1. Beam Spin Asymmetry Measurements from Deeply Virtual Meson Production
B. Zhao1, R. De Masi2, K. Joo1, V. Kubarovsky3, P. Stoler3, M. Ungaro1
and the CLAS Collaboration
1. University of Connecticut, USA,
2. Saclay-CEA, France
3. RPI, USA
Introduction
Experiment
Preliminary Results
DVMP with CLAS12
Summary
The title of this talk is Beam Spin Asymmetry measurements from Deeply Virtual Meson Production with CLAS at Jefferson Lab.

2. ? How is the proton charge density related to
its quark momentum distribution?
D. Mueller, X. Ji, A. Radyushkin, A. Belitsky, …
M. Burkardt, … Interpretation in impact parameter space
DIS
Elastic
Correlated quark momentum
and helicity distributions in
transverse space - GPDs
Structure functions,
quark longitudinal
momentum & helicity
distributions
Proton form factors, transverse charge & current densities ?
As shown in the left hand side, from elastic scattering, we can study proton’s transverse charge and current densities. As shown in right hand side, deeply inelastic scattering we probe quark’s longitudinal momentum distributions, but has no sensitivities in transverse dimension. The recently developed formalism of Generalized Parton Distributions (GPDs) now holds promise to correlate the proton charge density related to its quark momentum distribution.

3. [ ] ò [ Link to DIS and Elastic Form Factors ] å ò ) ( , ~ q H D = ) ,
Form factors (sum rules) [ ) ( , ~
) Dirac f.f. 1 t G x E dx H F1 q P A = ] ò å -
) Pauli f.f. F2
DIS at x
=t=0 ) ( , ~ q H D = ) , ( ~ t x E H q
GPDs has 4 terms, H, E, H-tilde, E-tilde. They depend three kinematic variables, x, zeta, t. The second moments of GPDs and the total angular momentum carried by quarks are related via Ji’s sum rule. The quark’s total angular momentum can be decomposed in a spin part (Delta Sigma) and an orbital momentum part (Lq). If one makes enough measurements to extract the second moment of GPDs, the sum rule will determine the orbital momentum contribution to the nucleon spin. In the forward direction, the H and H-tilde reduce to the usual parton distributions q(x) and del q(x). On the other hand, the first moments of GPDs and the elastic form factors are related the following relations.
Ji’s Angular Momentum Sum Rule 1 1 1 ò [ ]
J q =
DS+Lq =
xdx H q( x , x , ) + E q( x , x , ) 2 2 - 1
X. Ji, Phy.Rev.Lett.78,610(1997)

4. What can we measure? Deeply Exclusive Processes:
To extract GPDs, we measure deeply exclusive processes such as deeply virtual compton scattering and deeply virtual Compton scattering. DVCS is the simplest and cleanest. More detailed talk will be given by Franck later this session. Deeply virtual meson production will be complementary to DVCS and helps to separate flavor helicity components of GPDs.
Deeply Virtual (Exclusive) Meson Production (DVMP): helps to separate flavor helicity components of GPDs
Deeply Virtual Compton Scattering (DVCS): Clean (Talk by F. Sabatie)
, but limited flavor separation

5. p0, h: Unique sensitivity to polarized GPDs
0, +

For example, pi0 and eta channels are sensitive to a polarized GPDs H-tilde.
Different mesons select different combinations of quark flavors. (Difficult to isolate in DVCS)
Extract unique information about GPDs, complementing DVCS.

