1. for GPDs studies at COMPASS
Status and prospects
for GPDs studies at COMPASS
Nicole d’Hose, CEA-Saclay
On behalf of the COMPASS collaboration
- Generalized Parton Distributions (GPDs)
- Sensitivity to COMPASS kinematics
- High luminosity and recoil Detection
- Meson production (present ρ studies)
- DVCS with polarized μ+ and μ-
"Hadron Structure and Hadron Spectroscopy", Prague, August 1st-3rd, 2005

2. Burkard,Belitsky,Müller,Ralston,Pire Parton Distribution H( x,,t )
GPDs  a 3-dimensional picture of the partonic nucleon structure Px
ep eX
Deep Inelastic Scattering
Q²xBj x p γ*
Parton Density q ( x )
x boost
x P z 1 y
Hard Exclusive Scattering
Deeply Virtual Compton Scattering
Burkard,Belitsky,Müller,Ralston,Pire
Generalized
Parton Distribution H( x,,t )
ep ep p
GPDs * Q²
x+
x-  t
x P y z r
x boost
( Px, ry,z )

3. Nucleon spin: ½ = ½ ΔΣ + ΔG + < Lz >
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 t p q
Nucleon spin: ½ = ½ ΔΣ + ΔG + < Lz >
quark contribution
gluon contribution
orbital angular momentum
RHIC, HERMES,
COMPASS
FUTURE
EMC, SMC, SLAC,
HERMES

4. 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 < mπ/mp=0.14
COMPASS domain

5. GPDs and relations to the physical observables
γ, π, ρ, ω…
factorization
x+ξ
x-ξ 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,ξ,t)
“ordinary” parton density
Elastic Form Factors
Ji’s sum rule
Intro
2Jq =  x(Hq+Eq)(x,ξ,0)dx x x
H(x,0,0) = q(x) (x,0,0) = Δq(x)
 H(x,ξ,t)dx = F(t)

6. Parametrization of GPDs
Model 1: H(x,ξ,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
<b2> = α’ln 1/x transverse extension of partons in hadronic collisions
H(x,0,t) = q(x) e t <b2> = q(x) / xα’t (α’slope of Regge traject.)
This ansatz reproduces the
Chiral quark-soliton model: Goeke et al., NP47 (2001)

7. Necessity of factorization to access GPDs
Collins et al.
Deeply Virtual Compton Scattering (DVCS): γ γ* Q2 p p’
GPDs
x + ξ
x - ξ
t =Δ2
meson
Gluon contribution L γ* Q2 γ
hard
x + ξ
x - ξ
soft
GPDs
Q2 large
t << Q2
+ γ* p p’
t =Δ2
Hard Exclusive Meson Production (HEMP):
meson p p’
GPDs γ*
x + ξ
x - ξ
hard
soft L Q2 L
t =Δ2
Quark contribution

8. Complementarity of the experiments in the world
100GeV
At fixed xBj, study in Q2
0.0001< xBj < 0.01
Gluons
Valence and sea quarks
And Gluons
Valence quarks
JLab
PRL87(2001)
Hermes PRL87(2001)
COMPASS plans
H1 and ZEUS
PLB517(2001) PLB573(2003)

9.  At fixed xBj, study in Q2 Benefit of a higher muon intensity
for GPDs study
if Nμ  5  Q2 < 17 GeV2
for DVCS
if Nμ  2  Q2 < 11 GeV2
for DVCS
E=190,
100GeV
Limitation by luminosity
now Nμ= 2.108μ per SPS spill
Q2 < 7.5 GeV2
for DVCS 
At fixed xBj, study in Q2

10. 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
>>LINAC4
From Lau Gatignon

11. Additional equipment to the COMPASS setupμ’ p’ μ
DVCS μp  μ’p’
At these energies (for μ, μ’, and )
the missing mass technique is not adapted
M2required = (mp+mπ)2-mp2
= 0.25 GeV2
M2observed > 1 GeV2
 ECal 1 or 2
  12°
Recoil detector
to insure exclusivity
to be designed and built
+ additional calorimeter
at larger angle
Nμ=2.108/SPS cycle
(duration 5.2s, each 16.8s)
2.5m liquid H2 target
to be designed and built
L = cm-2 s-1

