
F. Bradamante 3

4. OUTLOOK the COMPASS experiment results from 6LiD data
unpolarised azimuthal asymmetries
Collins asymmetry
Sivers asymmetry
other TMD asymmetries
first results from NH3 data
Collins and Sivers asymmetries
conclusions
Collins and Sivers mechanisms have been firmly established
IWHSS09
F. Bradamante 4

5. Unpolarised Target SIDIS Cross-Section
A Bacchetta, M Diehl, K Goeke, A Metz, P Mulders, M Schlegel (06)
3 independent azimuthal modulations in h , the hadron azimuthal angle in GNS
pQCD contributions expected to be important at pt > 1 GeV/c
IWHSS09
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6. Unpolarised Target SIDIS Cross-Section
Cahn effect + Boer-Mulders DF
Boer- Mulders x Collins FF + Cahn effect
IWHSS09
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7. Unpolarised Target SIDIS Cross-Section
Cahn effect kinematical effect due to quark intrinsic momentum
Boer- Mulders DF leading order DF
quark with spin parallel to the nucleon spin in an unpolarised nucleon
Cahn effect + Boer-Mulders DF
Boer- Mulders x Collins FF + Cahn effect
IWHSS09
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8. Unpolarised Azimuthal Asymmetries
data sample: part of the 2004 data collected with L and T target polarisation
with both target orientation configurations to cancel possible polarisation effects
event selection:
Q2>1 (GeV/c)2
0.1<y<0.9
W>5 GeV/c2
0.2 < z < 0.85
0.1 < pT < 1.5 GeV/c
final statistics:
IWHSS09
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9. Unpolarised Azimuthal Asymmetries
to extract the asymmetries
the azimuthal distributions have to be corrected by the apparatus acceptance  dedicated MC simulations for L and T target polarisation data
acceptance
initial azimuthal distribution
final azimuthal distribution
IWHSS09
F. Bradamante 9

10. Unpolarised Azimuthal Asymmetries
to extract the asymmetries
the azimuthal distributions have to be corrected by the apparatus acceptance  dedicated MC simulations for L and T target polarisation data
acceptance
final azimuthal distribution
the final azimuthal distributions are fitted with the function:
IWHSS09
F. Bradamante

11. Unpolarised Azimuthal Asymmetries
systematic errors evaluated from:
compatibility of results with L and T target polarization (different experimental conditions, different MCs)
comparison of results obtained using two 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 target polarizations
different geometrical regions for the scattered muon
most important contribution
IWHSS09
F. Bradamante

12. Unpolarised Azimuthal Asymmetries
sin modulation
error bars: statistical errors bands: systematical errors
IWHSS09
F. Bradamante

13. Unpolarised Azimuthal Asymmetries
cos modulation
IWHSS09
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14. Unpolarised Azimuthal Asymmetries
cos modulation
comparison with theory
IWHSS09
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15. Unpolarised Azimuthal Asymmetries
cos2 modulation
IWHSS09
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16. Unpolarised Azimuthal Asymmetries
cos2 modulation
comparison with theory
IWHSS09
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17. Unpolarised Azimuthal Asymmetries
summary
positive hadrons
negative hadrons
error bars: statistical errors only
IWHSS09
F. Bradamante

18. OUTLOOK the COMPASS experiment results from 6LiD data
unpolarised azimuthal asymmetries
Collins asymmetry
Sivers asymmetry
other TMD asymmetries
first results from NH3 data
Collins and Sivers asymmetries
conclusions
Collins and Sivers mechanisms have been firmly established
IWHSS09
F. Bradamante 18

19. Transversity Distribution Function
is chiral-odd: observable effects are given only by the product of Tq (x) and an other chiral-odd function
can be measured in SIDIS on a transversely polarised target
via “quark polarimetry”
“Collins” asymmetry
“Collins” Fragmentation Function
“two-hadron” asymmetry
“Interference” Fragmentation Function
Λ polarisation
Fragmentation Function of q↑Λ
all explored in COMPASS I will concentrate on the first
IWHSS09
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20. Collins Asymmetry
Collins effect  azimuthal distribution of the hadrons produced in C
“Collins angle”
C = h - s’ = h + S - p
h,s’,S azimuthal angles of hadron momentum, of the
spin of the fragmenting quark and of the nucleon in the GNS
± refer to the opposite orientation of the transverse spin of the nucleon
PT is the target polarisation; DNN is the transverse spin transfer coefficient initial  struck quark
from the azimuthal distribution
of the hadrons one measures
the “Collins Asymmetry”
IWHSS09
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21. Collins Asymmetry
using different targets (p, d, n) and identifying the final hadron one can perform flavour separation
unique feature of SIDIS
has been measured by the HERMES experiment on proton
different from zero for charged hadrons
of opposite signe for positive and negative charge particles
from the azimuthal distribution
of the hadrons one measures
the “Collins Asymmetry”
IWHSS09
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22. Collins Asymmetry - Deuteron data
charged hadrons
2004: first results from 2002 data [PRL94 (2005) ] confirmed in
2006: final results from data [NPB765 (2007)31]
asymmetries compatible with zero within the statistical errors (syst. errors much smaller)
the same with leading h
with HERMES results, cancellation between u and d quark contributions
IWHSS09
F. Bradamante

