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Experimental Overview of Deeply Virtual Exclusive Reactions
Charles HydeOld Dominion University
Norfolk VA
INT-12-49W: Orbital Angular Momentum in QCD February 6 - 17, 2012
Nucleon spin comes from the spin and orbital motion
of quarks and gluons--- Chairman Mao
A universally correct statement
for the nucleon spin
J. Phys. Conf. Ser. 299:012006, 2011, arXiv:1101.2482
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Deeply Virtual Exclusive Processes - Kinematic Coverages
H1, ZEUS
JLab Upgrade
H1, ZEUS
JLab @ 12 GeV27
GeV
200
GeV
W =
2 GeV
0.7
HERMES C
OMPASS
Study of high xB domain requires high luminosity
3/25/09 2Volker Burkert, Workshop on Positrons at JLab
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Bethe-Heitler and Virtual Compton Scattering (VCS)
• BH-DVCS interference Access to DVCS amplitude, linear in GPDs
e pe p g
p'p
k k'
q' = +
+
Bethe-Heitler (BH)
VCSq
D
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Leading Order (LO) QCD Factorization of DVCS
• Symmetrized Bjorken variable:
• SCHC Tranversely polarized virtual photons dominate to O(1/Q)
e pe p g
(x+P (xP
GPDg(x,ξ ,t=D2
)
+PΔ/2
GPDf(x,ξ ,t=D2)PΔ/2
GPDf(x,ξ ,t=D2)
(x+P (xP +
PΔ/2 PΔ/2
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HERA-H1 DVCS-dominated and BH-dominated events
epegXX is ultra-forward,no visible energy dominated by exclusive e+
p
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HERA DVCS, fits by D.Müller et al., 2012 for EIC whitepaper
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What do DVCS experiments measure?• ds(epepg) = twist-2 (GPD) terms + Sn [twist-n]/Qn-2
Isolate twist-2 terms cross sections vs Q2 at fixed (xBj, t).
• GPD terms are `Compton Form Factors’
Re and Im parts (accessible via interference with BH):
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DVCS, GPDs, Compton Form Factors(CFF), and Lattice QCD
Cross-section (s) measurementand beam charge difference (ReT)
integrate GPDs with 1/(x±) weight
Beam or target spin Dscontain only ImT,
therefore GPDs at x = and
(at leading order:)
D.R.
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Exploiting the harmonic structure of DVCS with polarization
The difference of cross-sections is a keyobservable to extract GPDs
With polarized beam and unpolarized target:
With unpolarized beam and Long. polarized target:
With unpolarized beam and Transversely polarized target:
~
~
Separations of CFFs H(± ,t), E(± ,t),…
~
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HERMES overview
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HERMES-Transversely Polarized H(e,e’ )g X, SSA
• Azimuthal moments• Differential in
xBj, Q2, or t, integrated over other 2 variables.
• sinf moments Sensitive to E(,,t
• sin2f moments ≈ 0 ≈ Twist 3
• sin3f moments ≈Gluon Transversity
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HERMES Recoil Detector
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HERMES Recoil Exclusivity
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HERMES Recoil Detector Results
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HERMESsummary 2011
• next to final• averaged
over Q2 and t• Transversely
polarized H-target sensitivity to E(,,D2),≈0.1
DVCSAsymmetries
e+e–
BeamSpinAsymmetryTransversetargetSingle Spin
Beam &Transverse TargetDouble Spin
LongitudinalTarget
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DVCS in CLAS @ 6 GeV
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CLAS: Longitudinally Polarized Protons
S.Chen, et al, PRL 97, 072002 (2006)
JLab/Hall B – Eg1 Non-dedicated experiment(no inner calorimeter), butH(e,e’gp) fully exclusive.
AUL
fully exclusive
Higher statistics and larger acceptance (Inner Calorimeter) run Feb-Sept. 2009
fitGPD
Fig. 5.
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e’
p
DVCS@Hall B
epa epg
g
5 Tesla Solenoid420 PbWO4 crystals :
~10x10x160 mm3 APD+preamp readout
Orsay / Saclay / ITEP / Jlab
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CLAS 6 GeV: Exclusivity and Kinematics• H(e,e’gp’)x• Overcomplete
triple coincidence
• Example angular distribution of Beam Spin Asymmetry• One (Q2,xB) bin
• Two t-bins.