6. How to extract GPDs
Beam spin asymmetries, longitudinal and transverse target spin asymmetries for DVCS and DVMP.
(get imaginary part of the amplitudes)
Absolute cross section measurements.
(get real part of amplitudes)
Separations of different GPDs.
(E and H and H-tilde separations)
To extract GPDs, we measure beam asymme

7. Structure Functions ep->epp0, ep->eph ALU = s+ + s-
ds/dWp = sT + esL + esTTcos2f + √2e(e+1)sLTcosf + h√2e(e-1)sLT’sinf
ep->epp0, ep->eph
s+ - s-
ALU =
s+ + s- N
High-luminosity
Large acceptance
Good Resolution
p, h
This picture pictures shows deeply virtual meson production of pi0 and eta channels. Here Incoming and scattered electrons produce virtual photons and we measured all final states. Beam spin asymmetry ALU will be determined by measuring the ratio of the difference and sum of two electron helicity states. To measure exclusive channels, we need high luminosity. We also need a spectrometer with large acceptance and good resolution to identify all final states.

8. CEBAF at Jefferson Lab CLAS Emax ~ 6 GeV Imax ~ 200 mA
Duty Factor ~ 100%
sE/E ~
Beam P ~ 80%
Eg(tagged) ~ GeV
CLAS

9. CEBAF Large Acceptance Spectrometer (CLAS)
Six identical sectors
5 T toroidal B-field
Δθ= degrees
Δφ = 0-50 degrees
Δp/p =

10. Event Reconstruction p - K+ p
Particles either inbend or outbend depending the polarity of particle and toroidal field. Momentum is determined from the curvature of the track and pid is done using momentum and tof. p

11. Additions to CLAS Inner calorimeter (PbWO4)
Superconducting solenoid magnet

12. New Forward Calorimeter.
(E DVCS)
Need to reconstruct 0 and  from 2 decays

13. Inner Calorimeter (IC)
photon detection at small angles
(4-15; 1-5 GeV)
Front view from the target
424 PBWO4 crystals, 16 mm long,
pointing geometry, ~ 1.4 degree/crystal,
APD readout

14. Preliminary Mass Reconstruction of gg
Only from new forward calorimeter

15. epepp0 event selection
MM2 of epepX
Data Analysis by R. De Masi
Preliminary

16. Integrated ALU for p0 channel
Data Analysis by R. De Masi
ALU
Preliminary f
Integrated for all Q2 and x
Fit to a*sinf

17. Preliminary Preliminary ALU for p0 channel Q2 xB
4.5
Data Analysis by R. De Masi
3.5
3.0
Preliminary
Preliminary
2.5
2.0
1.5
1.0 xB
0.10
0.15
0.20
0.30
0.40
0.55

18. epeph event selection
MM2 of epepX
Data Analysis by B. Zhao
Preliminary

19. Integrated ALU for h channel
Data Analysis by B. Zhao
ALU
Preliminary f
Integrated for all Q2 and x
Fit to a*sinf/(1+b*cosf + c*cos2f)

20. CLAS12 Superconducting Forward Calorimeter Preshower Calorimeter
Forward Cerenkov
(LTCC)
Forward
Time-of-Flight
Detectors
Forward Drift Chambers
Superconducting
Torus Magnet
Inner Cerenkov
(HTCC)
Central Detector
Beamline
Instrumentation
Inner Calorimeter
* Reused detectors from CLAS

21. Hard exclusive production of  and  with CLAS12 PR12-06-108
DVMP of  and  with CLAS12 PR
K. Joo, M.Ungaro
University of Connecticut, USA
V. Kubarovsky, P.Stoler
Department of Physics, Rensselaer, USA
High Q2, Low t
High Q2, High t
Hard exclusive production of  and  with CLAS12 PR

22. Acceptance of Q2 vs xB with CLAS12
Increase in luminosity by a factor ~5
Larger acceptance

23. Acceptance for p0 events using FASTMC for CLAS12

24. Summary
An extensive program is underway to study GPDs by measuring exclusive channels.
Measured large ALU in ep ->epp0, ep->eph.
The second half of the run (e1-dvcs) is scheduled in 2008.
The new experiment (E05-114) using polarized proton target is also scheduled in 2008.
Deeply virtual meson production (DVMP) measurements with CLAS12 as well as DVCS is approved and will be one of flagship experiments with the JLAB 12 GeV upgraded program.