1. Generalized Parton Distributions @
« Expression of Interest » SPSC-EOI-005
 a complete proposal for autumn 2008
The main objectives for GPD at COMPASS
The detectors and target to be implemented
Roadmap for the realisation of this experimental program
GPD 2008 workshop,
Trento, 13 June 2008
Nicole d’Hose, Saclay, CEA/DAPNIA
On behalf of the COMPASS collaboration

2. In DVCS and meson production we measure Compton Form Factor
hard
soft p p’ γ
GPDs γ*
x + ξ
x - ξ
t =Δ2 Q2
In DVCS and meson production
we measure Compton Form Factor
For example at LO in S:
t, ξ~xBj/2 fixed
DGLAP
q(x)
DGLAP
DGLAP
ERBL

3. +  At COMPASS with + and -  access both ImH and ReH θ μ’ μ *  p
with DVCS + BH with polarized and charged leptons
and unpolarized target  θ μ’ μ *  p μ p
BH calculable
DVCS +
dσ(μpμp) = dσBH + dσDVCSunpol + Pμ dσDVCSpol
+ eμ aBH Re ADVCS eμ Pμ aBH Im ADVCS
Twist-3 M01
Twist-2 M11
Twist-2 gluon M-11
 Known expression
eμ Pμ 
eμ 
Pμ 
>>
Belitsky,Müller,Kirchner

4. +  At COMPASS with + and -  access both ImH and ReH θ μ’ μ *  p
with DVCS + BH with polarized and charged leptons
and unpolarized target  θ μ’ μ *  p μ p
BH calculable
DVCS +
dσ(μpμp) = dσBH + dσDVCSunpol + Pμ dσDVCSpol
+ eμ aBH Re ADVCS eμ Pμ aBH Im ADVCS
cos nφ sin nφ
n=0,1,2, n=1,2

5. +  At COMPASS with + and -  access both ImH and ReH θ μ’ μ *  p
with DVCS + BH with polarized and charged leptons
and unpolarized target  θ μ’ μ *  p μ p
BH calculable
DVCS +
Analysis through dependence in  and Q2
Absolute cross section measurements

6. 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), …)
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

7.  3-dim picture of the partonic nucleon structure t=-2
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
d/dt for DVCS or meson production
IM(H), Re(H) with DVCS and Beam Charge and Spin Difference

8. 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
b or r

9. 2 Parametrizations of GPDs
Factorization: H(x,ξ,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 <b2>
<b2> ~ 8 α’ 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
(α’slope of Regge traject.)
for valence quark α’ ~ 1 GeV-2 to reproduce FF  meson Regge traj.
for gluon α’ ~ GeV-2 (J/ at Q2=0)
α’ ~ 0.02 GeV-2 (J/ at Q2=2-80 GeV2)
<< α’ ~ 0.25 GeV-2
for soft Pomeron

10. 0.65 0.02 fm ??? measurements of d/dt @ COMPASS
b or r
0.65 0.02 fm ??? measurements of d/dt
H1 - PLB659(2008)
@ COMPASS
at large distance : the gluon density generated by the pion cloud
increase of the N transverse size for xBj < mπ/mp=0.14
(chiral dynamics prediction)

11. μ’  * μ  p Beam Charge and Spin Asymmetry
at E = 100 GeV COMPASS prediction * μ  p
BC&SA
6 month data taking in 2010
250cm H2 target
25 % global efficiency Q2
xBj 7 6 5 4 3 2

12. μ’  * μ  p Beam Charge and Spin Asymmetry
at E = 100 GeV COMPASS prediction * μ  p
VGG: Double-Distribution in x,
BC&SA
model 1: H(x,ξ,t) ~ q(x) F(t)
model 2 and 2*:
Regge-motivated t-dependence
<b2> = α’ ln 1/x
H(x,0,t)=q(x)et <b2>= q(x)/xα’t
α’val=0.8 GeV-2
α’val=1.1 GeV-2
Guzey and Teckentrup:
Dual parametrization
model 3:
Regge-motivated t-dependence
with α’val=1.1(1-x) GeV-2
α’sea=0.9 GeV-2
α’gluon=0.5 GeV-2

13. 2nd ultimate goal with GPD
Contribution to the nucleon spin knowledge
½ = ½ ΔΣ + ΔG + < Lzq > + < Lzg >
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,ξ,0) +Eq (x,ξ,0) ) dx
 with a transversely polarized target DVCS et MV t p q E

14. How to get the GPD E ? With a transversely polarized target DVCS:
Vector meson (M) production:
transversely polarized target inside the recoil detector
 study for a second stage of the experiment

