Transcript [ppt]
Experimental requirements for GPD
measurements at JLab energies.
• Detector that ensures exclusivity of process,
measurement of complete final state
• Measure single γ (DVCS) & π0 simultaneously up
to 9 GeV/c
• Vector mesons & pseudoscalar mesons
• Double DVCS (e+e-, µ+µ-)
• Large kinematics coverage in Q2, x, t
• Polarized beam, longitudinal & transverse
polarized targets.
GPDs with Exclusive DVCS and
ρ0/ρ+ production at CLAS12
Exclusive Processes
ep → epγ + X
ep → epρ0 + X
with EX = 0
Need γ/π0 identification for momentum ≤ 9 GeV/c
CLAS12 – Complete event reconstruction
Forward Calorimeter
Preshower Calorimeter
Forward Cerenkov
(LTCC)
Forward
Time-of-Flight
Detectors
Forward Drift Chambers
Superconducting
Torus Magnet
New TOF
Layer
Inner Cerenkov
(HTCC)
Central Detector
Beamline
Instrumentation
* Reused detectors from CLAS
Inner Calorimeter
Optimized for
unpolarized &
polarized target
experiments.
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Exclusive Deeply Virtual Compton Scattering ep→e’p’γ
Polarized beam, unpolarized target:
~
DsLU ~ sinf{F1H( x,x,t) + x(F1+F2)H +kF2E}
Kinematically suppressed at small t
Unpolarized beam, longitudinally polarized target:
DsUL
~
~ sinf{F H+x(F +F )(H +..
1
1
2
}
x = xB/(2-xB )
k = t/4M2
Kinematically suppressed
Unpolarized beam, transversely polarized target:
DsUT ~ sinf{k(F2H – F1E) + ….. }
Kinematically suppressed
GPD combinations accessible as azimuthal moments of the total cross section.
Deeply Virtual Exclusive Processes Kinematics Coverage of the 12 GeV Upgrade
H1, ZEUS
H1, ZEUS
COMPASS
11 GeV
HERMES
Study of high xB domain
requires high luminosity
0.7
Exclusive DVCS with polarized beam
ep
E = 11 GeV
DsLU~sinfIm{F1H+..}df
Selected Kinematics
L = 1x1035
T = 2000 hrs
DQ2 = 1 GeV2
Dx = 0.05
epg
Projected results
Exclusive DVCS on longitudinal target @ 6GeV
Exclusive DVCS on longitudinal target @ 12GeV
ep
epg
Longitudinally polarized target
~
Ds~sinfIm{F1H+x(F1+F2)H...}df
CLAS Exclusive DVCS
PRL97, 072002 (2006)
L = 2x1035 cm-2s-1
T = 1000 hrs
DQ2 = 1GeV2
Dx = 0.05
Exclusive DVCS on transverse target
Sample kinematics
ep
epg
E = 11 GeV
Transverse polarized target
Ds ~ sinfIm{k1(F2H – F1E) +…}df
AUTx Target polarization in scattering plane
AUTy Target polarization perpedicular to
scattering plane
Asymmetry highly sensitive
to the u-quark contributions to
proton spin.
Q2=2.2 GeV2, xB = 0.25, -t = 0.5GeV2
Exclusive r0 production on transverse target
AUT = -
K. Goeke, M.V. Polyakov,
M. Vanderhaeghen, 2001
2D (Im(AB*))/p
|A|2(1-x2) - |B|2(x2+t/4m2) - Re(AB*)2x2
r0
A ~ 2Hu + Hd
B ~ 2Eu + Ed
r+
A ~ Hu - Hd
B ~ Eu - Ed
Q2=5 GeV2
r0
Eu, Ed needed for
angular momentum
sum rule.
Exclusive r+ production on transverse target
A ~ Hu - Hd
B ~ Eu - Ed
Strong sensitivity
to d-quark
contributions.
CLAS
5.7 GeV
r+
AUT
r+
n
CLAS12 - Acceptance for DVCS
E = 11 GeV
ep
epg
Q2 > 2.5 GeV2
Central Detector
For small t, protons
recoil at large polar
angles.
Forward Detector
CLAS12 – Acceptance for DVCS
Ee = 11 GeV, xB = 0.325-0.375
ep
epγ
(πo)
EC
IEC
DVCS photons
need detection
for Qg > 2o
γ
EC
EC
IEC
IEC
Kinematics and Acceptance with CLAS + IC
DVCS studies require:
high Q2
low t
The much improved acceptance for photon detection, and the longer running time
will allow us to extend the kinematics range to higher Q2, and to map out the xB
and t dependence in small bins
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Target Spin Asymmetry: f Dependence
6 GeV run with NH3 longitudinally polarized target
(CLAS + IC) 60 days of beam time
CLAS eg1 (preliminary)
• CLAS (eg1+IC) projected
A dedicated CLAS experiment with longitudinally polarized
target will provide a statistically significant measurement of
the kinematical dependences of the DVCS target SSA
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