1. COMPASS plans to measure GPDs
Jean-Marc Le Goff
DAPNIA/CEA-Saclay
24 Feb 2006, Albuquerque
workshop on Orbital Angular Momentum
The COMPASS experiment
GPDs at COMPASS
now: deep r production
≥2010: DVCS and HEMP

2. The COMPASS experiment

3. The COMPASS Collaboration (230 Physicists from 12 Countries)
Bielefeld, Bochum, Bonn (ISKP & PI), Erlangen, Freiburg, Heidelberg, Mainz, München (LMU & TU)
Dubna (LPP and LNP), Moscow (INR, LPI, State University), Protvino
CERN
Warsaw (SINS),
Warsaw (TU)
Helsinki
Prag
Nagoya
CEA-Saclay
30% der Mitglieder = deutsch
Lisboa
Torino(University,INFN),
Trieste(University,INFN)
Burdwan,
Calcutta
Tel Aviv

4. COMPASS  COmmon Muon and Proton Apparatus
COMPASS fixed target experiment at CERN
muons hadrons π/K, p
Beam: µ+/ spill (4.8s / 16.2s) h/spill
Beam polarisation: 80%
Beam momentum: GeV/c GeV/c
COMPASS
SPS
LHC
COMPASS  COmmon Muon and Proton Apparatus
for Structure and Spectroscopy

5. Physics goals muon beam hadron beams nucleon spin structure
Quark and Gluon Polarization in (longitudinally) polarized nucleons
transverse spin distribution
function Tq(x)
Flavor dependent polarized
quark helicity densities q(x)
Lambda polarisation
Diffractive vector-meson production
hadron beams
nucleon spectroscopy
Primakoff-Reactions
polarizability of  and K
Exotics : glueballs and hybrids
charmed mesons and baryons
semi-leptonic decays
double-charmed baryons
Large detector: 40 x 6 x 4 m3
Data taking starts 2001
5-10 years physics program
Cycle: 16.8 s
800 Gbyte/shift

6. Spectrometer 2002  2004 trigger-hodoscopes SM2 dipole SM1 dipole DW45
straws
SM2 dipole
Muon-filter2,MW2
RICH_1
HCAL1
Gem_11
SM1 dipole
ECAL2,HCAL2
Polarised Target
MWPC Gems Scifi
Muon-filter1,MW1
Veto
straws,MWPC,Gems,SciFi
Gems,SciFi,DCs,straws
Silicon
SciFi
Micromegas,DC,SciFi
BMS

7. COMPASS and GPDs

8. measurement of GPDs γ γ* γ* γ γ* 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

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

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

11. m flux at COMPASS in 2010 ?  sharing CNGS/FT  new Linac 4 
up to 10 times more p
+improve m line
COMPASS
>>LINAC4
From Lau Gatignon

12. What can we measure now ?

13. DVCS, HEMP ? sDVCS small, bckg Vanderhagen et al.:
HEMP factorization: gL
vector meson decay
 R=sT/sL
ρ0 largest s
ρ0  π+ π- , charged particules
Vanderhagen et al.:
1/Q4
1/Q6
vector mesons pseudo-scalar

14. Diffractive r0 productionW t
Q 2 * N N’
μN μ’N’ρ
π+π-
Exp (NMC, E665, Zeus, H1, Hermes):
lr ≈ lg S-channel helicity conservation SCHC
exchanged object has natural parity : P=(-1)J NPE

15. Spin properties of amplitudes
angular dist. of ρ  π+π- : spin density matrix elt
they are bilinear combinations of
helicity amplitudes :
λγ=  1, λρ=  1,  9 amplitudes
if NPE  5 amplitudes
T00, T11 >> T01, T >> T-11
SCHC helicity flip flips

16. r spin density matrix elt04 00
0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q² < 10 GeV2
Distribution :
in terms of amplitudes:

17. Determination of R=sL/sT
2002
If SCHC holds :
sL is dominant at Q2>2
2002: high stat
in large Q2 range
2003 and 2004 data :
- much more stat
- better high Q2 coverage

