1. SINGLE HADRON TRANSVERSE SPIN ASYMMETRIES FROM COMPASS
Franco Bradamante University of Trieste and INFN Trieste
on behalf of the COMPASS Collaboration
Kyoto, October 3, 2006

2. THE SPECTROMETER two stages spectrometer
Large Angle Spectrometer (SM1)
Small Angle Spectrometer (SM2)
tracking, calorimetry, PID
SciFi
Silicon
Micromegas
GEMs
Straws
SDC
MWPC
W45
MuonWall
SM2
E/HCAL
E/HCAL
SM1
Polarised Target
MuonWall
RICH
m beam
high energy beam
broad kinematical range
large angular acceptance
Franco Bradamante

3. solid target operated in frozen spin mode
superconductive
Solenoid (2.5 T)
3He – 4He Dilution
refrigerator (T~50mK)
Dipole (0.5 T)
two 60 cm long cells
with opposite polarization (systematics)
THE TARGET SYSTEM
solid target operated in frozen spin mode
dN/dz
2000
zvtx (mm)
-1000
1000
4000
THE RICH DETECTOR
5 m
6 m
3 m
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
Franco Bradamante

4. DATA TAKING
transversely polarised deuteron target ~ 20% of the running time
2002
11 days of data taking (19),
2 periods
2003
9 days of data taking (14),
1 period
2004
14 days of data taking (24),
Collins and Sivers asymmetries
published results 2002
prel results
with RICH PID
Two hadron asymmetries
Λ polarimetry
prel results
trigger (large x, Q2)
DAQ, on line filter
DIS
events
(106)
Franco Bradamante

5. SINGLE HADRON ASYMMETRIES
unpol B and T
transvely polarised
target
SIDIS cross-section
long pol B and/or T
h azimuthal angle of the hadron
s’ azimuthal angle of the transverse spin of the struck quark
s azimuthal angle of the transverse
spin of the initial quark
s’ = p - s FS
at leading order
Collins
Sivers
beam polarization
independent angles  independent extraction of
the asymmetries
Franco Bradamante

6. EVENT SELECTION DIS events Q2 > 1 GeV2 W2 > 25 GeV2
0.1 < y < 0.9
hadrons
pht > 0.1 GeV/c
z > (all h)
z > (leading h)
Q2 = 2.4 (GeV/c)2
W = 9.4 GeV/c2
xBj  0.035
2002 data
Franco Bradamante

7. increase in statistics by a factor of 7!
ASYMMETRIES vs 2002
results from data released in April (DIS2006)
comparison with the published 2002 data
PRL 94 (2005)
increase in statistics by a factor of 7!
Franco Bradamante

8. COLLINS ASYMMETRIES 2002-2004 small errors (~1%) small asymmetries
DIS2006
DIS2006
small errors (~1%)
small asymmetries
cancellation between p and n
(lh and ah)
Franco Bradamante

9. COLLINS ASYMMETRY results from model calculations:
good fits to the proton HERMES DATA with Collins FF: favoured ~ – unfavored, at variance with unpolarised case
comparison with BELLE DATA ok: universality of the Collins effect, hints on Tu
comparison with the deuteron COMPASS 2002 DATA ok Td ~ unconstrained
comparison with the new COMPASS data
Vogelsang and Yuan
preliminary COMPASS
Efremov et al
preliminary COMPASS
Anselmino et al, Tq = q
preliminary COMPASS
Anselmino et al, SB
preliminary COMPASS
agreement not so good as with COMPASS 2002 data
from a global fit of all the present data probably stronger constrains on the transversity PDFs and in particular on Td (x)
RICH + AGS annual Users’ Meeting June 6,
2006, ags/users meetings/workshops/basp , and paper in
preparation.
Franco Bradamante

10. SIVERS ASYMMETRIES 2002-2004 small errors (~1%) small asymmetries
DIS2006
DIS2006
small errors (~1%)
small asymmetries
cancellation between p and n
(lh and ah)
Franco Bradamante

11. What have we learnt over the past 2 years ? Sivers Asymmetry
comparison of the same calculations with the COMPASS data
preliminary COMPASS
preliminary COMPASS
Collins et al
Vogelsang and Yuan
preliminary COMPASS
as in the Collins case, agreement not always so good as for 2002 data
from a global fit of the present HERMES and COMPASS data probably stronger constrains on the Sivers DFs
RICH + AGS annual Users’ Meeting June 6,
2006, ags/users meetings/workshops/basp , and paper in
preparation.
Anselmino et al
Franco Bradamante

12. p±, K± ASYMMETRIES
same DIS event selection and hadron definition as before
plus
PID based on RICH response 3%
protons
12%
kaons
77%
pions 5%
no RICH information
100%
hadrons
after all cuts
final sample
positive
all
leading
pions
5.3 M
3.4 M
kaons
0.9 M
0.7 M
negative
all
leading
pions
4.2 M
2.8 M
kaons
0.6 M
0.4 M
additional systematic tests: RICH stability
 systematic errors smaller than statistical ones
Franco Bradamante

13. p±, K± SIVERS ASYMMETRIES 2003-2004
GPD2006
Franco Bradamante

14. p±, K± Collins Asymmetries 2003-2004
GPD2006
again, difficult to see an effect …
Franco Bradamante

15. SUMMARY New precise DIS deuteron data from COMPASS are now available
In all the channels investigated in so far
the measured deuteron asymmetries are very small, compatible with zero
Collins and Sivers asymmetries
for positive and negative hadrons, p±, K±
PRESENT PICTURE
Collins: u quark dominance DT0D(fav.) ~ - DT0D(unfav) for u quark
DTd from global analysis (NEW COMPASS DATA)
Sivers: D0Tu ~ - D0Td
Franco Bradamante

16. One of the most recent citations
COMPASS 2002 data
Evidence for the Absence of Gluon Orbital Angular Momentum in the Nucleon
S.J. Brodsky, S. Gardner
SLAC-PUB-12062, Aug Subm. Phys.Lett.B, hep-ph/
Due to the smallness of the anomalous magnetic moment of the
deuteron the orbital motion of the quarks should not contribute to
the Sivers SSA of the deuteron. Thus
Franco Bradamante

17. riserva
Franco Bradamante

18. COLLINS ASYMMETRY
important theoretical work on the interpretation of the data
assumptions on Tq and fit of HERMES data
to exctract the Collins FF
Vogelsang Yuan, PRD 72, (2005)
Soffer bound
Efremov et al, PRD 73, (2006)
chiral quark-soliton model
Anselmino et al, BNL UM 2006
Soffer bound
& Tq = q
RICH + AGS annual Users’ Meeting June 6,
2006, ags/users meetings/workshops/basp , and paper in
preparation.
Franco Bradamante

19. SIVERS ASYMMETRY
important theoretical work on the interpretation of the data
use of the HERMES data to exctract/check the Sivers DF
Vogelsang Yuan, PRD 72, (2005)
result: d SDF ~ -2 u SDF
Collins et al, PRD 73, (2006)
input: d SDF = - u SDF
Anselmino et al, PRD 72, (2005)
u SDF ~ - d SDF
fit using the
COMPASS 2002 data too
RICH + AGS annual Users’ Meeting June 6,
2006, ags/users meetings/workshops/basp , and paper in
preparation.
Franco Bradamante