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Prospects for GPD studies at COMPASSE. Burtin CEA-Saclay Irfu/SPhN
On behalf of the COMPASS CollaborationCERN , March 4th, 2010
• Physics Motivations • Sensitivity of observables• Experimental issues• Beam tests results
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Generalized Parton Distributions
contains pdfH(x,0,0) = q(x) measured in DIS
contains form factors measured in elastic scattering
Factorisation:Q2 large, -t<1 GeV2
Generalized Parton Distributions
GPDs
x+ x-
P P’
hard
soft
g* g, ,p r
t
€
F(t) = dxH(x,ξ , t)∫
€
x(Hq (x,ξ, t = 0) + E q (x,ξ, t = 0))dx = J q∫contains information on the nucleon spin :Ji’s sum rule :
for quarks : 4 functions H,E,H,E(x,,t)~~
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3-D partonic structure of the nucleon (Pz,ry,z)
Hard Exclusive Scattering Deeply Virtual Compton Scattering
Burkardt,Belitsky,Müller,Ralston,Pire
GPDs : H( x,,t )
ep ep
pGPDs
* Q²
x+ x-
p
t
x P
y
z
r
x boost
access to correlations : ( Px, ry,z )
Fourier (=0)
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What makes Compass unique ?
CERN High energy muon beam • 100 - 190 GeV• 80% Polarisation• μ+ and μ- available
Opposite polarization
Will explore the intermediate xBj region
Uncovered region between ZEUS+H1 and HERMES+Jlab
Gluon, sea and valence quarks
Luminosity limit
Foreseen program :DVCS and meson production offa liquid H2 target (unpolarized)
xBj
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Comparison of BH and DVCS at 160 GeV
BH dominates BH and DVCS at the same level DVCS dominates
excellent access DVCS amplitude study of dDVCS/dt reference yield through the interference
θμ’μ
*
p
DVCS : Bethe-Heitler :
x=0.01 x=0.04 x=0.1
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)sin( DVCSDVCSpol
sQy
ed
122
6
Azimuthal angular dependence analysis
dσ(μpμp) = dσBH + dσDVCSunpol
+ Pμ dσDVCSpol
+ eμ aBH Re ADVCS + eμ Pμ aBH Im ADVCS
Twist-3 M01
from Belitsky, Kirchner, Müller : polarized beam off unpolarized target
Twist-2 M11 Twist-2 gluon M-11
DVCSBH Aa m
DVCSBH Aa e
eμPμ
eμ
Pμ
>>
)coscos()()(
),,(
2
21021
2BHBHBHBBH cCc
PP
tQxd
Known expression
)coscos( 221022
6DVCSDVCSDVCSDVCS
unpolcCc
Qye
d
)coscoscos()()(
323210
213
6IntIntIntInt cccc
PtPxye
)sinsin()()(
221
213
6IntInt ss
PtPxye
φ
θμ’μ
*
p
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Angular dependence analysis
Case of COMPASS : μ+(P=-0.8) and μ-(P=+0.8)
unpolarized H2 target φ
θμ’μ
*
p
DU,CS : dσμ+- dσμ-= 2 Pμ dσDVCSpol
+ eμ aBH Re ADVCS
€
s1DVCS sinϕ
€
c0Int + c1
Int cosϕ + c2Int cos2ϕ + c3
Int cos3ϕ
=> Re (F1H)
€
c0DVCS+BH + c1
DVCS+BH cosϕ + c2DVCS+BH cos2ϕ
€
s1Int sinϕ + s2
Int sin2ϕ
SU,CS : dσμ++ dσμ-= 2(dσBH+dσDVCSunpol ) + 2 eμ Pμ aBH Im ADVCS
=> Im (F1H)=> dσ/dt
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From SU,CS : t-slope measurement Using SU,CS : dDVCS / dt ~ exp(-Bt) B ~ ½ <r2>
Ansatz at small x :B(x) = b0 + 2 α’ ln(x0/x)α’ =0.125 GeV-2 (FFS)
