Status and prospects

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Status and prospects for GPDs studies at COMPASS
Nicole dHose, CEA-Saclay On behalf of the
COMPASS collaboration
- Generalized Parton Distributions (GPDs) -
Sensitivity to COMPASS kinematics - High
luminosity and recoil Detection - Meson
production (present ? studies) - DVCS with
polarized µ and µ-
"Hadron Structure and Hadron Spectroscopy",
Prague, August 1st-3rd, 2005
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GPDs ? a 3-dimensional picture of the partonic
nucleon structure
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Why GPDs are promising?
Goal correlation between the 2 pieces of
information -distribution of longitudinal
momentum carried by the partons -distribution
in the transverse plane
Implication of orbital angular momentum to
the total spin of a nucleon

Nucleon spin ½ ½ ?S ?G lt Lz gt

quark contribution
gluon contribution
orbital angular momentum
RHIC, HERMES, COMPASS
FUTURE
EMC, SMC, SLAC, HERMES
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What do we learn from the 3 dimensional picture
( Px,ry,z ) ? 1. Lattice calculation Negele et
al., NP Proc. Suppl. 128 (2004) ? fast
parton close to the N center ? small valence
quark core ? slow parton far from the N
center ? widely sea q and gluons
2. Chiral Dynamics Strikman et al., PRD69
(2004) at large distance, the gluon
density is generated by the pion cloud
significant increase of the N
transverse size if xBj lt mp/mp0.14

COMPASS domain
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GPDs and relations to the physical observables
?, p, ?, ?
factorization
x?
x-?
t
The observables are some integrals of GPDs over x
Dynamics of partons in the Nucleon
Models Parametrization
Fit of Parameters to the data
H, ,E, (x,?,t)
ordinary parton density
Elastic Form Factors
Jis sum rule
2Jq ? x(HqEq)(x,?,0)dx
x
x
H(x,0,0) q(x)
(x,0,0) ?q(x)
? H(x,?,t)dx F(t)
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Parametrization of GPDs
Model 1 H(x,?,t) q(x) F(t) Model 2
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)
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Necessity of factorization to access GPDs
Collins et al.
Deeply Virtual Compton Scattering (DVCS)

?
?
Q2

hard
x ?
x - ?
soft


GPDs
Q2 large t ltlt Q2 ?
p
p
t ?2
Hard Exclusive Meson Production (HEMP)
meson
L
t ?2
Quark contribution
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Complementarity of the experiments in the world
At fixed xBj, study in Q2
Valence quarks
Valence and sea quarks And Gluons
0.0001lt xBj lt 0.01 Gluons
JLab PRL87(2001)
Hermes PRL87(2001) COMPASS plans
H1 and ZEUS PLB517(2001) PLB573(2003)
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if Nµ ? 2 ? Q2 lt 11 GeV2 for
DVCS
At fixed xBj, study in Q2
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In 2010 ? ? sharing CNGS/FT operations
? new Linac4 (high intensity H- source)
as injector for the PSB
improvements on the muon line
what could be the available proton/muon flux?
COMPASS
gtgtLINAC4
From Lau Gatignon
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Additional equipment to the COMPASS setup
DVCS µp ? µp?
? ECal 1 or 2 ?? ? 12
Nµ2.108/SPS cycle (duration 5.2s, each 16.8s)
2.5m liquid H2 target to be designed and built
L 1.3 1032 cm-2 s-1
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Competing reactions to DVCS DVCS µp ?
µp? HEpP µp ? µpp ?
?? Dissociation of the proton µp ?
µNp ? Np DIS µp ?µpX
with 1?, 1p, 2p,? Beam halo
with hadronic contamination Beam
pile-up Secondary interactions External
Bremsstrahlung
Selection DVCS/DIS with PYTHIA 6.1 Tune
parameters -maximum angle for photon detection
30 -threshold for photon detection
50MeV -maximum angle for charged particle
detection 30
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Key role of the Calorimetry
ECAL2 from 0.4 to 2 mainly lead glass
GAMS ECAL1 from 2 to 12 good energy and
position resolution
for 2 photons separation
in a high rate environment ECAL0 from 12 to
30 to be designed for
background rejection

Careful study of photon and p0 production
linked to the hadron program
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possible solution to complete the COMPASS setup
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ECAL0
12
4m