12. Competing reactions to DVCS Selection DVCS/DIS
DVCS: μp  μp
HEπ°P: μp  μpπ°
 
Dissociation of the proton:
μp  μN*π°
 Nπ
DIS: μp μpX
with 1, 1π°, 2π°,η…
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°

13. 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 π0 production
linked to the hadron program

14. possible solution to complete the COMPASS setup
30°
ECAL0
12° 4m :
Received funding by EU FP6 (Bonn-Mainz-Warsaw-Saclay)
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

15. Before 2010, i.e. right now
with the present COMPASS setup
and the polarized 6LiD target
Hard "exclusive" meson production studies

16. Hard exclusive meson production (ρ,ω,…, π,η… )
GPDs γ*
x + ξ
x - ξ
t =Δ2 L
Scaling predictions:
hard
soft
1/Q6
1/Q4
Collins et al. (PRD ):
1.factorization applies only for γ*
2. σT << σL L
vector mesons pseudo-scalar mesons
ρ0 largest production present study
ρ0  π+ π with COMPASS

17. Description of the diffractive ρ° production
μN μ’N’ρ
π+π-
At Q2 > 1 GeV2
QCD calculations (for σL)
with GPDs and
with exchange of
2 quarks or 2 gluons W t
Q 2 * N N’
Regge theory:
at low energy W < 5 GeV:
exchange of Reggeon
ρ,ω (JP=1-),
a2, f2 (JP=2+),
a3, f3 (JP=3-), …
at higher energy :
exchange of Pomeron
COMPASS 2002: 10-2<Q2<10 GeV2,
<W>=10 GeV, t small
Experimental observations (NMC, E665, ZEUS, H1, HERMES):
the helicity of γ* is approximatively retained by the ρ° meson  SCHC
the exchange object has natural parity P=(-1)J  NPE

18. Spin properties of the production amplitudes
Angular Distribution of the production and decay of ρ  π+π-
 Spin density matrix elements  bilinear combinations
of the helicity amplitudes A( γ*(λγ ) ρ(λρ ) )  Tλρ λγ
λγ=  1, λρ=  1, 0
if NPE T- λ ρ -λγ = (-1) λρ-λγ Tλρ λγ
9 helicity amplitudes reduce to five 5 independent amplitudes :
A(L L), A(TT) >> A(T L) > A(L T) > A(T -T)
i.e. T00 , T >> T > T > T-11
SCHC >> single helicity flip > double helicity flip
SCHNC

19. Incoherent exclusive ρ0 production
Assuming
both hadrons are p
0.5 < Mpp< 1 GeV
Mpp *
Q 2 W N N’
Emiss
Exclusivity of
the reaction
Emiss=(M²X-M²N) /2MN
-2.5 < Emiss < 2.5 GeV t
6LiD polarized target
Kinematics:
ν > 30 GeV
Eμ’ > 20 GeV
Q² > 0.01 GeV²
pt²
Incoherent production
0.15 < pt²< 0.5 GeV²
scattering off a
quasi-free nucleon
Background ~12%

20. ρ° angular distributions W(cosθ, φ, Φ)
depends on the Spin density matrix elements
 23 (15) observables with polarized (unpolarized) beam φ
This analysis:
only one-dimensional
angular distribution
We will use also:
ψ= φ - Φ

21. Angular distributions
0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q²
0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q² < 10 GeV2 φ
Preliminary :
- Corrected
for Acceptance, smearing and efficiency
(MC:DIPSI gen)
- Background
not subtracted
Statistical error only, limited by MC

22. Measurement of r Distribution : Spin density matrix elements:04 00
0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q² < 10 GeV2
Distribution :
Spin density matrix elements:
Tλρ λγ are helicity amplitudes
meson photon

23. Determination of Rρ° =sL/sT
2002
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 Q2>2 GeV2
- High statitics from
γ-production to hard regime
- Better coverage at high Q2
with 2003 and 2004 data

24. Measurement of r and Im r04
1-1 3
1-1 φ
Distribution :
beam polarisation
weak violation
Spin density matrices:
If SCHC holds

