For a GPD program

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After the GPD workshop held at Trento, 13 June
2008, How to emphasize the role of COMPASS
How to be as much as possible independent of GPD
modeling
For a GPD program _at_
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1rst measurement 3-dim picture of the
partonic nucleon structure
density of quark with longitudinal momentum
fraction x and transerse distance b from the
center of momentum
z
t-?2
x P
b
y
x boost
?
independent of models

• measurement of the transverse size via
• - direct measurement with d?/dt for DVCS or
  meson production
• - indirect measurement with
• IM(H), Re(H) with DVCS and Beam Charge and
  Spin Difference

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1rst measurement 3-dim picture of the
partonic nucleon structure
density of quark with longitudinal momentum
fraction x and transerse distance b from the
center of momentum
t-?2
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2 B
At Q28 GeV2 B 5.45 ? 0.19 ? 0.34 GeV-2
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at large distance the gluon density generated
by the pion cloud increase of the N
transverse size for xBj lt mp/mp0.14
(chiral dynamics prediction)
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2 main classes of GPD Parametrization
Factorization H(x,?,t) q(x) F(t) or
Regge-motivated t-dependence more realistic
with x-t correlation it considers
that fast partons in the small valence core
and slow partons at larger distance
(wider meson cloud) H(x,0,t)
q(x) e t ltb?2gt ltb2?gt
8 a ln 1/x 2 B C (1-x)2
transverse extension of partons
transverse size in hadronic collisions
has to be finite
due to
confinement (aslope of Regge
traject.) for valence quark a 1 GeV-2 to
reproduce FF ? meson Regge traj. for gluon
a 0.164 GeV-2 (J/? at Q20) a
0.02 GeV-2 (J/? at Q22-80 GeV2) B5.45 ? 0.19
GeV-2 a 0 GeV-2 (DVCS at Q28 GeV2)
ltlt a 0.25 GeV-2 for soft
Pomeron
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2nt measurement

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
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At COMPASS with ? and ?- ? access both ImH and
ReH
with 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
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At COMPASS with ? and ?- ? access both ImH and
ReH
with 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

• cos nf ?sin nf
• n0,1,2,3
  n1,2

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At COMPASS with ? and ?- ? access both ImH and
ReH
with DVCS BH with polarized and charged
leptons and
unpolarized target
s1Int sin ?

c1Int cos ?

Analysis through dependence in ? and Q2 Absolute
cross section measurement Dominance of H
dominance with a H2 target
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• Importance to measure both Im H and Re H
• a dispersion relation
• After a few GPD models for DVCS
• (VGG (1998), Freund et al. (2003), Guzey et
  al. (2006), )
• 2007-2008 many theoretical papers promising
• Fitting procedure (Mueller) or GPD quintessence
  function (Polyakov)
• based on dual parametrization
• partial wave expansion with respect to the
  angular momentum
• partial wave amplitude of mesonic exchanged in
  the t-channel
• ? Regge phenomenology is a guide line towards
  realistic GPD ansatze

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µ
?
?
Beam Charge and Spin Asymmetry at E? 100 GeV
COMPASS prediction
µ
p
?
BCSA
6 month data taking in 2010 250cm H2 target 25
global efficiency
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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 Regge-motivated t-dependence
ltb2?gt a ln 1/x
H(x,0,t)q(x)et ltb?2gt q(x)/xat
aval0.8 GeV-2 aval1.1 GeV-2
Guzey and Teckentrup Dual
parametrization model 3 Regge-motivated
t-dependence with aval1.1(1-x) GeV-2
asea0.9 GeV-2 agluon0.5 GeV-2
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3rd measurement sensitivity to the
GPD E
With a transversely polarized target
DVCS
Vector meson (M) production

• transversely polarized target inside the foreseen
  recoil detector
• another recoil detector inside the present
  transv. polarized target
• ? study for a second stage of the
  experiment

OR
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THE PROPOSAL For next meeting, Monday,
Septembre 8, 1700 To do list I- GPD
physics at COMPASS II- experimental
realisation III- feasibility and simulation

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II- Experimental realisation 1- ??
and ?-? beams (Lau Gatignon CERN,,Saclay,) 2-
Target - 2.5 m long H2 with minimum thickness
- long polarized target inside the
recoil detector? 3- Control of luminosity
(Freiburg, Mainz, Saclay,) 4- Scattered muon
trigger veto system (Bonn, Mainz, Saclay,
) 5- Recoil detector - foreseen proton
detection with scintillators
(Bonn, Freiburg, Mainz,
Saclay, Warsaw, ) - a tricky proton
detection with the present polarized target?
- photon detection around the recoil
detector? 6- Calorimetry (Calorimeter Group,
Dubna, Protvino, ) - ECAL12 - ECAL0
to be build
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III- Feasibility and simulation (Dubna,
Freiburg, Saclay, Warsaw, ) Selection of
the DVCS events from the background Final
kinematical fits (Mmiss coplanarity
) Efficiency (comparison to Bethe-Heitler)
Tests in 2008 IV- Impact of the GPD
program or Comparison of the projected
measurements with different models DVCS Model
VGG, Guzey, FFS, Mueller Mesons Model Kroll and
Goloskokov Impact of our experiment within the
statistical errors