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

Beam Spin Asymmetry Measurements from Deeply Virtual Meson Production

Introduction

Experiment

Preliminary Results

DVMP with CLAS12

Summary

Proton form factors, transverse charge & current densities

D. Mueller, X. Ji, A. Radyushkin, A. Belitsky, …M. Burkardt, … Interpretation in impact parameter space

Structure functions,quark longitudinalmomentum & helicity distributions

?

Correlated quark momentum and helicity distributions in transverse space - GPDs

How is the proton charge density related to its quark momentum distribution?

ElasticDIS

Link to DIS and Elastic Form Factors

),,(~ ,~ , , txEHEH qqqq

DIS at =t=0

)()0,0,(~)()0,0,(

xqxH

xqxHq

q

Form factors (sum rules)

)(),,(~ , )(),,(~

) Dirac f.f.(),,(

,

1

1,

1

1

1

tGtxEdxtGtxHdx

tF1txHdx

qPq

qAq

q

q

) Pauli f.f.(),,(1

tF2txEdxq

q

Lq = 1

1

)0,,q()0,,q(21

21 xE xHxdxJ q

X. Ji, Phy.Rev.Lett.78,610(1997)

Ji’s Angular Momentum Sum Rule

• Deeply Exclusive Processes:

Deeply Virtual Compton Scattering (DVCS): Clean (Talk by F. Sabatie), but limited flavor separation

Deeply Virtual (Exclusive) Meson Production (DVMP): helps to separate flavor helicity components of GPDs

What can we measure?

L

0, : Unique sensitivity to polarized GPDs

•Different mesons select different combinations of quark flavors. (Difficult to isolate in DVCS)

•Extract unique information about GPDs, complementing DVCS.

0, +

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)

Structure Functions

N

d/d = T + L + TTcos2 + √LTcos + h√LT’sin

• High-luminosity

• Large acceptance

• Good Resolution

ep->ep, ep->ep + - -

ALU = + + -

Emax ~ 6 GeV

Imax ~ 200 A

Duty Factor ~ 100% E/E ~ 2.5

10-5

Beam P ~ 80%E(tagged) ~ 0.8- 5.5 GeV

CLAS

CEBAF at Jefferson Lab

/q e e

p

/e

•Six identical sectors

•5 T toroidal B-field

•Δθ=15-140 degrees

•Δφ = 0-50 degrees

•Δp/p = 10-2-10-3

CEBAF Large Acceptance Spectrometer (CLAS)

K+

p

Event Reconstruction

Inner calorimeter (PbWO4)

Superconducting solenoid magnet

Additions to CLAS

New Forward Calorimeter.(E06-003 DVCS)

• Need to reconstruct 0 and from 2 decays

424 PBWO4 crystals, 16 mm long, pointing geometry, ~ 1.4 degree/crystal,

APD readout

photon detection at small angles (4-15; 1-5 GeV)

Front view from the target

Inner Calorimeter (IC)

Mass Reconstruction of

Only from new forward calorimeter

Preliminary

epep0 event selection

Data Analysis by R. De Masi MM2 of epepX

Preliminary

Integrated ALU for 0 channel

• Integrated for all Q2 and x• Fit to asin

Data Analysis by R. De Masi

Preliminary

ALU

ALU for 0 channel

Q2

xB

Data Analysis by R. De Masi

Preliminary

1.0

1.5

2.0

2.5

3.0

3.5

0.10

0.15

0.20

0.30

0.40

Preliminary

4.5

0.55

epep event selection

Data Analysis by B. ZhaoMM2 of epepX

Preliminary

Integrated ALU for channel

• Integrated for all Q2 and x• Fit to asin/(1+bcos + ccos2)

Data Analysis by B. Zhao

Preliminary

ALU

Beamline Instrumentation

Preshower Calorimeter

Forward Drift Chambers

Inner Cerenkov(HTCC)

SuperconductingTorus Magnet

Central Detector

Forward Calorimeter

Forward Time-of-Flight

Detectors

* Reused detectors from CLAS

Forward Cerenkov(LTCC)

Inner Calorimeter

CLAS12

K. Joo, M.UngaroUniversity of Connecticut, USA

V. Kubarovsky, P.StolerDepartment of Physics, Rensselaer,

USA

1. High Q2, Low t

2. High Q2, High t

DVMP of and with CLAS12

PR12-06-108

Acceptance of Q2 vs xB with CLAS12

• Increase in luminosity by a factor ~5• Larger acceptance

Acceptance for 0 events using FASTMC for CLAS12

Summary

1. An extensive program is underway to study GPDs by measuring exclusive channels.

2. Measured large ALU in ep ->ep, ep->ep

3. The second half of the run (e1-dvcs) is scheduled in 2008.

4. The new experiment (E05-114) using polarized proton target is also scheduled in 2008.

5. 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.