GPDs measurements at JLab with 12 GeV

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GPDs measurements at JLab with 12 GeV

• Silvia Niccolai for
  the CLAS Collaboration

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GPDs measurements at JLab with 12 GeV

• Interest of GPDs
• How to access GPDs experimentally
• Current status of GPD studies at JLab
• JLab_at_12 GeV
• Plans for GPDs in Hall A
• CLAS12 and GPDs measurements
• Conclusions and outlook

• Silvia Niccolai for
  the CLAS Collaboration

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Studying nucleon structure with electron
scattering
Form factors transverse quark distribution in
coordinate space
Parton distributions longitudinal quark
distribution in momentum space
x
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Studying nucleon structure with electron
scattering
Form factors transverse quark distribution in
coordinate space
Parton distributions longitudinal quark
distribution in momentum space
x
Accessible in hard exclusive processes
? H(x,?,t)dx F1(t) (? ?)
? E(x,?,t)dx F2(t) (? ?)
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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
 Handbag  factorization valid in the Bjorken
regime high Q2 , ? (fixed xB), tltltQ2
Vector H (x,?,t) Tensor E (x,?,t)
flip nucleon helicity

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

Quark angular momentum (Jis sum rule)
X. Ji, Phy.Rev.Lett.78,610(1997)
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Extracting GPDs from DVCS spin observables
x xB/(2-xB) k-t/4M2
Polarized beam, unpolarized target


DsLU sinf ImF1H x(F1F2)H kF2Edf
H, H, E
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
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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
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Hard exclusive meson production and GPDs
Different mesons ? different sensitivity to GPDs
H H E E
e

p, ?, ?
t
e

gL
(Q2)
x?
x-?
Vector mesons (r, w, f)
Pseudoscalar mesons (p, h)
Factorization proven only for longitudinally
polarized virtual photons and valid at high Q2
and small t
quark flavor decomposition accessible via meson
production

• L(M) ? dx GPD(x,x,t) 2 1/Q6

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GPDs where we stand, where we are going

• Pioneering dedicated experiments on DVCS (Hall
  A, CLAS), show evidence for handbag (twist-2)
  dominance (asymmetry sinf) and unexpected
  scaling at Q2 2 GeV2 (Hall A)

CLAS (ep?epg)
lt-tgt0.26 GeV2, ltxBgt0.36
Hall A (ep?epg)
Twist-2 Twist-3
Fit ? sin?/(1?cos??
C. M. Camacho et al., PRL97, 262002 (2006)
Small Q2 range (but small error bars)
F. X. Girod et al., arXiv 0711.4805, submitted
to PRL
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GPDs where we stand, where we are going

• Pioneering dedicated experiments on DVCS (Hall
  A, CLAS), show evidence for handbag (twist-2)
  dominance (asymmetry sinf) and unexpected
  scaling at Q2 2 GeV2 (Hall A)
• GPD models fail to reproduce consistently the
  DVCS cross section and asymmetry data

Hall A data for xB 0.36 Q2 2.3 GeV2 t -0.28
GeV2
Fit to the data
Standard H VGG
Variations on H VGG
VGG GPD model by Vanderhaegen, Guidal, Guichon
M . Guidal, arXiv 0803.1592
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GPDs where we stand, where we are going

• Pioneering dedicated experiments on DVCS (Hall
  A, CLAS), show evidence for handbag (twist-2)
  dominance (asymmetry sinf) and unexpected
  scaling at Q2 2 GeV2 (Hall A)
• GPD models fail to reproduce consistently the
  DVCS cross section and asymmetry data
• DVMP experiments at CLAS (r, w, p0) and Hall A
  (p0) confirm that scaling cannot be
• reached for Q2 as low as for DVCS and that
  something is missing in GPDs parameterizations

Preliminary
CLAS gp?pp0
Arbitrary units Statistical errors only
Interference terms different from
zero transverse and longitudinal contributions
Q2 2.25 (GeV/c)2 , xB 0.34
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GPDs where we stand, where we are going

• Pioneering dedicated experiments on DVCS (Hall
  A, CLAS), show evidence for handbag (twist-2)
  dominance (asymmetry sinf) and unexpected
  scaling at Q2 2 GeV2 (Hall A)
• GPD models fail to reproduce consistently the
  DVCS cross section and asymmetry data
• DVMP experiments at CLAS (r, w, p0) and Hall A
  (p0) confirm that scaling cannot be
• reached for Q2 as low as for DVCS and that
  something is missing in GPDs parameterizations