23. Collins Asymmetry - Deuteron data
identified hadrons
2007: final results from data [PLB 673 (2009) 127]
again, difficult to see a signal …
IWHSS09
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24. Collins Asymmetry - Fits to Data
new results
using
HERMES (p) and
COMPASS (d)
pion data, and
BELLE data
IWHSS09
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25. Collins Asymmetry - Fits to Data
new results
using
HERMES (p) and
COMPASS (d)
pion data, and
BELLE data
IWHSS09
F. Bradamante

26. OUTLOOK the COMPASS experiment results from 6LiD data
unpolarised azimuthal asymmetries
Collins asymmetry
Sivers asymmetry
other TMD asymmetries
first results from NH3 data
Collins and Sivers asymmetries
conclusions
Collins and Sivers mechanisms have been firmly established
IWHSS09
F. Bradamante 26

27. Sivers Asymmetry the “Sivers DF” the most famous of the TMD parton DF
appears in SIDIS as a modulation in the “Sivers angle” S = h - S
h azimuthal angle of hadron momentum
S azimuthal angle of the spin of the nucleon
the “Sivers angle” S and the “Collins angle” C are independent
 the Collins and Sivers asymmetries can be disentangled and extracted from the same data in SIDIS on a transversely polarised target
the “Sivers DF”
the most famous of the TMD parton DF
J. Collins, Nucl. Phys. B396 (1993) 161
F. Baldrachini, N.S. Craigie, V. Roberto, M. Socolovsky, Fortschritte der Physik 29 (1981) 505
the Sivers asymmetry
has been measured by the HERMES experiment on proton
different from zero for positive charge particles
compatible with zero for negative charge particles
IWHSS09
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28. Sivers Asymmetry - Deuteron data
charged hadrons
2004: first results from 2002 data [PRL94 (2005) ] confirmed in
2006: final results from data [NPB765 (2007)31]
asymmetries compatible with zero within the statistical errors
(systematic errors much smaller)
cancellation between u and d quark contributions in the dueteron
IWHSS09
F. Bradamante

29. Sivers Asymmetry - Deuteron data
identified hadrons
2007: final results from data [PLB 673 (2009) 127]
IWHSS09
F. Bradamante

30. Sivers Asymmetry - Fits to Data
new results using HERMES (p) and COMPASS (d) pion and kaon data
IWHSS09
F. Bradamante

31. OUTLOOK the COMPASS experiment results from 6LiD data
unpolarised azimuthal asymmetries
Collins asymmetry
Sivers asymmetry
other TMD asymmetries
first results from NH3 data
Collins and Sivers asymmetries
conclusions
Collins and Sivers mechanisms have been firmly established
IWHSS09
F. Bradamante 31

32. SIDIS cross-section 18 structure functions 8 modulations Sivers
A Bacchetta, M Diehl, K Goeke, A Metz, P Mulders, M Schlegel (06)
Sivers
Collins
8 modulations
(4 LO)
all measured by COMPASS on deuteron
IWHSS09
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33. again cancellation between the u and d quarks in the deuteron ?
one example …
beam polarization
g1T is chiral-even, T-even, leading twist
again cancellation between
the u and d quarks in the deuteron ?
all these asymmetries are compatible with zero; same result for charged pions and kaons
IWHSS09
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34. Conclusions from COMPASS Deuteron Data
the unpolarized azimuthal asymmetries are different from zero and different for positive and negative hadrons
all the transverse spin asymmetries have been measured to be compatible with zero
from HERMES, COMPASS, and BELLE data a consistent picture
first fits
extractions of the transversity and Sivers DFs and Collins FF
predictions
still, a lot of expectation for the higher energy COMPASS proton data
IWHSS09
F. Bradamante