25. ~ ~ ‘’ Longitudinal ’’ Meson production : filter of GPDs
Cross section:
Vector meson production (ρ,ω,…)  H & E
Pseudo-scalar production (π,η… )  H & E ~ ~
Hρ0 = 1/2 (2/3 Hu + 1/3 Hd + 3/8 Hg)
Hω = 1/2 (2/3 Hu – 1/3 Hd + 1/8 Hg)
H = /3 Hs - 1/8 Hg
with present COMPASS data
can be investigated
Single spin asymmetry ~ E/H
for a transverse polarized target

26.  comfortable statistics until Q2= 20 GeV2     Q2= 7 GeV2
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 < 4GeV2 and large xBj~ 0.4

27. towards a complete experimental program
with a liquid H2 target,
a recoil detector,
an extended calorimetry
for both HEMP and DVCS
in 2010

28. Polarized μ+ and μ- beams
to maximise the muon flux (Lau Gatignon)
Pπ=110GeV and Pμ=100GeV
Pol(μ+) = -0.8 and Pol(μ-) = +0.8
Nμ+ ~ 2. Nμ-
Muon section 400m
Hadron decay section 600m
Compass
target
scrapers
Be absorbers
Protons
400 GeV
T6 primary
Be target
Collimators
H V H V
2.108 muons/spill
protons/spill

29. DVCS+ Bethe Heitler φ θ μ’ μ *  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: DVCS>>BH
 DVCS Cross section
Smaller energy: DVCS~BH
Interference term will provide
the DVCS amplitude φ θ μ’ μ *  p

30. dσ(μpμp) = dσBH + dσDVCSunpol + Pμ dσDVCSpol
Advantage of and
for Deeply virtual Compton scattering (+Bethe-Heitler )
t, ξ~xBj/2 fixed
dσ(μpμp) = dσBH + dσDVCSunpol + Pμ dσDVCSpol
+ eμ aBH Re ADVCS eμ Pμ aBH Im ADVCS
 cos nφ  sin nφ φ θ μ’ μ *  p
Pμ+=-0.8 Pμ-=+0.8
Diehl

31. DVCS Beam Charge Asymmetry (BCA) measured with the 100 GeV muon beam at COMPASS
Q2=40.5 GeV2
x = 0.05 ± 0.02
Model 1: H(x,ξ,t) ~ q(x) F(t)
Model 2:
H(x,0,t) = q(x) e t <b2>
= q(x) / xα’t φ φ
In 2010
BCA
L = cm-2 s-1
efficiency=25%
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 φ

32. Advantage of the kinematical domain of COMPASS
Model 1: H(x,ξ,t) ~ q(x) F(t)
BCA 
model 1
model 2
COMPASS
Model 2:
H(x,0,t) = q(x) e t <b2>
= q(x) / xα’ t
sensitivity to the different spatial
distribution of partons  when xBj 
range of COMPASS

33. 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 (xBj<10-2)
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

34. <x>=0.11 <Q2>=2.6
COMPASS
6 angular distributions
among 18: 3 bins in xBj=0.05, 0.1, 0.2
6 bins in Q2 from 2 to 7 GeV2
BCA in DVCS
projections
for 1 year
HERMES
one single bin
<x>=0.11 <Q2>=2.6

35. Roadmap for GPDs at COMPASS
This initiative is now an "Expression of Interest": SPSC-E0I-005 :
Present COMPASS studies with the polarized target
Complete analysis of ρ production
 SCHC study in a large range in Q GeV2
 NPE study with longitudinal double spin asymmetry
 E/H investigation with the transverse polarized target
Other channels: , 2π … :
Realization of the recoil detector prototype within the EU FP6
: 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

36. Not presented…
Other measurements with the high intensity and the unpolarised target

37. until Q2 = 10-3
COMPASS
x =
Kinematical domains
for colliders and
fixed target
experiments

38. Success of QCD
The NLO DGLAP equations describe
the Q2 evolution of F2
proton
Possible
New
Accurate
Measurement
At COMPASS

39. Coherent interaction of partons Log1/x in the QCD evolution
Understanding of
low x physics
ZEUS H1
y=Q2/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)

40. Quark and gluon contributions
Preliminary errors
for COMPASS 2003 (6LiD)
NMC 94
E665 97
ZEUS 93+95
Gluon GPD calculations:
Frankfurt et al. PRD54 (1996)
Quark GPD calculations:
Vanderhaeghen et al. PRD60 (1999)
Gluon contribution
Quark contribution