15. Hard exclusive meson production at COMPASS
GPDs g*
x + ξ
x - ξ
t =Δ2 L
hard
soft
Collins et al. (PRD ):
-factorization applies only for *L
-probably at high Q2
Filter of GPDs:
,, production
presently studied
at COMPASS
Vector Meson (,,…): H and E
Pseudo-scalar Meson (,,…) : H and E
~ ~
Different flavor contents:
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
measurement of
d /dt
and ratio of
meson production

16. Competition in the world and COMPASS role
HERA
Ix2
100GeV
E=190,
COMPASS at CERN-SPS
High energy muon beam
100/190 GeV
Unique availability of
μ+ and μ-
polar(μ+)=-0.80
polar(μ-)=+0.80
change once per day
2.108 μ per SPS cycle
if intensity  2,
Q2 range up to 11 GeV2
after 2010,
which improvements
on the muon line ?
Gluons valence quarks valence quarks
and sea quarks
and gluons
COMPASS JLab 12 GeV, FAIR,…

17. Additional equipment to the COMPASS setupμ’ p’ μ
DVCS μp  μ’p’
SciFi, Si, μΩ, Gem, DC, Straw, MWPC
all COMPASS trackers:
 ECal1 + ECal2
  10°
2.5m cryogenic target
to be designed and built
- 2011: long H2 target
later: transversely polarized
+ additional calorimeter
ECal0 at larger angle
4m long Recoil proton detector
to insure exclusivity
to be designed and built
Nμ=2.108/SPS cycle
(duration 5.2s, each 16.8s)
Possibility for an increase
of intensity?

18. Recoil detector + calorimetry

19. Requirements for the recoil detector
1) Time of Flight measurement  t and  (sectorization)
(ToF) < 300 ps   P/P ~ 3 à 15 %
t = (p-p’)²= 2m(m-Ep’)
 t/t ~2  P/P  10 bins in t from tmin to 1 GeV2
t is the Fourier conjugate of the impact parameter r
t is the key of the measurement
315  12 ps have been achieved during the 2006 test
intrinsic limit due to the thin layer A
2) Hermiticity + huge background + high counting rates
Rejection of extra ° at large angle
 study of rejection with angular and coplanarity constrains
if not, detection of extra ° at a reasonable cost

20. Present 1m long Recoil Proton Detector in COMPASS
for the hadron program (spectroscopy)

21. Calorimeter acceptance
xbj Q2
Existing Calorimeters
Xbj-bins
+ 3m x 3m ECAL0
+ 4m x 4m ECAL0
y > 0.05
E. Burtin

22. Kinematical ranges for photons in ECALs
Min=5GeV
Min=10GeV
A.Sandacz
Max=60 GeV
Max>100 GeV
x<0.03
Kinematical ranges for photons in ECALs
for /°separation E threshold = 1GeV = 2GeV

23. Physical background to DVCS
Source : Pythia 6.4 generated DIS events
exclusive 0 production
in COMPASS data
One scattered muon
One photon with E1 > 3 GeV
-2.5 < Emiss < 2.5 GeV
0.03<xBj<0.3 and 0.1<y<0.9
No other photon in calorimeters:
1GeV < E>1 < 3 GeV
One proton in recoil detector
No charged tracks in spectrometer
GeV
Emiss=(MX2-MP2)/2MP
in this case
DVCS is dominant
But ideal case
Only acceptance
No pile up
+ angular correlation
+ coplanarity
E. Burtin Q2

24. μ p  μ  p 2008: DVCS feasibility study in the COMPASS environment
in the RPD
in ECAL1+2
2008: Request to the collaboration for switch-over
from pion beam to muon beam for ~5 days
Goal 1: a better knowledge of the + and - beams
Goal 2: a better knowledge of the RPD and its extrapolation to a larger size
hadrons (5107  / 10s (5 MHz)) : muons (25 108  / 5s (50 MHz))
with the exclusive rho production (~1000 evts in 5 days)
Goal 3: a better knowledge of the Calorimetry
low photon energy threshold necessary for a good /° separation

25. and if in 2009 (?)
we get 30 days of dedicated beam
to perform a « GPD » pilot run

26. μ’  * θ μ  p at E = 100 GeV Beam Charge and Spin Asymmetry Q2 xBj
6 month data taking in 2010
250cm H2 target
25 % global efficiency
at E = 100 GeV μ  p
“GPD” pilot run in 2009
1 month data taking
40cm H2 target
25 % global efficiency
Beam Charge and Spin Asymmetry Q2
xBj 7 6 5 4 3 2
14000 evts
780 evts