Co-linearity of g with q-p’
Missing Energy Ex
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CLAS, 6 GeV Beam Helicity Asymmetry
• F.X. Girod et al, Phys.Rev.Lett.100, 162002, 2008
• sinf moments of ALU
Solid blue curves: VGG GPD model
• Data set doubled by Fall/Winter 2008/2009 run
Q2
xB
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CLAS DVCS Longitudinal Target w/ Inner Calorimeter
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(e,e’)X HRS trigger
132 PbF2Digital Trigger Validation
16chan VME6U: ARS128 samples@1GHz
DVCS: JLab Hall A 2004, 2010L ≥ 1037 cm2/sPrecision cross sections•Test factorization•Calibrate Asymmetries
e-
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Beam helicity-independent cross sections at Q2=2.3 GeV2, xB=0.36
d4 s
nb/G
eV4 )
•Contribution of Re[DVCS*BH] + |DVCS|2 large.•Positron beam or measurements at multiple incident energies to separate these two terms and isolate Twist 2 from Twist-3 contributions
PRL97:262002 (2006) C. MUNOZ CAMACHO, et al.,
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DVCS-Deuteron, Hall A• E03-106:
D(e,e’g)X ≈ d(e,e’g)d+n(e,e’g)n+p(e,e’g)p
Sensitivity to En(,,t) in Im[DVCS*BH]
• E08-025 (5.5 GeV- 2010) Reduce the systematic errors
• Expanded PbF2 calorimeter for p0 subtraction
Separate the Re[DVCS*BH] and |DVCS|2 terms on the neutron via two beam energies.
Q2=2.3 GeV2, xB=0.36
neutron
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CLAS12
• GPDs & TMDs• Nucleon Spin Structure • N* Form Factors• Baryon Spectroscopy • Hadron Formation
Central Detector
Forward Detector
CLAS12
2m
3/25/09 25Volker Burkert, Workshop on Positrons at JLab
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DVCS with CLAS at 12 GeV
• 80 days on H2 target at ~1035 /cm2/s
• 120 days on Longitudinally Polarized NH3 target Total Luminosity 1035 /cm2/s, dilution factor ~1/10
• D(e,e’gn)pS
• Ambitions/options for Transversely polarized targets NH3 target has 5 T transverse field
• need to shield detectors from “sheet of flame”• Reduce (Luminosity)(Acceptance) by factor of 10 (my guess)
HD-ice target (weak holding field, less dilution)• Currently taking data with photon beam• Polarization measurements incomplete• Test with electron beam in 1-2 months.
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DVCS at 12 GeV in Hall A:100 days HRS ×PbF2
All equipment in-hand.Ready for beam !
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COMPASS 2014+ DVCS & DVES
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COMPASS Recoil Proton Detector + ECAL0
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COMPASS DVCS Projections
• 160 GeV
• Spin×Charge averaged
• Spin×Charge difference
• Spin×Charge difference
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Leading Order (LO) QCD Factorization of DVES
P-Δ/2
D
x+ξ x-ξ
P +Δ/2 GPD(x,ξ ,t=D2)
DA(z)z
Gluon and quark GPDs enter to same order in S.
SCHC: sL~ [Q2] sT~ [Q2]
Spin/Flavor selectivity
+
+
[Diffractive channels only]
GPD(x,ξ ,t=D2)
DA(z)z
x+ξ x-ξ
g*
x+ξ x-ξ
GPD(x,ξ ,t=D2)
DA(z)z
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Semi Universal behavior of exclusive reactions at high W2
• Two views: Extracting leading twist
information is hopeless for Q2+q’2<10 GeV2
Perturbative t-channel exchange even for modest Q2, but convolution of finite size of nucleon and probe.
• Fitting data (cf C.Weiss) requires setting scale of gluon pdf 2 <<Q2
Finite transverse spatial size b≈1/ of gV amplitude
GWolf 0907.1217
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sL/sT in vector meson production at HERA
• SCHC: rpp, wppp, f KK Validate SCHC from decay angular distribution (Schilling & Wolf)
Extract dsL from
• Rapid rise in r04 vs Q2: Validation of
perturbativeexchange in t-channel.
• Sub-asymptotic saturation of dsL/dsT
Extra mechanism for dsT?
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Vector Mesons at JLab
• Deep r SCHC observed at 20% level Anomalous rise in dsL at low W
• Deep w SCHC strongly violated in CLAS data No (??) SCHC tests from HERMES or HERA.