18. Conclusions on rho SCHC → R stot + R → sL
when Q2 > 2 → R > 1 : accurate sL
we have transv. target spin asym → E/H
important for Ji sum rule (∫E+H)
exploratory measurement
(no exclusivity, nuclear target)

19. Towards a dedicated experiment for DVCS and HEMP

20. Additions to COMPASS setupμ’ p’ μ
2.5m liquid H2 target
to be designed and built
L = cm-2 s-1
 ECal 1 or 2
  12°
Exclusivity: at high energy
dDM2 = d[(mp+mπ)2-mp2] > 1 GeV2
need 0.25 GeV2 for DM cut
 hermetic detector
Recoil detector
to insure exclusivity
to be designed and built
+ additional calorimeter
at larger angle

21. A possible solution
30°
ECAL0
12° 4m :
Funding by European Union (Bonn-Mainz-Warsaw-Saclay)
45° sector recoil detector
- scintillating material studies (200ps ToF Resolution over 4m)
- fast triggering and multi-hit ADC/TDC system

22. DVCS background not included: Beam halo with hadronic contamination
Beam pile-up
Secondary interactions
External Bremsstrahlung
DVCS: μp  μp
with PYTHIA 6.1 simulate:
HEπ°P: μp  μpπ°
 
Dissociation of the proton:
μp  μN*π°
 Nπ
DIS: μp μpX
with 1, 1π°, 2π°,η…
Acceptance cuts:
qgmax= 30°
Egmin= 50 MeV
qchargedmax= 30°

23. GPD measurement xxB/(2-xB) g* γ TDVCS: and t are fixed loop over x
GPDs
x + ξ
x - ξ
t =Δ2 g*
TDVCS:
xxB/(2-xB)
and t are fixed
loop over x

24. DVCS + Bethe-heitler φ θ μ’ μ *  p BH calculable μ μ p p
Higher energy: DVCS>>BH
 DVCS Cross section
Smaller energy: DVCS~BH
Interference term will provide
the DVCS amplitude

25. DVCS + BH with and . dσ(μpμp) = dσBH + dσDVCSunpol + Pμ dσDVCSpol
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

26. at 100 GeV assuming: L = 1.3 1032 cm-2 s-1 φ φ φ
BCA
Q2=40.5 GeV2
x = 0.05 ± 0.02
Model 1: H(x,0,t) ~ q(x) F(t)
Model 2:
H(x,0,t) = q(x) e t <b2>
= q(x) / xα’t
assuming:
L = cm-2 s-1
150 days
efficiency=25%
2 bins shown out of 18:
3 bins in xBj= 0.05, 0.1, 0.2
6 bins in Q2 from 2 to 7 GeV2 φ φ
BCA
x = 0.10 ± 0.03 φ

27. advantage of COMPASS kinematics
model 1
COMPASS
model 2
Model 1: H(x,0,t) ~ q(x) F(t)
Model 2: H(x,0,t) = q(x) e t <b2>
= q(x) / xα’t
sensitive to different spatial distributions at different x

28. HEMP: 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
Transverse single spin asymmetry : ~ E/H

29. HEMP in 2010 HEMP requires higher Q2 than DVCS
with liq H target and same m flux as now:
r  Q2= 20 GeV2
w, p, h, f  Q2= 7 GeV2
if more m  higher Q2

30. Roadmap for DVCS at COMPASS
: r production: sL and transverse spin asym
2005 : "Expression of Interest": SPSC-E0I-005
: recoil detector prototype
2008 ? : proposal
: construction of the recoil detector
cryogenic target, ECal0
2010: dedicated GPD measurement

31. Measure OAM ? Ji sum rule relates total quark spin to GPDs H and E :
similar sum rule for gluons
To do :
Measure H and E
Perform flavor decomposition
Extrapolate to t=0
Integrate over x at fixed x
Sivers asymmetries are also measured at COMPASS

32. SPARE

33. Key role of calorimetry
ECAL2 from 0.4 to 2°: mainly lead glass GAMS
ECAL1 from 2 to 12° : good energy and position resolution
for 2-g separation in a high rate environment
ECAL0 from 12 to 30°: to be designed
for background rejection
Careful study of g and π0 production
 COMPASS program with hadron beam

34. 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

35. 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

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

37. <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