35. OUTLOOK the COMPASS experiment results from 6LiD data
unpolarised azimuthal asymmetries
Collins asymmetry
Sivers asymmetry
other TMD asymmetries
first results from NH3 data
Collins and Sivers asymmetries
conclusions
Collins and Sivers mechanisms have been firmly established
IWHSS09
F. Bradamante 35

36. 2007 run: transversely polarized NH3 target
data taking: May to November equally shared between transverse and longitudinal
transverse polarization data taking:
Period
“Weeks”
Target Polarization 1
25 / 26 / 2
27 / 28 3
30 / 31
+ - + / 4
39 / 40 5
41 / 42a 6
42b / 43
data used for these results
several stability tests have been performed
detectors and triggers performances, event reconstruction, K0 reconstruction,
distributions of kinematical variables (zvtx, Em’, fm’, xBj Q2,y, W, Ehad, fhadLab ,
qhadLab , fhadGNS , qhadGNS , pt)
~20% of the total collected data has been used
IWHSS09
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37. Collins asymmetry – proton data
integrated over x and z
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
statistical errors only; systematic errors ~ 0.3 sstat
IWHSS09
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38. Collins asymmetry – proton data
comparison with M. Anselmino et al. predictions
IWHSS09
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39. Sivers asymmetry – proton data
statistical errors only; systematic errors ~ 0.5 sstat
the measured symmetries are small, compatible with zero
IWHSS09
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40. Sivers asymmetry – proton data
comparison with the most recent predictions from M. Anselmino et al.
IWHSS09
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41. Sivers asymmetry – proton data
comparison with predictions from
S.Arnold, A.V.Efremov, K.Goeke, M.Schlegel and P.Schweitzer arXiv:
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42. CONCLUSIONS new COMPASS results for
unpolarised hadron asymmetries on deuteron for positive and negative hadrons
Collins and Sivers asymmetries on protons
Collins asymmetry different from zero the effect is there at COMPASS energies!
Sivers asymmetry: smaller, compatible with zero to be understood
near future: analysis of the whole 2007 proton data sample
longer term: transverse spin physics is one of the items being spelled out for the future COMPASS program
the study of transverse spin effects needs further precise measurements and the COMPASS facility is the only place where SIDIS can be measured at high energy
IWHSS09
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43. CONCLUSIONS one full year of data taking (>2009)
the study of transverse spin effects needs further precise measurements and the COMPASS facility is the only place where SIDIS can be measured at high energy
IWHSS09
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44. THANK YOU !

45. COMPASS

46. COMPASS fixed target experiment at the CERN SPS
broad physics programme
data taking since 2002:
muon beam
deuteron (6LiD) polarised target
2002
2003
2004
L/T target polarisation
4:1
2006
L target polarisation only
proton (NH3) polarised target
2007
L /T target polarisation
1:1
hadron beam
LH target
2008
muon beam: 160 GeV/c longitudinal polarisation -80%
intensity 2·108 µ+/spill (4.8s/16.2s)

47. COMPASS high energy beam large angular acceptance
broad kinematical range
two stages spectrometer
Large Angle Spectrometer (SM1)
Small Angle Spectrometer (SM2)
variety of tracking detectors to cope with different particle flux from θ = 0 to θ ≈ 200 mrad
SciFi
Silicon
Micromegas
GEMs
Straws
SDC
MWPC
W45
MuonWall
SM2
E/HCAL
E/HCAL
SM1
MuonWall
Polarised Target
calorimetry, PID
RICH
RICH detector
m beam 47

48. COMPASS 48 radiator C4F10 threshold: p ~2 GeV/c K ~ 10 GeV/c
Polarised Target
RICH
RICH detector
m beam 48

49. COMPASS
Polarised Target
m beam 49

50. The Target System solid state target operated in frozen spin mode
: 6LiD (polarised deuteron)
dilution factor f = 0.38
polarization PT = 50%
two 60 cm long cells
with opposite polarisation (systematics)
2006:
PTM replaced with the large acceptance COMPASS magnet (180 mrad)
2 target cells  3 target cells
2007: NH3 (polarised protons)
dilution factor f = polarization PT = 90%
during data taking with transverse polarisation,
polarisation reversal in the cells after ~ 4-5 days

51. SIDIS event selection and kinematics
data
DIS cuts:
Q2>1 (GeV/c)2
0.1<y<0.9
W>5 GeV/c2
hadrons
energy deposit in HCALs>Thr. ( ~5 GeV )
pT>0.1 GeV/c
z>0.2 (all), z>0.25 (leading)

