Writing the proposal,

1
Writing the proposal, discussing with Alain,
some remarks about
the COMPASS goal,
the different predictions,
the existing experimental
results Nicole dHose
CERN 26 March 2008
2
In DVCS and meson production we measure Compton
Form Factor

For example at LO in ?S
DGLAP
t, ?xBj/2 fixed
q(x)
DGLAP
DGLAP
ERBL
3
1rst ultimate goal with GPD
3-dim picture of the partonic nucleon structure
or probability densities of quarks
and gluons in impact parameter space
H(x,?0,t-?2)
? measurement of d?/dt or IM(H), Re(H) via
VCS and BSA, BCA or Beam Charge and Spin
Difference
z
x P
b
y
x boost
4
2nd ultimate goal with GPD

• Contribution to the nucleon spin knowledge
• ½ ½ ?S ?G lt Lzq gt lt Lzg gt
• the GPDs correlation between the 2 pieces of
  information
• -distribution of longitudinal momentum carried
  by the partons
• -distribution in the transverse plane
• the GPD E is related to the angular
  momentum
• 2Jq ? x (Hq (x,?,0) Eq (x,?,0) ) dx
• ? with a transversely polarized target DVCS
  et MV
• ? with a deuterium or neutron target DVCS

E
5
GPDs in Lattice
From Schierholz, JLab May 2007
probability densities of quarks and gluons
in impact parameter space
6
Sensitivity to the 3-D nucleon picture
Lattice calculation (unquenched QCD) Negele et
al., NP B128 (2004) 170 Göckeler et al., NP B140
(2005) 399 ? fast parton close to the N
center ? small valence quark core ?
slow parton far from the N center ?
widely spread sea q and gluons
m?0.87 GeV
x
Last result on 29 May 2007 First comprehensive
full lattice QCD In the chiral regime with m?
0.35 GeV Hägler et al., hep-lat
07054295 MIT, JLab-THY-07-651,
DESY-07-077, TUM-T39-07-09
0.5 fm at small x
0.15fm at large x

7
Predictions in chiral dynamics
Strikman et al., PRD69
(2004) 054012 Frankfurt
et al., Ann. Rev. Nucl. Part. Sci. 55 (2005) 403
at large distance gluon density
generated by the pion cloud increase of the
N transverse size for xBj lt mp/mp0.14
0.6 0.4 fm
Promising COMPASS domain
8
2 Parametrizations of GPDs
Factorization H(x,?,t) q(x) F(t)
boundary conditions or Regge-motivated
t-dependence is more realistic
it considers that fast partons in the small
valence core and slow partons at
larger distance (wider meson cloud)
it includes correlation between x and t ltb2?gt
aln 1/x transverse extension of partons in
hadronic collisions
H(x,0,t) q(x) e t ltb?2gt q(x) / xat (aslope
of Regge traject.)
This ansatz reproduces the
Chiral quark-soliton model Goeke et
al., NP47 (2001)
More correct behavior at small and large x
ltb2?gt a (1-x) ln1/x B(1-x)2
to reproduce perfectly the proton form
factor
9
2
3 frameworks or models for GPD
)
Q

t,
?,
(x,

• Quark domain Double distribution x,? a la
  Radyushkin
• - Vanderhaeghen, Guichon, Guidal (VGG)
• PRD60 (1999) 094017,
  Prog.Part.Nucl.Phys.47(2001)401-515
• Goloskokov and Kroll for meson production with
  q and g contribution
• Eur. Phys. JC53 (2008) 367

Gluon quark domain very
promising Dual parametrization
with Mellin moments decomposition
QCD evolution separation x, ? and ?,
t - Polyakov (2002) Müller, NPB794
(2008) - Guzey, PRD74 (2006) 054027
hep-ph/0607099 - Polyakov and Vanderhaeghen,
hep-ph/0803.1271
Gluon domain

• GPDS,V,g(x,?) ? QS,V,g(x)
• Dependence generated via the QCD evolution

• Freund, Frankfurt, Strikman (FFS)
• Schoeffel PLB658 (2007) 33 GPD _at_ HERA and
  prospects for COMPASS

10
DVCS cross sections ?(?p??p) at HERA

• FFS predictions HS,V,g(x,?) ? QS,V,g(x) _at_ 1
  GeV² PDF used ? CTEQ6M
• ? Dependence
  generated via the QCD evolution
• Predictions at NLO

Good description of the Q² and W dependences
11
DVCS cross sections ?(?p??p) at HERA
Guzey predictions No difference between ----
Factorized and Regge motivated
t-dependence in the small x domain
12
New H1 results on d?/dt PLB659 (2008)
d?/dt d?/dtt0 exp(-bt)
b(Q²)A(1-Blog(Q²/2)) A 6.98 /- 0.98
GeV-2 B 0.12 /- 0.03
Global value at Q28 GeV2