2004-2007
Received funding by EU FP6 (Bonn-Mainz-Warsaw-Sac
lay) Goal full test of feasibility of a 45
sector recoil detector - scintillating material
studies (200ps ToF Resolution over 4m) - fast
triggering and multi-hit ADC/TDC system
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Before 2010, i.e. right now with the
present COMPASS setup and the
polarized 6LiD target Hard
"exclusive" meson production studies
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Scaling predictions
hard
soft
1/Q6
1/Q4
Collins et al. (PRD56 1997) 1.factorization
applies only for ? 2. sT ltlt sL
L
vector mesons pseudo-scalar mesons
?0 largest production present study ?0
? p p- with COMPASS
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Description of the diffractive ? production
At Q2 gt 1 GeV2 QCD calculations (for sL) with
GPDs and with exchange of 2 quarks or 2 gluons

Regge theory at low energy W lt 5 GeV
exchange of Reggeon ?,?
(JP1-), a2, f2 (JP2),
a3, f3 (JP3-), at higher energy
exchange of Pomeron
COMPASS 2002 10-2ltQ2lt10 GeV2,
ltWgt10 GeV, t small
Experimental observations (NMC, E665, ZEUS, H1,
HERMES) the helicity of ? is
approximatively retained by the ? meson ? SCHC
the exchange object has natural parity
P(-1)J ? NPE
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Spin properties of the production amplitudes
Angular Distribution of the production and decay
of ? ? pp-
? Spin density matrix elements ? bilinear
combinations
of the helicity amplitudes A( ?(?? ) ??(?? ) )
? T?? ??

• ?? ? 1, 0 ?? ? 1, 0

• if NPE T- ? ? -?? (-1) ??-?? T?? ??

9 helicity amplitudes reduce to five 5
independent amplitudes

• A(L? L), A(T?T) gtgt A(T? L) gt A(L? T) gt
  A(T? -T)

i.e. T00 , T11 gtgt T01
gt T10 gt T-11
SCHC gtgt single helicity flip
gt double helicity flip

• SCHNC

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Incoherent exclusive ?0 production
Assuming both hadrons are p 0.5
lt Mpplt 1 GeV
Mpp

Q 2
W
N
N
Emiss
Exclusivity of the reaction
Emiss(M²X-M²N) /2MN -2.5 lt Emiss lt 2.5 GeV
t
6LiD polarized target
Kinematics ? gt 30 GeV Eµ gt 20 GeV Q² gt 0.01
GeV²
Incoherent production 0.15 lt pt²lt 0.5 GeV²
scattering off a quasi-free nucleon
pt²
Background 12
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? angular distributions W(cos?, f, F)
depends on the Spin density matrix elements ?
23 (15) observables with polarized (unpolarized)
beam
f
This analysis only one-dimensional
angular distribution
We will use also ? f - F
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Angular distributions
0.01 lt Q² lt 0.05 lt Q² lt 0.3 lt Q² lt 0.6 lt Q² lt
2.0 lt Q² lt 10 GeV2

f
Preliminary - Corrected for Acceptance,
smearing and efficiency (MCDIPSI gen) -
Background not subtracted

Statistical error only, limited by MC
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Measurement of r
04 00
0.01 lt Q² lt 0.05 lt Q² lt 0.3 lt Q² lt 0.6 lt
Q² lt 2.0 lt Q² lt 10 GeV2
Distribution
Spin density matrix elements
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Determination of R? sL/sT
If SCHC holds only T00?0 T11?0 Then
Impact on GPD study easy determination of
sL factorisation only valid for sL sL is dominant
at Q2gt2 GeV2
- High statitics from ?-production to
hard regime
- Better coverage at high Q2 with 2003
and 2004 data
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Measurement of r and Im r
04 1-1
3 1-1
f
Distribution
beam polarisation
weak violation
Spin density matrices
If SCHC holds
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 Longitudinal  Meson production filter of
GPDs
Cross section
Vector meson production (?,?,?) ? H
E Pseudo-scalar production (p,? ) ? H
E


H?0 1/?2 (2/3 Hu 1/3 Hd 3/8 Hg) H? 1/?2
(2/3 Hu 1/3 Hd 1/8 Hg) H?
-1/3 Hs - 1/8 Hg
can be investigated with present COMPASS data
Single spin asymmetry E/H
for a transverse polarized target
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Meson Production in 2010
With a liquid Hydrogen target and the same muon
flux than now Measurement of hard
exclusive meson production ?
comfortable statistics until Q2 20 GeV2
? ? ? ? Q2 7 GeV2

