DVCS beam-spin asymmetries and meson electroproduction with CLAS

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DVCS beam-spin asymmetriesand meson
electroproduction with CLAS

• Silvia Niccolai for
  the CLAS Collaboration

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DVCS beam-spin asymmetriesand meson
electroproduction with CLAS

• GPDs and DVCS
• First CLAS results
• The e1-dvcs experiment
• Event selection
• Results BSA
• Preliminary DVCS cross sections
• DVMP current status
• Conclusions and outlook

• Silvia Niccolai for
  the CLAS Collaboration

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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
GPDs Correlation between quark distributions
both in momentum and coordinate space
 Handbag  factorization valid in the Bjorken
regime high Q2 , ? (fixed xB)
Quark angular momentum (Jis sum rule)
X. Ji, Phy.Rev.Lett.78,610(1997)
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Accessing GPDs through DVCS
?s s - s - ? I(DVCSBH)
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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
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
Vanderhaegen, Guidal, Guichon
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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

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The CLAS detector

• Toroidal magnetic field (6 supercond. coils)
• Drift chambers (argon/CO2 gas, 35000 cells)
• Time-of-flight scintillators
• Electromagnetic calorimeters
• Cherenkov Counters (e/p separation)

• Performances
• large acceptance for charged particles
• 8ltqlt142, ppgt0.3 GeV/c, ppgt0.1GeV/c
• good momentum and angular resolution
• Dp/p 0.5- 1.5, Dq, Df 1 mrad

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DVCS _at_ CLAS first results
NON-DEDICATED experiments
ep?epX
ep?epg
S. Chen et al., PRL 97 (2006)
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 handbag
dominance confirmed
Low statistics Limited kinematics Uncertainties
on determination of epg final state
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Hall A (JLab) first dedicated DVCS experiment
C.M. Camacho et al., PRL 97 (2006)
ep?egX
Polarized cross section difference
Q2 evolution? evidence for scaling
(small Q2 range)
Unpolarized cross section
Formalism of Belitsky, Mueller, Kirchner
ltQ2, -tgt (2.3,-0.28)
High resolution?exclusivity High
luminosity?precision Limited phase space
Fit
BH2
Inteference BH-DVCS
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The e1-dvcs experiment at CLAS

• Experiment March - May 2005
• Ee 5.77 GeV
• Polarization 76 - 82
• Current 20-25 nA
• Integrated luminosity 3.33? 107 nb-1

424 PbWO4 crystals 16cm x 1.3cm x 1.3cm 18
radiation lengths APD light readout
Superconducting solenoid magnet (shielding for
Moeller electrons)
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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
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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)

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BSA coverage and f distributions
Data integrated over t

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

Fit ? sin?/(1?cos??
Ph.D. Thesis of F.X. Girod
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BSA a vs. t
() Guidal, Polyakov, Radyushkin, Vanderhaegen,
PRD 72 (2005)
() Cano and Laget, PL B551 (2003)
GPD model overestimates the data
arXiv 0711.4805 hep-ex Submitted to PRL
Ph.D. Thesis of F.X. Girod
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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
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Difference of Polarized 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
Largest kinematical coverage ever obtained for
DVCS 0.1ltxBlt0.58, 1ltQ2lt4.6, 0.09lt-tlt2
Ph.D. Thesis of H.S. Jo
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Hard exclusive meson production and GPDs
4 Generalized Parton Distributions (GPDs) H
H conserve nucleon helicity E
E flip nucleon helicity
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
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ep?epp0 in CLAS channel selection
MM2(epp0)
IM(gg)
MM2(ep)
MM(ep0)
p0
w
p0
h
p
Before cuts
M2(GeV2/c4)
M(GeV/c2)
M(GeV/c2)
M2(GeV2/c4)
After cuts
p0
p
p0
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ep?epp0Kinematical coverage and binning

• W gt 2 GeV/c2
• 7 Q2 bins (1 - 4.6 GeV2/c2)
• 7 xB bins (0.1 0.55)
• 8 -t bins (0.09 2 GeV2/c2)
• 20 f bins

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ep?epp0 reduced cross sections
NO sT/sL separation
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sTesL vs. t (GeV2/c2)
Preliminary
Arbitrary units Statistical errors only
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sLT vs. t (GeV2/c2)
Preliminary
Arbitrary units Statistical errors only
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sTT vs. t (GeV2/c2)
Preliminary
Arbitrary units Statistical errors only
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Arbitrary units Statistical errors only
Preliminary
Interference terms different from
zero transverse and longitudinal contributions
Q2 2.25 (GeV/c)2 , xB 0.34
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Beam-spin asymmetry
BSA fit asinf
Rita De Masis Bo Zhaos analysis
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Beam-spin asymmetry
a vs. t
a 0.08
Rita De Masis Bo Zhaos analysis
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Conclusions

• DVCS is the cleanest reaction giving access to
  GPDs
• First pioneering experiments 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