• gt ?ltrT²gt0.65 fm
• gtgt valence quarks value
• b measurements _at_ H1 (low x)
• dominated by glue and sea quarks

no W dependence
b measurements _at_ COMPASS
13
0.65 fm ??? b measurements _at_
COMPASS
14
DVCS BH with polarized and charged leptons
and unpolarized target
ds(µp?µp?) dsBH dsDVCSunpol Pµ
dsDVCSpol eµ aBH Re ADVCS
eµ Pµ aBH Im ADVCS
? Known expression
Pµ ?
eµ ?
eµ Pµ ?
Twist-3 M01
Twist-2 M11
Twist-2 gluon M-11
gtgt
Belitsky,Müller,Kirchner
15
with
F1H dominance with a proton target F2E
dominance with a neutron target (F1ltlt) (studied
at JLab) very attractive for Jis sum rule
study
16
DVCS at JLab Hall A with polarized
electrons
Beam Spin cross section difference (2006)
Twist-2 Twist-3
PRL97, 262002 (2006)
Analysis in F
- twist-2 and twist-3 extraction - twist-3
contributions very small
Corrected for realvirtual RC (P.
Guichon) Checked elastic cross-section _at_ 1
17
DVCS at JLab Hall A with polarized
electrons
Q2 dependence and test of scaling
lt-tgt0.26 GeV2, ltxBgt0.36
Twist-2 Twist-3
PRL97, 262002 (2006)
No Q2 dependence strong indication for scaling
behavior and handbag dominance
18
Black bands JLab/HallA PRL97, 262002
(2006) Predictions by Vanderhaeghen
Double Distribution (1999-2002)
dual parametrization
(2008)
Good improvement
19
DVCS at JLab Hall B- CLAS with polarized
electrons
Beam Spin Asymmetry ALU
Denominator(BHVCS)
E1-DVCS kinematical coverage and binning
W2 gt 4 GeV2 Q2 gt 1 GeV2
hep-ph/0711.4805
20
hep-ph/0711.4805
Exclusivity selection for e p ? e p ? -
missing transverse momentum lt 0.09 GeV -
missing energy lt 0.3 GeV - angle between ?p
and ?p planes lt 1.5 - angle between the ?
direction and the direction inferred by e and p lt
1.2
e p ? e p Y
e p ? e p ? X
Good Work for ? contamination rejection pi0
21
Good improvement just on 19 March 2008
Predictions by Vanderhaeghen
Double Distribution
(1999-2002) dual
parametrization
(2008)
Q22 GeV2 xB0.25
hep-ph/0711.4805 and PRL97, 262002 (2006)
22
DVCS at HERMES Beam Charge Asymmetry AC

PRD75 (2007) 011103
23
DVCS at HERMES Beam Charge Asymmetry AC
PRD75 (2007) 011103
Double Distribution

dual parametrization
24
µ
?
?
Beam Charge and Spin Asymmetry at E? 100 GeV
COMPASS prediction
µ
p
?
6 month data taking in 2010 250cm H2 target 25
global efficiency
BCSA
25
µ
?
?
Beam Charge and Spin Asymmetry at E? 100 GeV
COMPASS prediction
µ
p
?
BCSA
VGG double-distribution in x,?
model 1 H(x,?,t) q(x) F(t)
model 2 and 2 correl x and t
ltb2?gt a ln 1/x
H(x,0,t) q(x) e t ltb?2gt q(x) /
xat a slope of Regge traject.
a0.8 a1.1
Guzey Dual parametrization
model 3 also Regge-motivated
t-dependence with a1.1
26
Model-Dependent Constraint on Ju and Jd
Through the modeling of GPD E
1-Transversaly polarised target
In Meson production
In DVCS
2-Neutron target - liquid deuterium target

27
Attempt for Ju and Jd determination
Be very caution!
HERMES DVCS TTSA on proton hep-ex/0802.2499
JLab, HallA, DVCS SSDiff on
neutron, PRL99 (2007) 242501 In
the near future same experiment HallA and TTSA
with CLAS
28
Proposal progress physics envisaged at
COMPASS d?/dt and BCSA and difference
comparison to the other experiments and
predictions Experimental realisation Recoil
muRex (2006), RPD (2007- 2008 hadron run 2008
GPD tests) Calorimetry working group on
ECAL1 and ECAL2 ECAL0
study Simulation fast MC Etienne and
Andrzej with different generators
(VGG, Guzey, FFS, Pythia) complete
simulation Sebastian after Marcin Tests in
2008 to control ? ?-, RPD efficiency,
Calorimetry performences
Necessity of - BMS Stations
- Muon Trigger - SciFi
(Yann) 0102 04 (same frame) 05 06

(than pixelGEM)
29
Next GPD meeting Wednesday 23 April at
1700