DVCS and Generalized Parton Distributions with CLAS

1
DVCS and Generalized Parton Distributions with
CLAS

• Silvia Niccolai for
  the CLAS Collaboration

QCD08 Montpellier - 7/7/08
2
DVCS and Generalized Parton Distributions with
CLAS

• GPDs and DVCS
• The CLAS e1-dvcs experiment
• CLAS BSA
• DVCS with CLAS at 12 GeV
• Conclusions and outlook

• Silvia Niccolai for
  the CLAS Collaboration

QCD08 Montpellier - 7/7/08
3
Deeply Virtual Compton Scattering and GPDs
e

• Q2 - (e-e)2
• xB Q2/2Mn nEe-Ee
• x?, x-? longitudinal momentum fractions
• t (p-p)2
• x ? xB/(2-xB)

p
p
conserve nucleon helicity
Vector H (x,?,t) Tensor E (x,?,t)
 Handbag  factorization valid in the Bjorken
regime high Q2 , ? (fixed xB)
flip nucleon helicity

• 3D quark/gluon
• image of
• the nucleon

Quark angular momentum (Jis sum rule)
X. Ji, Phy.Rev.Lett.78,610(1997)
4
Accessing GPDs through DVCS
?s s - s - ? I(DVCSBH)
5
DVCS asymmetries and GPDs
x xB/(2-xB) k-t/4M2
Polarized beam, unpolarized target


DsLU sinf ImF1H x(F1F2)H kF2Edf
H, H, E
at twist-2
Kinematically suppressed at low t
Unpolarized beam, longitudinal target


H, H
DsUL sinfImF1Hx(F1F2)(H df
Unpolarized beam, transverse target
H, E
DsUT sinfImk(F2H F1E) .. df
Beam charge asymmetry


H, H, E
DsC cosfReF1Hx(F1F2)H-kF2Edf
6
DVCS observables and GPDs
Spin asymmetry (beam/target) Im(T) GPDs at xx
t0
Cross section Re(T) integral of GPDs over x
Vanderhaegen, Guidal, Guichon
7
Jefferson Laboratory Newport News, USA
Continuous Electron Beam Accelerator Facility
Imax 200 mA Duty Factor 100 sE/E 2.5
10-5 Beam Pol 80
Emax 6 GeV
Hall B

8
DVCS _at_ CLAS first observations
NON-DEDICATED experiments
ep?epX
ep?epg
S. Chen et al., PRL 97 (2006)

• JLab Hall A first dedicated DVCS experiment
  (ep?egX)
• polarized cross sections differences and
  unpolarized cross sections
• small kinematic coverage, but high precision
• evidence for twist-2 dominance and scaling at
  low Q2 (2 GeV2)

ltQ2gt 1.82 GeV2 ltxBgt 0.16 lt-tgt 0.31 GeV2

Target-spin (L) asymmetry (H, H)
Beam-spin asymmetry (H)
BSA, BCA and TSA (T L) at HERMES sinf behavior
confirmed
Low statistics Limited kinematics Uncertainties
on determination of epg final state
9
The e1-dvcs experiment at CLAS

• Experiment March - May 2005
• Ee 5.77 GeV
• Polarization 76 - 82
• Current 20-25 nA
• Integrated luminosity 33 fb-1

424 PbWO4 crystals 16cm x 1.3cm x 1.3cm 18
radiation lengths APD light readout
Superconducting solenoid magnet (shielding for
Moeller electrons)
10
Channel selection
TOF
ep?epg in CLAS
CC
DC
All 3 particles are detected
EC

• electron ID
• EC, CC, DC and TOF
• proton ID
• DC and TOF
• photon ID
• IC or EC

IC
11
Selection of the DVCS final state
epp0?epg(g) background calculated with Monte
Carlo simulation and experimental epp0?epgg
data 5 on average

• Exclusivity cuts
• PXT lt 90 MeV/c (150 MeV/c) ep?epgX
• Cone angle ?(?X) lt1.2 (2.7) ep?epX
• Coplanarity angle between (?p) et (?p)lt1.5
  (3)
• EX lt 300 MeV (500 MeV)

12
BSA coverage and f distributions
Data integrated over t

• 13 Q2, xB bins
• 5 t bins
• 12 f bins

Fit a sin?/(1c cos? d cos2f?
( d 0 )
Ph.D. Thesis of F.X. Girod
13
BSA a vs. t
() Guidal, Polyakov, Radyushkin, Vanderhaegen,
PRD 72 (2005)
() Cano and Laget, PL B551 (2003)
VGG model overestimates the data
F.X. Girod et al., PRL 100 (2008)
14
Unpolarized Cross Sections
0.09lt-tlt0.2 GeV2
0.2lt-tlt0.4 GeV2
0.4lt-tlt0.6 GeV2
PRELIMINARY (arbitrary units)
0.6lt-tlt1 GeV2
1lt-tlt1.5 GeV2
1.5lt-tlt2 GeV2
Ph.D. Thesis of H.S. Jo
15
Difference of Polarized Cross Sections
0.09lt-tlt0.2 GeV2
0.2lt-tlt0.4 GeV2
0.4lt-tlt0.6 GeV2

• Upcoming experiments in Hall B
• p-DVCS with unpolarized target (more statistics)
• p-DVCS with longitudinally polarized target