• Benefit of an increase in intensity
• for an extension of the range in Q2

NB for ? results from JLab the SCHC was not
observed at Q2 lt
4GeV2 and large xBj 0.4
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towards a complete experimental program
with a liquid H2 target, a recoil
detector, an extended calorimetry
for both HEMP and DVCS
in 2010
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Polarized µ and µ- beams

• to maximise the muon flux (Lau Gatignon)
• Pp110GeV and Pµ100GeV
• Pol(µ) -0.8 and Pol(µ-) 0.8
• Nµ 2.? Nµ-

2.108 muons/spill
1.3 1013 protons/spill
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µ
µ
DVCS Bethe Heitler
p
p
BH calculable
The high energy muon beam at COMPASS allows to
play with the relative contributions
DVCS-BH which depend on 1/y 2 mp El xBj
/Q2
Higher energy DVCSgtgtBH ? DVCS Cross section

• Smaller energy DVCSBH
• Interference term will provide
• the DVCS amplitude

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Advantage of and
for Deeply virtual Compton scattering
(Bethe-Heitler )
t, ?xBj/2 fixed
Pµ-0.8 Pµ-0.8
Diehl
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DVCS Beam Charge Asymmetry (BCA) measured with
the 100 GeV muon beam at COMPASS
BCA
Q24?0.5 GeV2
x 0.05 0.02
Model 1 H(x,?,t) q(x) F(t)
Model 2
H(x,0,t) q(x) e t ltb?2gt
q(x) / xat
f
f
In 2010
BCA
L 1.3 1032 cm-2 s-1 efficiency25 150
days data taking
x 0.10 0.03
Only 2/18 data sets
In total 3 bins in xBj 0.05, 0.1, 0.2
6 bins in Q2 from 2 to 7 GeV2

f
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Advantage of the kinematical domain of COMPASS
Model 1 H(x,?,t) q(x) F(t)
Model 2
H(x,0,t) q(x) e t ltb?2gt
q(x) / xa t
sensitivity to the different spatial
distribution of partons ? when xBj ?
range of COMPASS
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Competition to COMPASS
measurements at COMPASS in 2010
compared to HERMES in the same xBj
intermediate range but reduced
kinematical domain in Q2 H1, ZEUS
(xBjlt10-2) about 2 data years
until 2007 equivalent integrated
luminosity/year with new recoil
detection JLab 6 GeV (large xBj) ? 11 GeV
in 2011? very high luminosity

e-RHIC in the far future around 2015?
high energy in the collider mode
high luminosity
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COMPASS
6 angular distributions among 18
3 bins in xBj0.05, 0.1, 0.2
6 bins in Q2 from 2 to 7 GeV2
BCA in DVCS projections for 1 year
HERMES
one single bin
ltxgt0.11 ltQ2gt2.6
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Roadmap for GPDs at COMPASS
This initiative is now an "Expression of
Interest" SPSC-E0I-005
2004-2009 Present COMPASS studies with the
polarized target Complete analysis of ?
production ? SCHC study in a large range in
Q2 0.02-20 GeV2 ? NPE study with
longitudinal double spin asymmetry ? E/H
investigation with the transverse polarized
target Other channels ?, 2p
2004-2007 Realization of the recoil detector
prototype within the EU FP6

• 2007-2009 construction of the recoil detector
• cryogenic target, ECal0
• 2010 complete experiment for GPDs study at
  COMPASS
• benefit directly of every
  improvement of the muon flux
• and of the intensity
  upgrade of the SPS

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Not presented
Other measurements with the high intensity and
the unpolarised target
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Kinematical domains for colliders and fixed
target experiments
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Success of QCD The NLO DGLAP equations describe
the Q2 evolution of F2
proton
Possible New Accurate Measurement At COMPASS
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Understanding of low x physics
ZEUS H1
yQ2/xs
New phenomena Coherent interaction of
partons Log1/x in the QCD evolution
Transition from high to low Q2 to understand
confinement
Saturation model
New data at low x low Q2 with COMPASS
Saturation model
Bartels, Golec-Biernat, Kowalski PRD66 (2002)
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Quark and gluon contributions
Gluon GPD calculations Frankfurt et al. PRD54
(1996) Quark GPD calculations Vanderhaeghen
et al. PRD60 (1999)
Gluon contribution
Quark contribution
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