1. Experimental review of DG Alain Magnon DAPNIA/SPhN (CEA Saclay) & COMPASS (CERN)
A.Magnon
Alain Magnon

2. Content The nucleon spin problem; why measure DG ?
Present knowledge of DG
Ongoing experiments :
RHIC spin, HERMES, COMPASS
Summary
A.Magnon
Alain Magnon

3. Quark & gluon spin Nucleon spin: ½ = ½ ΔΣ + ΔG +<Lz >
contribution
gluon
contribution
Orbital angular
momentum
Naive quark parton model + relativistic corrections
DS ~ 0.60
QCD ; Ellis-Jaffe assuming Ds = 0 ,
A.Magnon
Alain Magnon

4. Spin structure function g1
Various pQCD global analysis of world
data on g1 ( p, d, n ) with different
assumptions and parametrisations
Need knowledge of F & D meson octet
decay constant & assumption of SU(3)F
 “DS” ~
Quark contribution to spin DS small,
very different from expectations ~0.6
EMC  “DS” = / /- .14
xBJ
A.Magnon
Alain Magnon

5. (*)Scheme dependent, Q2 dep. omitted
Interpretation
In polarised DIS, one measures flavor singlet axial matrix element
a0 = ΔΣ – (as/2p)nf ΔG(*)
All experiments (SLAC,EMC,SMC,
SLAC, HERMES) confirm:
a0 ~ 0.2 ÷ 0.3
Imperative to measure ΔG
2.5 ?
(*)Scheme dependent, Q2 dep. omitted
A.Magnon
Alain Magnon

6. Dq (DG) from inclusive data
Duv(x) and Ddv(x) well constrained
DG(x) not : shape and sum unknown
∫DG(x)dx
- SMC global QCD analysis
at Q02 = 1 GeV2 : /- 1.2
- Blümlein-Böttcher
at Q02 = 2 GeV2: /- 0.6 1
A.Magnon
Alain Magnon

7. Dq & DG from inclusive data
g1p
F2p
Need more precise data on g1(x,Q2)
and better coverage in x and Q2
Need direct measurements
A.Magnon
Alain Magnon

8. Direct Measurements p -> p HERMES, COMPASS e, m -> N
RHIC (polarized) collider
p -> p
HERMES, COMPASS
e, m -> N
A.Magnon
Alain Magnon

9. RHIC polarized protons collider
STAR
A.Magnon
Alain Magnon

10. ΔG/G – RHIC p p A.Magnon Alain Magnon dG in partonic scattering
Qg scatter to jets know quark pol from DIS all from pQCD
Other rare channels have advantages
But L vs time start with pi0s and jets
A.Magnon
Alain Magnon

11. pp -> π0/prompt g + X π0 prompt γ
Excellent agreement with NLO & NLL
calculation with collider data !
A.Magnon
Alain Magnon

12. ΔG - Phenix: ALL for π0
2003/4
Outlook 2005/7
A.Magnon
Alain Magnon

13. ΔG - Phenix: ALL for π0
Y. Fukao, spin2004
A.Magnon
Alain Magnon

14. ΔG – Star: ALL for jets (2005)
0.2
0.4
0.6
0.8
1.0
0.0 gg qq qg
Inclusive Jets :LO
W. Vogelsang
2005
pT (GeV)
A.Magnon
Alain Magnon

15. ΔG at RHIC – summary & outlook
Various channels, large coverage in xg
- Jet/leading hadron production :
First results of ALL pp p0 +X from PHENIX
Very good perspective for p+- at PHENIX and jets at STAR
- Direct photon production:
hard scale 100 GeV 2, xg reconstructed in STAR
- Heavy flavours, ...
RHIC Spin program on track, major effort to reach performance
Run s1/2(GeV) Pbeam L(pb-1)
>
>
A.Magnon
Alain Magnon

16. ΔG/G from Photon-Gluon Fusion
Strategies to suppress background:
1) Open charm production:
q = c unique, no background
charm fragmentation, 1.2 D0 per event
D0 -> K- π+ (BR 4%), D*+(~20%) -> D0 π+
2) High-PT hadron pair production: q=u,d,s
skT(intr) = 450 MeV/c
spT(frag) = 350 MeV/c
pQCD scale set by:
1) ŝ > 4mc2 or
2) ŝ > (pt1+pt2)2
Photon-Gluon Fusion
A.Magnon
Alain Magnon

17. ΔG/G from high-PT hadron pair production
e, m h1 h2 N
Photon Gluon Fusion
Leading Order Process
QCD-Compton
measured
aLL : calculable partonic asymmetries
R : Monte-Carlo (Lepto/Phytia depending on Q2)
A.Magnon
Alain Magnon

18. RHIC Spin Roadmap HERMES & DESY 27.5 GeV e Pure atomic H or D targets
Spectrometer with complete
hadron identification
A.Magnon
Alain Magnon

19. HERMES ALL from high PT hadrons
Proton target
Statistics decreases with pT
Need good description of
background (also polarized)
A.Magnon
Alain Magnon

20. ΔG/G from HERMES 27.5 GeV e on p <xg> = 0.17
<Q2> = 0.06 (GeV/c)2
<p2T> = 2.1 (GeV/c)2
Promising channel, but …
theoretical uncertainty
might be large
A.Airapetian et al.
A.Magnon
Alain Magnon

21. LHC COMPASS SPS COMPASS EXPERIMENT
Beam: µ+/ spill (4.8s / 16.2s)
Luminosity: ~ cm-2 s-1
Beam polarisation: 76%
Beam momentum: GeV/c
LHC
COMPASS
SPS
A.Magnon
Alain Magnon