PRELIMINARY (arbitrary units)
0.6lt-tlt1 GeV2
1lt-tlt1.5 GeV2
1.5lt-tlt2 GeV2
Largest kinematical coverage ever obtained for
DVCS 0.1ltxBlt0.58, 1ltQ2lt4.6, 0.09lt-tlt2
Ph.D. Thesis of H.S. Jo
16
Large phase space (x,t,Q2) and high luminosity
Valence region
Sea/gluon region
JLab Upgrade
17
Hall B _at_12 GeV CLAS12
Concurrent measurement of deeply virtual
exclusive, semi-inclusive, and inclusive
processes
Forward Electromagnetic Calorimeter
Forward Time-of-Flight
Low threshold Cerenkov Counter
Drift Chambers
High threshold Cerenkov Counter
Preshower Calorimeter
Q2 gt 2.5 GeV2
Central detector
Inner Electromagnetic Calorimeter
Design luminosity 1035 cm-2s-1
18
Hall B _at_12 GeV CLAS12
Concurrent measurement of deeply virtual
exclusive, semi-inclusive, and inclusive
processes
Forward Electromagnetic Calorimeter
Forward Time-of-Flight
Low threshold Cerenkov Counter
Drift Chambers
ep egp
High threshold Cerenkov Counter
Preshower Calorimeter
Q2 gt 2.5 GeV2

• Acceptance for
• charged particles
• Central (CD), 40oltqlt135o
• Forward (FD), 5oltqlt40o

Central Detector
Central detector
Forward Detector
Inner Electromagnetic Calorimeter
Design luminosity 1035 cm-2s-1
19
Hall B _at_12 GeV CLAS12
Concurrent measurement of deeply virtual
exclusive, semi-inclusive, and inclusive
processes
Forward Electromagnetic Calorimeter
Forward Time-of-Flight
Low threshold Cerenkov Counter
Drift Chambers
ep egp
High threshold Cerenkov Counter
qg
Preshower Calorimeter
Q2 gt 2.5 GeV2
xB 0.35

• Acceptance for
• charged particles
• Central (CD), 40oltqlt135o
• Forward (FD), 5oltqlt40o

EC
Central detector
IC

• Acceptance for photons
• IC 2oltqlt5o
• EC, 5oltqlt40o

Inner Electromagnetic Calorimeter
Q2
Design luminosity 1035 cm-2s-1
20
CLAS12 DVCS beam-spin asymmetry
e p epg
E 11 GeV
DsLUsinfImF1H..df

• L 1x1035
• T 2000 hrs

Q25.5GeV2 xB 0.35 -t 0.25 GeV2
21
CLAS12 DVCS target-spin asymmetry

• L 2x1035 cm-2s-1
• T 1000 hrs

E 11 GeV
Projected results
Longitudinally polarized target

DssinfImF1Hx(F1F2)H...df
S. Chen et al., PRL 97 (2006)
Ee5.75 GeV
ltQ2gt 1.82 GeV2 ltxBgt 0.16 lt-tgt 0.31 GeV2
Dedicated experiment at 6 GeV scheduled for next
year
22
CLAS 12 DVCS transverse target-spin asymmetry
E 11 GeV
e p epg
Projected results
Q22.2 GeV2, xB 0.25, -t 0.5GeV2
Transversely polarized target
Ds sinfImk1(F2H F1E) df
AUTx Target polarization in scattering plane
and CLAS12 will allow us to measure also DVCS
polarized and unpolarized cross sections, nDVCS,
vector and pseudo-scalar meson
electroproduction
AUTy Target polarization perpendicular to
scattering plane
Transverse-target spin asymmetry is highly
sensitive to the u-quark contributions to proton
spin.
H. Avakian
23
Conclusions

• DVCS is the cleanest reaction giving access to
  GPDs
• First pioneering experiments at CLAS and HERMES
  and dedicated Hall A experiment gave evidence for
  handbag dominance at modest Q2
• The e1-dvcs experiment at CLAS allowed for the
  first time to extract BSA and polarized and
  unpolarized cross sections (still preliminary)
  for DVCS over the widest phase space ever covered
  ? strong constraint for GPD models

• DVMP cross sections (and asymmetries) analyses
• are underway using e1-dvcs data, aiming to
• flavor decomposition of GPDs epp0, eph, epr
• DVCS asymmetry under study, to study N?D
• transition GPDs
• Next year at CLAS new DVCS experiment with
• longitudinally polarized target
• CLAS_at_12 GeV will explore DVCS and DVMP
• for Q2 up to 9 GeV2 and xB up to 0.7

Outlook
24
Conclusions

• GPDs are a unique tool to explore the internal
  landscape of the nucleon
• 3D quark/gluon imaging of the nucleon
• orbital angular momentum carried by quarks

• Their extraction from experimental data is very
  difficult
• they depend on 3 variables, only two (x, t)
  experimentally accessible
• they appear as integrals in cross sections

• We need to measure several exclusive channels
  and observables over a wide phase space to
  constrain the GPDs parameterizations

• Very promising first experimental results are
  coming from JLab _at_ 6 GeV
• twist-2 dominance, precocious scaling
• first constraints on GPD models
• first model-dependent extraction of Ju, Jd

• JLab _at_12 GeV will be the ideal facility to study
  GPDs in the valence region

• A very rich experimental program focused on GPDs
  is planned for the upgraded Hall A and Hall B

25
CLAS12 ep ?epr0
2D (Im(AB))/p
T
A (2Hu Hd)
AUT -
r0
A2(1-x2) - B2(x2t/4m2) - Re(AB)2x2
B (2Eu Ed)
Asymmetry depends on the GPD E, necessary for
Jis sum rule
Goeke, Polyakov, Vanderhaegen (2001)
and CLAS12 will allow us to measure also DVCS
polarized and unpolarized cross sections, nDVCS,
vector and pseudo-scalar meson
electroproduction
L1035 cm-2s-1 T2000 hrs