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Silvia Niccolai, IPN Orsay, for the CLAS Collaboration
QNP2012, Palaiseau, April 19th 2012
Deeply virtual Compton scattering on longitudinally polarized protons and neutrons at CLAS
k
k’
q’
GPDsN N’
Deeply Virtual Compton Scattering and GPDs
e’t
(Q2)
eL*
x+ξ x-ξ
H, H, E, E (x,ξ,t)~~
p p’
« Handbag » factorization validin the Bjorken regime:
high Q2 , (fixed xB), t<<Q2
• Q2= - (e-e’)2
• xB = Q2/2M=Ee-Ee’
• x+ξ, x-ξ longitudinal momentum fractions• t = (p-p’)2
• xB/(2-xB)
0,x ),( Ex q21 Hxdx qJG =
21J q
1
1)0 ,, (
Quark angular momentum (Ji’s sum rule)
X. Ji, Phy.Rev.Lett.78,610(1997)
Vector: H (x,ξ,t)
Tensor: E (x,ξ,t)
Axial-Vector: H (x,ξ,t)
Pseudoscalar: E (x,ξ,t)
~
~
conserve nucleon helicity
flip nucleon helicity
«3D» quark/gluonimage of
the nucleon
4 GPDs for each quark flavor
Polarized beam, longitudinal target:
LL ~ (A+Bcos)Re{F1H+x(F1+F2)(H + xB/2E)…}d~ Re{Hp, Hp}~
Im{Hn, En, En}
x= xB/(2-xB) k=-t/4M2
leptonic planehadronic
planeN’
e’
e
LU ~ sin Im{F1H + x(F1+F2)H -kF2E}d~Polarized beam, unpolarized target: Im{Hp, Hp, Ep}
~
UL ~ sinIm{F1H+x(F1+F2)(H + xB/2E) –xkF2 E+…}dUnpolarized beam, longitudinal target:
~ Im{Hp, Hp}~
Unpolarized beam, transverse target:
UT ~ sinIm{k(F2H – F1E) + ….. }d Im{Hp, Ep}
Sensitivity to GPDs of DVCS spin observables
dxxx
txHtxHPRe qq
1
0
q
11),,(),,(
2
qe H
),,(),,(q tHtHIm qq 2qe H
Im{Hn, Hn, En}
~
Proton Neutron
~
Re{Hn, En, En}~
Im{Hn}
~
Polarized beam, longitudinal target:
LL ~ (A+Bcos)Re{F1H+x(F1+F2)(H + xB/2E)…}d~ Re{Hp, Hp}~
Im{Hn, En, En}
x= xB/(2-xB) k=-t/4M2
leptonic planehadronic
planeN’
e’
e
LU ~ sin Im{F1H + x(F1+F2)H -kF2E}d~Polarized beam, unpolarized target: Im{Hp, Hp, Ep}
~
UL ~ sinIm{F1H+x(F1+F2)(H + xB/2E) –xkF2 E+…}dUnpolarized beam, longitudinal target:
~ Im{Hp, Hp}~
Unpolarized beam, transverse target:
UT ~ sinIm{k(F2H – F1E) + ….. }d Im{Hp, Ep}
Sensitivity to GPDs of DVCS spin observables
dxxx
txHtxHPRe qq
1
0
q
11),,(),,(
2
qe H
),,(),,(q tHtHIm qq 2qe H
Im{Hn, Hn, En}
~
Proton Neutron
~
Re{Hn, En, En}~
Im{Hn}
~
CLAS pDVCSBSAs
CLASpDVCSTSAseg1 (2000),not a DVCS-dedicatedexperiment
What we have learned from the published CLAS asymmetries
Model-independent fitat fixed xB, t, Q2
of DVCS observables
S. Chen et al, PRL 97, 072002 (2006)
M. Guidal, Phys. Lett. B 689, 156-162 (2010)
ImH has steeper t-slope than ImH: is axial charge more
concentrated than the electromagnetic charge?