160 GeV muon beam2.5m LH2 target2 yearsL = 1222 pb-1
εglobal = 10 %
1 < Q2 < 8 GeV2
3 σ slope measurement for :
α’ > 0.30 (ECAL 1+2 ) α’ > 0.16 (ECAL
0+1+2)
Assuming 3% systematic error on BH
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Angular dependence analysis
Case of COMPASS : μ+(P=-0.8) and μ-(P=+0.8)
unpolarized H2 target φ
θμ’μ
*
p
DU,CS : dσμ+- dσμ-= 2 Pμ dσDVCSpol
+ eμ aBH Re ADVCS
€
s1DVCS sinϕ
€
c0Int + c1
Int cosϕ + c2Int cos2ϕ + c3
Int cos3ϕ
=> Re (F1H)
€
c0DVCS+BH + c1
DVCS+BH cosϕ + c2DVCS+BH cos2ϕ
€
s1Int sinϕ + s2
Int sin2ϕ
SU,CS : dσμ++ dσμ-= 2(dσBH+dσDVCSunpol ) + 2 eμ Pμ aBH Im ADVCS
=> Im (F1H)=> dσ/dt
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DU,CS : Beam Charge & Spin Difference
φ
θμ’μ
*
p
160 GeV muon beam2.5m LH2 target2 yearsL = 1222 pb-1
εglobal = 10 %
Bj
=> Re (F1H)
€
....+c1Int cosϕ + ...
DU,CS : dσμ+- dσμ-= 2 Pμ dσDVCSpol
+ eμ aBH Re ADVCS
€
s1DVCS sinϕ
€
c0Int + c1
Int cosϕ + c2Int cos2ϕ + c3
Int cos3ϕ
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BCSA() over the kinematical domain
(deg)
1 < Q2 < 2
2 < Q2 < 4
4 < Q2 < 8
0.03< x < 0.070.02 < x < 0.030.01< x < 0.020.005< x < 0.01
Points: VGG prediction Phys. Rev. D60:094017,1999
Statistical errors onlyCurves: FFS prediction Phys. Rev. D59:119901,1999
160 GeV muon beam2.5m LH2 target2 yearsL = 1222 pb-1
εglobal = 10 %
BCSA = DU,CS /SU,CS
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statistical precision of the cosϕ modulation
B B B
B B B
BCSA = DU,CS /SU,CS = A0 + ACS,U cos ϕ + A2 cos2ϕ
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Meson production : filter of GPDs
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 = -1/3 Hs - 1/8 Hg
Transversely polarized target asymmetry on vector meson : E/H ( studied at COMPASS without RPD )
ρ : ω : 9 : 1 : 2 at large Q2
Cross section measurement :Vector meson : ρ,ω,… H & EPseudo-scalar : π,η… H & E ~ ~
Would allow for flavor separation :
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Continuation of the GPD program :constrain the GPD E
with +, - beam and transversely polarized NH3 (proton) target
DT,CS dT ( +) - dT ( -)
Im(F2 H – F1 E) sin(- S) cos 160 GeV muon beam1.2 m polarized NH3 target (f=0.26)2 years - εglobal = 10 %
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GPD program : new equipments
μ’
p’
μ
DVCS μp μ’p’
ECal1 + ECal2
10° 2.5 m liquid H2 target
4m long Recoil Proton Detector
Hermetic calorimetrynew ECAL0
Later stage… Transversely polarized targetAssociated RPD
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Recoil Proton Detector
Gandalf Project:1 GHz digitalisation of the PMT signal tocope for high rate
• 4 m long scintillator slabs• ~ 300ps timing resolution• Full scale prototype tested successfully
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ECAL 0Requirements- Photon energy range 0.2- 30 GeV- Size: 320cm x 320cm ; - Granularity 4x4 – 6x6cm2
- Energy resolution < 10.0%/√E (GeV) - Thickness < 50 cm, - Insensitive to the magnetic field.