52. Sivers Asymmetry
IWHSS09
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53. Collins Asymmetry
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54. unpolarised target SIDIS cross-section
Collins and Sivers mechanisms have been firmly established
EMC, J.J. Aubert et al., Phys. Lett. 130B (1983) 118
Charged hadrons, Muons of energy 120 and 280 GeV, hydrogen target
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55. COMPASS proton data
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56. SIDIS kinematics
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57. SIDIS kinematics
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58. Collins asymmetry for pions and kaons
preliminary
data
proton
(  DIS07)
data
deuteron
COMPASS
sign convention
IWHSS09
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59. Sivers asymmetry for pions and kaons d data
preliminary
data
proton
(  DIS07)
data
deuteron
COMPASS
sign convention
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60. Collins asymmetry– deuteron h± data
preliminary
naïve interpretation of the data (parton model, valence region)
proton data
 unfavored Collins FF ~ – favored Collins FF
at variance with unpol case
u quark dominance (d quark DF ~ unconstrained)
deuteron data
refs!
some (small) effect expected even if
 cancellation between Tu (x) and Td (x) access to Td (x)
IWHSS09
F. Bradamante 60

61. Sivers asymmetry
naïve interpretation of the data (parton model, valence region)
proton data
asymmetry for p+ > 0, asymmetry for p- ≈ 0
 Sivers DF for d-quark ≈ - 2 Sivers DF for u-quark 2
deuteron data
preliminary
refs!
the measured asymmetries compatible with zero suggest
IWHSS09
F. Bradamante 61

62. Extraction of the asymmetries
the Collins and Sivers asymmetries can be disentangled and extracted from the same data in SIDIS on a transversely polarised target
data NPB765(2007)31
to extract the Collins and Sivers asymmetries
the “ratio product” quantities
measured in each angular bin
-reduces acceptance variation effects
- at first order, spin independent effects cancel out
are fitted separately with the functions
check: fit of
with
(“2D fit”)
J. Collins, Nucl. Phys. B396 (1993) 161
F. Baldrachini, N.S. Craigie, V. Roberto, M. Socolovsky, Fortschritte der Physik 29 (1981) 505
confirmed by 8 parameter fit
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F. Bradamante 62

63. Collins Fragmentation Function
measurable in e+e– annihilation from the correlation between the azimuthal angles of p’s
first attempts to measured it from e+e– annihilation using LEP data
in the last years BELLE measurements
the asymmetries
are different from
zero and the
Collins FF
can be measured
spin2004
ref?
IWHSS09
F. Bradamante 63

64. L polarimetry systematic errors not larger than statistical errors
RICH ID not used yet; someother improvement in selection still foreseen
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65. two-hadron asymmetries
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66. two-hadron asymmetries
data
all +/- pairs x
preliminary
data
all +/- pairs
deuteron target
deuteron COMPASS kin
also the two hadron asymmetry ~ 0
HERMES: positive
similar results
with identified hadrons
ordering in pTh, z
A. Bacchetta, M. Radici
PRD74(2006) model for DiFF
IWHSS09
F. Bradamante 66

67. two-hadron asymmetries
identified hadrons on the deuteron target
p+p–
K+p–
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68. two-hadron asymmetries
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69. Transversity Distribution Function
three quark Distribution Functions are necessary to describe the structure of the nucleon at leading order
q(x)
f1q (x)
q=uv, vv, qsea
unpolarised DF
quark with momentum xP in a nucleon
well known – unpolarised DIS
Dq(x)
g1q(x)
helicity DF
quark with spin parallel to the nucleon spin in a longitudinally polarised nucleon
known – polarised DIS
DTq(x) = q↑↑(x) - q↑↓(x)
h1q(x),
dq(x),
dTq(x)
transversity DF
quark with spin parallel to the nucleon spin in a transversely polarised nucleon
largely unknown
ALL 3 OF EQUAL IMPORTANCE
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70. unknown for several years …
Collins asymmetry
asimmetry in the azimuthal distribution of the hadrons produced in on transversely polarised nucleons
Collins effect (J. Collins, 93)
a quark moving “horizontally” and polarized “upwards” would emit the leading meson preferentially on the “left” side of the jet
 the fragmentation function has a spin dependent part
“Collins Fragmentation Function”
the chiral-odd partner of the transversity DF in SIDIS
unknown for several years …
measurable in e+e– annihilation from the correlation between the azimuthal angles of p’s
first attempts using LEP data; in the last years BELLE measurements: the asymmetries are different from zero and the Collins FF can be measured
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71. Transversity Distribution Function
DTq(x)
contribution of the quarks to the transverse spin of the nucleon
properties:
is chiral-odd: decouples from incl DIS because helicity of quark must flip
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
Bakker, Leader, Trueman, PRD 70 (04)
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