~
F.-X. Girod et al, PRL. 100 162002 (2008)
The eg1-dvcs experiment at CLAS• Data taken from February to September 2009• Beam energies = 4.735, 5.764, 5.892, 5.967 GeV• Beam polarizaton ~ 85%• CLAS+IC to detect forward photons• Target: longitudinally polarized via DNP (5 Tesla, 1 Kelvin, 140 Ghz microwaves) NH3 (~80%) and ND3 (~30%) – Luminosity ~ 5∙1034 cm-2 s-1
• Target polarization monitored by NMR• ~75 fb-1 on NH3 (parts A, B), ~25 fb-1 on ND3 (part C)
Polarized ammonia
Carbon
Empty cell
C.D. Keith et al., NIM A 501 (2003) 327
pDVCS (ep→e’p’): particle IDEnergy deposited in EC for negative tracks
Electron ID cuts:Charge: -10.2 < E/p < 0.4 (energy deposited in EC)Ein > 0.06 (energy deposited in inner EC)p > 0.8Nphe(CC)>2Geometrical matching between EC, SC, CC z vertex cutEC, DC fiducial cuts
Proton ID cuts:Charge > 0z vertex cut¦Δβ¦ < 0.035
DC fiducial cuts
(DC/TOF) for positive tracks
22pTOF
TOF
mp
p
ct
d
+
protons
deuterons
IC photon ID cuts:E >2.5 GeVGeometrical fiducial cuts
EC photons not yet included inthe analysis (<10% events)
pDVCS: channel selection & coverage
Kinematical and exclusivity cuts to select DVCS events:
• Eγ>2.5 GeV Q2>1 GeV2 W>2 GeV• Cone Angle (angle between detected and predicted γ)• MM2 epX• Missing Energy• Coplanarity (angle between (γ*,p) and (γ,p) planes)• MM2 epX• Missing Transverse Momentum (in reaction frame)
Cone angle: before/after cuts
pDVCS- Sanity check: Beam Spin Asymmetry
)(
NNP
NNALU
Integrated over all kinematics,only IC photons includedOnly eg1-dvcs part B data (~2/3)No 0 background subtraction yet
Beam polarization: ~ 83 %
cos1
sin
1
0
p
pFit
F.-X. Girod et al, PRL. 100 162002 (2008)
eg1-dvcs
pDVCS: Target Spin Asymmetry
Dilution factor: f~ 0.76Target polarization: PT=-85%, +90%
Only IC photons includedonly eg1-dvcs part B data No 0 background subtraction yet
)(
NNPf
NNA
TLU
<xB>~0.3
<Q2>=2.3 (GeV/c2)2
Preliminary
Erin Seder, UConnGary Smith, Glasgow
pDVCS: Double (Beam-Target) Spin Asymmetry
Dilution factor: f~ 0.76Target polarization: PT=-85%, +90%Beam Polarizarion: PB= 83%
Only IC photons includedonly eg1-dvcs part B data No 0 background subtraction yet
)()(
)()(
NNNNPPf
NNNNA
TBLL
Gary Smith, Glasgow
2coscos 210 pppFit
First bin in -tp0 vs -t
p1 vs -t p2 vs -t
p0 vs xB
ALL THESE RESULTS ARE VERY PRELIMINARY!
DVCS on different targets
H2Free proton
NH3Free proton in nuclear medium
ND3Quasi-free proton in deuterium and in heavier nuclear medium
ND3
Quasi-free neutron in deuterium and in heavier nuclear medium
Calculate DVCS on a “free” neutronn
F.-X. Girod et al, PRL. 100 (2008) 162002
Daria Sokhan, IPNO
Sanity check: ALU – proton in NH3/ND3
Xpeep '' Xpeed ''
cos1
sin
1
0
p
pA
Raw beam-spin asymmetriesNo 0 background subtraction
Good agreement between the two analyses
Daria Sokhan, IPNO
nDVCS in ND3 – channel selection
< 0.95 (EC timing)
cGeVpGeVQ
cGeVWGeVE
X /4.01
/21
22
2
5
8
/ 4.0
)/( 15.0 222
cGeVp
cGeVm
n
Xepep
Standard PID cuts for electron and photon
Xneed ''pX
Exclusivitycuts:
nDVCS ALU beam-spin asymmetry from ND3
cos1
sin
1
0
p
pA
Xneed ''
Integrated over all kinematicsNo 0 subtraction yetStatistics very low, but ALU≠0! AUL analysis also underway
More data will be taken with CLAS12at 11 GeV, on liquid deuterium target
<xB>~0.3
<Q2>=2.3 (GeV/c2)2
Daria Sokhan, IPNOVery preliminary
Projections for 90 days of running with CLAS12
Summary and outlook
• Combining various DVCS spin observables for proton and neutron targets is necessary to provide constraints for model-independent extractions of Compton Form Factors (→GPDs)
• The eg1-dvcs experiment combined the CLAS-DVCS setup (CLAS+IC) with polarizedhydrogen and deuterium targets
• Preliminary results for TSA for pDVCS are in good agreement with existing data, and the statistics with respect to previous CLAS data has been improved by more than a factor 5
• Preliminary results for double-spin asymmetries show dominance of the constant term
• Very preliminary results for nDVCS (very low statistics) hint to non-zero beam-spin asymmetries
• A lot of work (mainly on background subtraction) still needs done
• Much more data for both pDVCS and nDVCS on a wider phase space will come from CLAS12
Thanks again to Erin Seder, Gary Smith, Daria Sokhan