Prototype under studies • Shaschlyk module with AMPD readout• Tested
DVCS+BH
ECAL
1&2
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2008 beam test
Ring A
Ring B
Compass hadron runTarget region
Selection of events : - one vertex with μ and μ’ - no other charged tracks - only 1 high energy photon (Δt<5ns) - 1 proton in RPD with p < 1. GeV/cDVCS Bethe-Heitler
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2008 beam test : exclusivity cuts
Δp =|Pμ’+γ|-|P
RPD|Δpperp < 0.2 GeV
Δϕ=ϕmiss - ϕRPD
Δϕ < 36 deg
Transverse plane
proton
γμ’
μ’+γ
Δϕ
miss
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2008 beam test : Bethe-Heitler signal
Monte-Carlo simulation of BH (dominant) and DVCS
After all cuts,Q2>1GeV2
Deep VCS Bethe-Heitler
Detection efficiency : ε μ+p->μ+p+γ = 0.32 +/- 0.13
Global efficiency :- μ+p->μ+p+γ efficiency - SPS & COMPASS availability- Dead time- trigger efficiency
εglobal = 0.13 +/- 0.05
Projections of errors are realistic
=> Bethe-Heitler observed
~ 10 times more data taken in 2009
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Conclusions & perspectives
COMPASS has a great potential in GPDs physics
• Study of the GPD H with a LH2 target : measurement ot t-slopes – transverse partonic structure of the nucleon measurement of Beam Charge and Spin differences & asymmetries
• Equipements needed : 4m long RPD, 2.5m LH2 target, Extended & improved calorimetry
• at a later stage :
study of the GPD E with a transversely polarized target
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DU,CS /SU,CS : Beam Charge & Spin Asymetry
φ
θμ’μ
*
pBCSA = DU,CS /SU,CS
160 GeV muon beam2.5m LH2 target2 yearsL = 1222 pb-1
εglobal = 10 %
Bj
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proton
γ
μ’
2008 DVCS beam testExperimental realisation
Compass hadron set-up
• New « superRPD » and H2 target• Hermetic calorimetry : Move ECALs upstream and/or complete ECAL2• New ECAL0 upstream of SM1
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proton
γ
μ’
2008 DVCS beam testECAL0 and ECAL1
Vertical
Horizontal
ECAL0
“superRPD” SM1
ECAL1 ECAL2
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Hadron program RPD
Proton identification in RPDElastic scattering (hadron beam)
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Kinematical consistency : ϑγ*γ
Events rejected
€
cosθγ *γ
μμ 'P =1
1+ 4MP2 x 2 /Q2
1+2MP
2 x
Q2
t +Q2
t +Q2 / x
⎛
⎝ ⎜
⎞
⎠ ⎟
proton
γ
μ’
μγ*
With μ, μ’ and γ :
With μ, μ’ and proton :
€
θγ*γ
μ+p->μ’+γ+p
€
θγ*γ
μμ 'γ
€
θγ*γ
μμ 'γ −θγ *γ
μμ 'P
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Exclusive photon production signal
φ
θμ’μ
*
p
Before cutsAll Q2
After cutsAll Q2
The peak at φ=0 remains => caracteristic of BH
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Liquid H2 Target & RPD
Ring A
Ring B
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RPD(2008) MuRex (2006)
B L=1m; th=1cmAtten length = 0.7mσB = 300 ps
L=4m; th=5cmAtten length = 4mσB = 200 ps
A L=50cm; th=5mmσA = 180 ps
L=2.83m; th=4mmσA = 270 ps
ToF σToF = 350 ps
RB-RA= 85-12 = 63 cm
σToF = 310 ps
RB-RA= 110-25 = 85cm
222222222
2)(cos
sin
1
1ToFBBAA
AB
cvvRRP
P
P
P
t
t
2
Measurements and Estimations for resolution
tmin=-0.06 GeV2
Good resolution in t
Importance for thethe transverse imaging