How to proceed to get GPDs

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How to proceed to get GPDs from the Transverse
Target Spin asymmetry on the ?0 production
Nicole dHose Mehdi Nehmé (for the L/T
separation and use of the VGG
code) CERN, 2 August 2007
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The experiment on Li6D
TTSA measured on coherent D incoherent p or n
non-exclusive bkgd
coh
semi-excl
incoh
0.01 lt PT2 lt 0.5 296 615 evts 0.1 lt PT2 lt
0.5 109 244 evts coherent 3
incoherent 70 non exclusive 27
? Andrzejs presentation
? Guillaumes present study
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The theory for meson production

• 1- Factorization for longitudinal photons only
• Suppression of transverse component ?T/?L
  1/Q2
• For COMPASS kinematics ltQ2gt2GeV2 R
  ?L/?T 1
• 2- Coherent estimation ? Pire,Cano, Strikmann?
  
• Incoherent estimation ? Kroll, Goloskokov
  (quark and gluon contribution)
• Guzey (quark
  and gluon contribution)
• VGG (mainly
  quark contribution)
• and proton neutron contribution ?

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quark and gluon enter at the same order of ?S
VGG
quarks
gluons
Diehl and Vinnikov (hep-ph/0412162) Evidence
for gluon contribution at xBj0.1
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modelisation of GPD in the VGG code
Factorization H(x,?,t) q(x) F(t) Or
Regge-motivated t-dependence H(x,0,t)
q(x) e t ltb?2gt q(x) / xat (aslope Regge
traject.)
the GPD E is related to the nucleon helicity
flip
E
known Hq (x,0,0) q(x) unknown Eq
(x,0,0) eq(x)Aqqval (x) Bq?(x) (based on
chiral soliton)
2 sum rules
?q ? eq (x) dx
2Jq ? x (q (x) eq (x) ) dx
? Aq and Bq are functions of Ju and Jd
? Eu - Ed Eg 0
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Definition of the asymmetry
Trento convention
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Diehl and Sapeta (2005) X-section decomposition
in terms of with photon helicity m,n0,?1 and
proton spin state i,j ? ½
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The polarized cross section
terms depending on the lepton polarisation
Plepton
Leading Terms suppressed by 1/Q by 1/Q2
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The polarized cross section and the measured
asymmetry
.
as sin?? very small
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The asymmetries
TTSA measured experimentally
TTSA estimated theoretically
? Use of the angular distribution of the rho
decay SCHC
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Diehl and Sapeta (2005)
a cos2? b
c cos2? d
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We have measured
other experiments NMC, E665
bin1 bin2
cos?L
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c cos2? d
c (2.25 r -0.75) ?tot
R?L/?T
d 0.75 (1-r) ?tot
At Q21.9 GeV2 R0.98?0.13
r0.46 ?0.03 ?tot 285 ? 106 nb
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c(r,?tot) d(r,?tot) measured
is determined and is independent of ?tot
precision on as a function on cos?L limit
between the 2 bins
?A0.065
?A0.045
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Results for for COMPASS Li6D and
comparison with VGG prediction for the proton
0.01GeV2ltPT2 full stat 0.1GeV2lt PT2
incoherent pn 2 bins in x or 2 bins in t
Ju0 0.2 0.4
Ju0 0.2 0.4
Jd0
Jd0
Ju0 0.2 0.4
Ju0 0.2 0.4
Jd0
Jd0
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Comparison full stat (released results) and
reduced sample for incoherent production
0.1 lt PT2 incoherent pn
?A0.09
0.01 lt PT2 coherent D incoherent pn
?A0.04
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Comparison COMPASS / HERMES
Ju0 0.2 0.4
Ju0 0.2 0.4
ltQ2gt1.9 GeV2
ltQ2gt1.9 GeV2
Jd0
Jd0
lt-tgt0.24GeV2
ltxgt0.03
DIS07
not the same scale about the same
statistical errors
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Just for an exercise NO MORE!!!
We compare our data on pn with the VGG code
available only for the proton!!!
COMPASS ?
HERMES DVCS p
Jlab DVCS n
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Estimation for a proton neutron target
and 54
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• In the future
• VGG code improvement
• proton and neutron contributions
• Quark and gluon contributions
• Predictions from Kroll
• Guzey

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Global analysis of the angular distributions ?
Need of a complete MC for acceptance correction