22. Spectrometer 2002 -> 2004 trigger-hodoscopes SPS 160 GeV
DW45
straws
SM2 dipole
Muon-filter2,MW2
RICH_1
HCAL1
Gem_11
SM1 dipole
ECAL2,HCAL2
Polarised Target
MWPC Gems Scifi
SPS 160 GeV
2.108m/spill
Muon-filter1,MW1
Veto
straws,MWPC,Gems,SciFi
Gems,SciFi,DCs,straws
Silicon
SciFi
Micromegas,DC,SciFi
A.Magnon
Alain Magnon

23. Inclusive longitudinal spin asymetry A1d
COMPASS has the potential to digest high luminosity and get high precision results
A.Magnon
Alain Magnon

24. Inclusive longitudinal spin asymetry A1d
Low x region important for precision on DS
A.Magnon
Alain Magnon

25. Pairs of hadrons with high pT
Photon Gluon Fusion
current fragmentation:
event cuts:
xF > 0.1
z > 0.1
2 high pt hadrons:
Q2 > 1 (GeV/c)2 , xBj < 0.05
0.1 < y < 0.9
pt > GeV/c
pt12+pt22 > 2.5 (GeV/c)2
m(h1h2) > GeV/c2
A.Magnon
Alain Magnon

26. Result for high pT asymmetry
Asymmetry for high-pt hadron pairs at Q2 > 1 (GeV/c)2:
2002/2003 data
Aγ* d = ± (stat.) ± (syst.)
Systematic uncertainty contains contributions from:
- target and beam polarisation measurement (5% resp. 3%)
- upper limit on false asymmetries (0.013)
- determination of depolarisation and dilution factors, D and f (5% each)
A.Magnon
Alain Magnon

27. Preliminary determination of ΔG/G
cut on xBj < 0.05:
Relative Fractions determined by Monte Carlo (LEPTO)
Photon Gluon Fusion
QCD-Compton
Leading Order
A.Magnon
Alain Magnon

28. Data Monte Carlo comparison
Use LEPTO Monte Carlo for Q2>1 (GeV/c)2 including RADGEN
Determination of the
fraction of PGF events
in LEPTO:
σPGF/σtot = 0.34 ± 0.07 (syst.)
Variation of the cut on pt2
Standard and modified* set of
fragmentation parameters
* SMC, B. Adeva et al., Phys. Rev. D70:012002, 2004
A.Magnon
Alain Magnon

29. COMPASS result for ΔG/G
ΔG/G = ± 0.31stat. ± 0.06syst.
2002/03 data
Q2 > 1 (GeV/c)2
at <xg> = 0.13 ± 0.08
10% of
statistics
A.Magnon
Alain Magnon
GSA,B,C: T. Gehrmann and W. Sterling, Z. Phys. C65 (1994) 461.

30. Q2 < 1 GeV/c2, additional background
for all Q2 : 10 times more data
Projection for data (all Q2):
Statistical error on
ΔG/G of 0.05
Relevant for Q2< 1 (GeV/c)2, uncertainty due to
unknown spin content of the resolved photon
A.Magnon
Alain Magnon

31. ΔG/G from open charm c D0 K- π+ (BR 4%) D*+ D0 π+ + c.c.
Special trigger: quasi real
photoproduction
charm contribution to the nucleon wave function is small
DsPGF at NLO:
Bojak, Stratmann NPB 540 (1999) 345; A.P. Contogouris and G. Grispos, Phys. Rev. D 62 (2000).
ŝ = m2cc
A.Magnon
Alain Magnon

32. ΔG/G from open charm Asymmetry determination with full 2002-2004
statistics:
Expected error on
ΔG/G from
data: 0.24
A.Magnon
Alain Magnon

33. Expected errors on ΔG/G
COMPASS Summary
First COMPASS result for
high pt hadron pairs with
Q2 > 1 (GeV/c)2
more data in 2004
10 times more data at Q2<1 (GeV/c)2
Good perspectives for
ΔG/G from open charm by
D0 mesons
COMPASS will resume data
taking 2006 and continue
until ≥ 2010
German-Sterling parametrization
A.Magnon
Alain Magnon

34. Future Fixed Target Programme at CERN
RHIC Spin Roadmap
Future Fixed Target Programme at CERN
Villars meeting, Sept.
CERN seminar Villars by John DAINTON
“ FT hadron program remains very competitive
…………………….
COMPASS complete in medium term
- ΔG/G, transversity, polarisability, spectroscopy
- SPSC p.o.t. concern  prioritise
COMPASS longer term
- GPD measurements would be unique”
Conclusions (personal) by Michelangelo MANGANO
“ COMPASS m beam … flagship experiment ”
A.Magnon
Alain Magnon

35. RHIC Spin Roadmap Summary Very exciting times ahead of us !
Two new facilities are operationnal
RHIC & COMPASS
First COMPASS result for DG/G from
high pt hadron pairs with Q2 > 1 GeV/c2
Excellent prospects for RHIC and COMPASS
Very exciting times ahead of us !
A.Magnon
Alain Magnon

36. RHIC Spin Roadmap
Additional slides
A.Magnon
Alain Magnon

37. Expected errors on ΔG/G
COMPASS preliminary,high PT, Q2 >1 (GeV/c)2
RHIC
error & 1/(LP4)1/2
proposal L=320 pb-1 P=70%
A.Magnon
Alain Magnon