• Deep f SHCH validated Model of P. Kroll consistent with world data set
• Perturbative t-channel exchange (2g), but factor of 10 suppression relative to colinear factorization from Sudakov effects in gf
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CLAS Deep rho
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Deep f
• Q2≈2 GeV2
CLAS, HERMES, HERA
• Model of S.Goloskokov and P. Kroll
Proposals/LOI in Hall B and Hall A
LOI for J/Y in Halls B and C.
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The next 20 years of DVCS experiments• First 5 years
Precision tests of factorization with Q2 range ≥ 2:1 for • xB∈[0.25,0.6]. tmin-t < 1 GeV2 + COMPASS : xB∈[0.01,0.1]• Proton unpolarized target observables• Im[DVCS*BH], Re [DVCS*BH], |DVCS|2.
Longitudinal, target spin observables• Primary sensitivity to H, ˜H, at x = ± = ±xB/(2xB) point.
Partial u,d flavor separations from quasi-free neutron. Coherent Nuclear DVCS on D, He
• 5-10 years Transversely Polarized H, D, 3He in JLab Halls A,B,C
• Optimize targets• Improved recoil/spectator detection?
Polarized targets at COMPASS• 10-15 years: Build electron ion collider with s≥1000 GeV2
and L > 21034 /cm2/s.
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Back-up Slides
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HERA DVCS
• Spatial imaging of gluons at small xB
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Unraveling DVCS observables• Twist-2 terms ≈
1, cosf, sinf • Twist-2 terms ≈
sin2f • Not a pure Fourier series:
1/[A + Bcosf Ccos2f]from BH propagators.
• GPDs enter with different weights for each azimuthal term for different polarization (lepton helicity, target-longitudinal or –transverse) observables Single and Double spin observables Beam charge difference (e+e– HERMES, JLab????; – COMPASS) Energy dependence (JLab)
• Complete separation of Re and Im parts of CFF of E, H,… in-principle possible ( D.Mueller, next)
• u, d flavor separations require neutron targets (or deep meson electroproduction)
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HERMES DVCS p(e,e’g)X
27 GeV polarized e± on Internal Gas Jet / Atomic Beam Source targets
2001 BSA
2006 BCA
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Hall A Helicity Dependent Cross Sections E00-110
Q2 = 2.3 GeV2
4 bins, Dt = 0.055 GeV2
Twist-2(GPD)+… Twist-3(qGq)+…Q
2 =2.
3 G
eV2
s1,2 = kinematic factors
PRL97:262002 (2006) C. MUNOZ CAMACHO, et al.,
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GPD results from JLab Hall A (E00-110) (C.MUNOZ CAMACHO et al PRL 97:262002)
• Q2-independance of Im[DVCS*BH]• Twist-2 Dominance (GPD)• Model « Vanderhaeghen-Guichon-Guidal (VGG) »
accurate to ≈30%
Compensate the small lever-arm in Q2 with precision in ds.
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CLAS12 – Central Detector SVT, CTOF
Charged particle tracking in 5T field
ΔT < 60psec in for particle id
Moller electron shield
Polarized target operation ΔB/B<10-4
3/25/09 44Volker Burkert, Workshop on Positrons at JLab
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Operates at T~500-750mK •Long spin relaxation times (months)•Weak transverse magnetic field
Heat extraction is accomplished with thin aluminum wires running through the target
HD ice : a transversely polarized target for CLAS
Test in 2010 with electron beam, Experiment conditionally scheduled in 2011
• 25+ years of development…
• Successful operation at LEGS photon beam
• Just in time for DVCS!!!!
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CLAS: Coherent 4He(e,e’g)
• A single GPD H(,,t)=(4/9)Hu+(1/9) Hu. GE=∫dx[(2/9)Hu-(1/9)Hu].
• E08-024, Autumn 2009 BoNuS GEM radial TPC
[t=0.0] EMC effect, [t=-0.1] GPD (Liuti & Taneja, Guzey & Strickman)
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DVCS in Hall A
• Elastic form factors, Real Compton Scattering: Correlated two-body final state, Spectrometers have the advantage over large acceptance:
• product of (Luminosity)(Acceptance)• Precision of absolute cross sections
• DVCS is a 3-body final state For –t/Q2<<1, final photon close to q-direction. Quasi two-body final state for limited t coverage