CLAS sL (gLp?prL0)
S. Morrow et al., in preparation
Models miss the data at low W (high xB)
VGG GPD model
Goloskokov Kroll GPD model
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GPDs where we stand, where we are going

• Pioneering dedicated experiments on DVCS (Hall
  A, CLAS), show evidence for handbag (twist-2)
  dominance (asymmetry sinf) and unexpected
  scaling at Q2 2 GeV2 (Hall A)
• GPD models fail to reproduce consistently the
  DVCS cross section and asymmetry data
• DVMP experiments at CLAS (r, w, p0) and Hall A
  (p0) hint that either scaling cannot be
• reached for Q2 as low as for DVCS or something is
  missing in GPDs parameterizations
• Hall As first attempt to measure nDVCS showed
  the importance of this channel for Jis sum rule
  and the extraction of Jq

n-DVCS access to E, the least known and
constrained GPD
Jd, Ju extraction is MODEL DEPENDENT
PhD thesis of M. Mazouz
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GPDs where we stand, where we are going

• Pioneering dedicated experiments on DVCS (Hall
  A, CLAS), show evidence for handbag (twist-2)
  dominance (asymmetry sinf) and unexpected
  scaling at Q2 2 GeV2 (Hall A)
• GPD models fail to reproduce consistently the
  DVCS cross section and asymmetry data
• DVMP experiments at CLAS (r, w, p0) and Hall A
  (p0) hint that either scaling cannot be
• reached for Q2 as low as for DVCS or something is
  missing in GPDs parameterizations
• Hall As first attempt to measure nDVCS showed
  the importance of this channel for Jis sum rule
  and the extraction of Jq

• More data needed on DVCS and DVMP
• High Q2 to verify scaling for DVCS on a wider Q2
  range, and to approach GPD validity regime for
  DVMP
• Wide xB coverage
• High accuracy on measured observables to test
  models (high luminosity required)
• Measurements of spin-asymmetries AND cross
  sections

JLab _at_ 12 GeV will be the optimal facility for
these goals
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Large phase space (x,t,Q2) and high luminosity
Valence region
Sea/gluon region
JLab Upgrade
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JLab Upgrade to 12 GeV
E 2.2, 4.4, 6.6, 8.8, 11 GeV
Beam polarization Pe gt 80
12 GeV
Enhance equipment in existing halls
Experiments on exclusive reactions to extract
GPDs are a central part of the physics programs
of the Halls A and B
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JLab_at_12 GeV DVCS setup for Hall A
H(e,e?)p

• e detected in HRS-L
• g detected in PbF2 calorimeter
• (upgraded with 76 additional elements)

Extraction of epg final state for published DVCS
_at_ 6 GeV
High resolution allows exclusivity without
requiring detection of recoil proton
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JLab12 Hall A with 3, 4, 5 pass beam
H(e,e?)p
High luminosity ? high accuracy Absolute
measurements d?(?e1) 250K events/setup
100 days
Twist 2 Twist 3 separation ImDVCSBH?DVCS2
ReDVCSBH ?DVCS2
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JLab12 projected results for DVCS in Hall A
Unpolarized cross sections (pb/GeV4)
400 hours
Ee8.8 GeV, Q2 4.8 GeV2, xB0.5
Ee11 GeV, Q2 9 GeV2, xB0.6
and exclusive p0 electroproduction will also be
measured
Polarized cross section differences (pb/GeV4)
C. Hyde-Wright et al., arXivnucl-ex/0609015
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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
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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
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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
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CLAS12 DVCS beam-spin asymmetry
e p epg
E 11 GeV
DsLUsinfImF1H..df

• L 1x1035
• T 2000 hrs
• DQ2 1 GeV2
• Dx 0.05

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

• L 2x1035 cm-2s-1
• T 1000 hrs
• DQ2 1GeV2
• Dx 0.05

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
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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
AUTy Target polarization perpedicular to
scattering plane
Transverse-target spin asymmetry is highly
sensitive to the u-quark contributions to proton
spin.
H. Avakian
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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
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Summary

• 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 parametrizations

• 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 extractions 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