27. Conclusion & prospects
Possible physics ouput
Sensitivity to transverse size of partons inside the nucleon
Sensitivity to the GPD E and total angular momentum
Working on a variety of models
to quantify the physics potential of a GPD exp. at COMPASS
Experimental requirements
Recoil Detection
Good calorimetry and Extension at larger angle
Roadmap
A global COMPASS proposal for the period
including transversity, longitudinal, spectroscopy + GPD + Drell-Yan
will be presented to SPSC at the end of the year 2008
2008-9: A small RPD and a liquid H2 target are available for the hadron program  tests of DVCS feasibility
from 2011: A complete GPD program at COMPASS with a long RPD
+ liquid H2 target (2011)
+ transversely polarized target (later)
very favourable timing window before the availability
of JLab 12 GeV, EIC, FAIR…

30. μ pμ p  global efficiency of 25% ? Data taking SPS availability 0.80
Spectro availability 0.90
DAQ livetime 0.93
Inter run 0.99
Spill gate 0.96
Quality checks 0.92
Trigger Veto livetime 0.80
Trigger efficiency 0.80
Reconstruction Incident muon 0.93
Scattered muon  31%
DVCS photon would be a guess!
Recoil proton 0.90
Control of absolute cross section within 5-10% :
- determination of F2 (already done in NMC within 2%)
- determination of Bethe-Heitler cross section (in 2009)

31. Naturally Polarized μ+ and μ- beams -switch: one per day
Solution proposed by Lau Gatignon:
To select Pπ=110GeV and Pμ=100GeV
to maximise the muon flux
2) To keep constant the collimator
settings which define
the π and μ momentum spreads
 Pol μ+ = -0.8 and Pol μ- = +0.8
Nμ+ ~ 1.6  Nμ-
2.108 μ- per SPS cycle (5.2s, 16.8s)
Requirements for DVCS:
-same energy
-maximum intensity
-opposite polarisation to a few %
-switch: one per day
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

32.  Criteria for a good proton Purity ~ 80%
Target
Inner
Layer
Outer Layer
Ai+1 Ai
Ai-1
Bi+1
Bi-1 
Criteria for a good proton
geometrical configuration
Correlation energy loss and measured
Good timing and good vertex with the muon
  
online (2001)
offline (2001)
protons
Zproton-Zmuon [cm]
Efficiency ~ 90%
Purity ~ 80%
Control with the present 1m long RPD

33. Requirements for calorimetry
Competing reactions to DVCS
DVCS: μp  μp 
HEπ°P: μp  μpπ° ~ 
 
Dissociation of the proton:
μp  μN* ~ /10
 Nπ
DIS: μp μpX dominant
with 1, 1π°, 2π°,η…
Beam pile-up
DVCS photon kinematics
ECAL1
ECAL0
Q2=1 GeV2
  E DVCS and E threshold for ° detection 
xBj= min EDVCS=10 GeV E thr. = 2 GeV in ECAL2
xBj= min EDVCS=5 GeV E thr. = 1 GeV in ECAL1

34. μ p  μ  p 2008: DVCS feasibility study in the COMPASS environment
in the RPD
2008: Request to the collaboration for switch-over
from pion beam to muon beam for ~5 days
Goal 1: a better knowledge of the + and - beams
beam size (Si),momentum (BMS), beam halo (forward spectro, RPD)
Goal 2: a better knowledge of the RPD and its extrapolation to a larger size
comparison: hadrons (5107  / 10s (5 MHz)) : muons (25 108  / 5s (50 MHz))
- rates in PMT, multipl., waveforms
using Rand., Incl., RPD triggers
with the exclusive rho production (~1000 evts in
5 days) with Incl. trigger
- efficiency, purity of the RPD
- kinematical fit, azimuthal correlation
- comparison t from recoil proton detector
or from COMPASS spectro
t = (p - p’)2 = (q - q’)2
in ECAL1+2
exclusive 0 production in
COMPASS data at 160 GeV
Emiss=(MX2-MP2)/2MP
GeV

35. low noise level and pile up rejection (with the SADC)
Goal 3: a better knowledge of the Calorimetry
° detection:
as a function of E° and the E threshold energy
in ECAL2 (x_Bj ~0.05 and E DVCS > 10 GeV)
in ECAL1 (x_Bj ~0.1 and E DVCS > 5 GeV)
thresholds of 1 GeV in ECAL1 and 2 GeV in ECAL2
should be necessary for a good /° separation
low noise level and pile up rejection (with the SADC)
study as a function of the beam intensit
necessity to measure with random trigger without/with beam
effect of the charged tracks given by the muons
selection of ° exclusif with exclusivity constrains