1. Longitudinal spin asym. and DG at COMPASS
JM Le GOFF, DAPNIA/Saclay
orbital momentum
quarks
gluons
valance
sea
HEP 2007, Manchester

2. The COMPASS experiment
≈ 240 physicists from 25 institutes
Polarized muon beam
Long. polarized target (& Transv.)
Quark polarization
g1, DS, Dq, valance and sea
Gluon polarization
Transversity
Production of r, F, J/Ψ, L, ...
Hadron beam
Primakoff: p, K polarizabilities
see A. Guskov presentation
Exotics q-states, glue balls
Double charmed hadrons
project:
Drell-Yann for transversity
project:
LH2 target + proton recoil det.
Generalized Parton Distributions

3. COMPASS at CERN M2 beam line Longitudinally polarized muons 160 GeV/c
2.108 m/ spill (4.8s / 16.8s)
PB= -80%
Longitudinally (or transversely) polarized deuteron target :
6LiD PT ~ 50%
Luminosity: ~ cm-2s-1
LHC
COMPASS at CERN
M2 beam line
SPS

4. COMPASS Spectrometer Q2>1 50 m NIM A 577(2007) 455
Trigger Hodoscopes
Q2>1
ECALs & HCALs
MWPCs
SM2
RICH
50 m
SM1
Drift Chambers
m Filters/Walls
6LiD target
Straws
GEMs Si
Micromegas
160 GeV m
NIM A 577(2007) 455
SciFi
veto

5. Spin asymmetry extraction
2-cell target  
every
8 hours
upstream
downstream  
weighted method:
gain in FoM:

6. (semi-)inclusive analysis

7. Inclusive asymmetries, md  m'X
Q2>1
PLB647(2007)8
good agreement with previous exp.
factor 4 gain in stat at low x
compatible with zero at low x
Q2<1 see PLB647(2007)330

8. g1 and contrib. to nucleon spin
G1=∫g1(x)dx has only 2% correction from unmeasured low x
(was a 50% negative correction before)
G1  DS=0.35±0.03±0.05 and Ds=-0.08±0.01±0.02 (CMPS alone)

9. QCD analysis DGLAP equations at NLO
Disagreement of new data with previous QCD fits
 new fit : 2 solutions, DG>0 and DG<0, |DG|≈

10. Semi-inclusive asym, m d  m' h X
separate valance and sea

11. Difference charge asym.
LO:

12. polarized valance quark distrib
for x>0.3
sea << valance
g1d(x)
Duv(x)+Ddv(x)

13. Moments if then Duv+Ddv=a8=0.58±0.03 Duv+Ddv=0.41±0.07±0.05 2 s below
Duv+Ddv G1 and a8
 favors asymmetric sea
Ds+Ds=-0.08±0.01±0.02 (inclusive)

14. Gluon polarization

15. Photon Gluon Fusion 2 signatures : qq=cc : open charm (D0)
- no physical bckg
- s small, BR(Kp)= 4%
 low stat
- scale mc2 (no Q2>1 cut)
high pT hadron pairs
- large stat
- phys bckg  needs MC
- scale pT2 (no Q2>1 cut)

16. D0 meson reconstruction
Thick target: no decay vertex
D0Kp mass reconstruction
 combinatorial bckg
kinematical cuts
RICH for particule id
D*  D0pslow
cut on M(Kpp)-M(Kp)

17. Extraction of DG/G Abckg measured to be zero outside D0 peak
event kin not fully
reconstructed 
parametrize aLL with NN

18. High pT hadron pairs + aLL : calculable partonic asymmetry
Photon
Gluon
Fusion
Leading
Order
QCD compton
Resolved g
Q2 < 1 (GeV/c)2
aLL : calculable partonic asymmetry
RPGF : Monte Carlo required for RPGF and Abckg

19. Contributions to cross-section (Q2<1)
Resolved photons

20. Data vs MC
excellent to good
agreement
for all variables

21. Systematic errors Dominant syst error due to MC
vary MC parameters in a range where data vs MC OK
largest effect for photon kT
nucleon
photon kT

22. Results for DG/G - open charm : DG/G=-0.57±0.41±0.17
- high pT Q2<1 : DG/G=-0.024±0.089±0.057
disfavors large values of DG (2-3 needed to get DS=a8)
PLB633(2006)25

23. Prospects all results presented 2002-04 (no data in 2005)
improved charm analysis with S/(S+B) in the weight
2006 data
- new PT magnet  larger acceptance
- improved RICH : faster electronics
PMs in the inner region
- 3-cell target kills false asym
2007 data on proton (NH3 target)
(not very good for DG since FoM=(fPT)2 reduced)
flavor separation in semi-inclusive analysis:
Duv and Ddv , Du and Dd

24. Improved D0 peak in 2006
2006
S/B improved by a factor ≈ 2

25. Conclusions
DS =0.30 ± 0.01 ± 0.02
error due to low x extrapolation strongly reduced
Ds+Ds=-0.08 ± 0.01 ± 0.02
Duv+Ddv =0.41 ± 0.07 ± 0.05
seems that Du+Dd ≈0
fit to inclusive data gives 2 solutions |DG|≈
direct measurement
DG not very large (anomaly contribution to a0 small)
but DG≈0.35 still possible (in which case Lz≈0)