Recent Results from Jefferson Lab Experiments

APCTP Workshop2007. 2. 28

Wooyoung KimKyungpook National University

OutlineOutline

Jefferson Lab, CLAS, Detector, KNU Contribution Pion Form Factor N-△ Transition Roper Resonance DVCS with Polarized Proton Target 12 GeV Upgrade

Thomas Jefferson National Accelerator Facility

The CEBAF Large Acceptance Spectrometer in Hall B

KNU Contribution

Laser Calibration of the TOF System

Detection Efficiency of Calorimeter

Reaction Analysis

Kijun Park Ph. D. Thesis

12 GeV Upgrade of TOF System

0 ,

( , ' )p e e n

Pion Form Factor arXv:nucl-ex/0607005 v1 7Jul 2006

Charged Pion Form at Q2 =1.60 and 2.45 (GeV/c)2

1H (e,e’ π+)n cross section measured

The simplest hadronic system available for study

Valence structure is abound state of a quark and antiquark

pQCD prediction

At low Q2: VMD model provides description of Fπ Transition from “soft” to “hard” physics expected at significantly lower Q2 for Fπ than for nucleon form factor

Fπ extracted from data by comparing the separated to Regge model prediction in which Fπ is a free parameter

Fπ deviates from the charge-radius constrained monopole form atthese values of Q2 by one σ

Fπ far from its pQCD predi

ction

Intrinsic Deformation of the Spin ½ Nucleon ; Inferred from Transition Amplitudes for the N- Δ excitation

Spherical Bag Model ; Δ+(1232) : A pure Spin-flip Transition through an M1 Excitation. D-state admixtures in the ground state of the Nucleon and/or Δ allo

ws the Quadrupole Excitation.

Glashow’s Suggestion ; A New Observable, REM(EMR) A non-zero Value for EMR : Clear Evidence for “Deformation”

Virtual Photon ; E2, C2(Coulomb, or Scalar), REM(EMR), RSM(CMR)

N-Δ Transition arXv:hep-ex/0606042 v1 19Jun 2006

Proton Structure

REM > 0

REM < 0

Asymmetry measurements

/

*

*

: Re( ) Re( ) Re( ) Im( ) Im( )

: Im( ) Re( ) Im( ) Im( ) Re( )

LT

LT

LT L T L T

LT L T L T

/m

LTe

m

AA

P

N NA

N N

/

*2 2* *

0* *2 (1 ) sin sinL LT

pdh

d k

2 * *

0

* *

sin cos 2

2 (1 ) sin cos

T L L TT

L LT

/

/

* *2 2

2 20

2 (1 ) sin sinL LTLT

d dA

d d

Q2 dependence of the electric (E1+) and scalar (S1+) quadrupole/magnetic dipole ratios from this experiment(dots)

Structure function versus cosθ*

π extracted for the p(e,e/ p)π0 reac-tion at Q2 =0.9 GeV2 .

Pion Deform the Proton’s Electric Charge Cloud

The photon (blue) emitted by the scattered electron (red) interacts directly with the pion cloud rather than with the quarks inside the proton

The measured shape of the deformed electric-charge cloud is slightly oblate (squashed), rather than prolate (elongated).

Dynamical Pion Rescattering Modes

Calculate a meson dressed vertex in terms of the underlying bare photocoupling form factors.

Fitted their dynamical model to photo-pion observables and Jlab/ Hall-C cross sections using a common parametrization for the bare charge GC(Q2) and electric quadrupole form factors.

Near Q2=0, GC(0) determined from GE(0) using the long wavelength limit. (Siegert’s Thm). T. Sata and T. S. Lee, PRC 63, 055201 (2001) S. S. Kamalov and S. N. Yang, PRL 83, 4494 (1999) “Pion cloud effects dominate the quadrupole transition”

*N

(1232)N

2( )EG Q N

ALT/- measurements

( , )p e e n Beam asymmetry ALT / vs φπ

* for the reaction at Q 2 =0.40 GeV2

and W=1.22 GeV.

for π0p channel (top) and for π+n channel (bottom), extracted at Q2 =0.40 GeV2 and W=1.18-1.26 GeV

*cosLT vs

AL

T/

- measurements

magnetic form factor

decreases with Q2 faster than the proton magnetic form factor

REM is small and negative, indicating strong helicity non-conservation

RSM is negative, while its magnitude increases with Q2

The results confirm the absence of pQCD scaling at these kinematics

Electron Beam Asymmetry

E = 5.754 GeVW=1.40, 1.69GeV @ Q2= 2.05GeV2

φCM φCM

A LT’

A LT’

MAID00

MAID03

SL04SL

Roper Resonance Kijun Park Ph. D. Thesis KNU (2006)

Structure Function

MAID00

MAID03

SL04SL

Δ(1232)

P11(1440)

S11(1535)

F15(1680)

Structure Function MAID00

MAID03

SL04SL

Δ(1232)

P11(1440)

S11(1535)

F15(1680)

Structure Function MAID00

MAID03

SL04SL

Δ(1232)

P11(1440)

S11(1535)

F15(1680)

Structure Function // MAID00

MAID03

SL04SL

Δ(1232)

P11(1440)

S11(1535)

F15(1680)

Photocoupling Amplitude A1/2 , S1/2

Quark Models

This Work

GWU (VPI) pion photoproduction

RPP etimation

ηelectro-, photo- production IM, DR

π electro- production IM, DR

Relativistic Quark ModelNonerlativistic Quark ModelBonn, DESY, NINA, Jlab(η)

Light-front calculation

q3G hybrid state

GPDs Formalism for the QCD description of deeply exclusive leptoproduc

tion reactions introduces Generalized Parton Distribution (GPDs)

Carry new Information about the dynamical degrees of freedom inside the Nucleon

In the Bjorken scaling regime(Q2→∞, xB finite), the amplitude for exclusive scattering reaction can be factorized into

A hard scattering part (exactly calcluable in pQCD) A nucleon structure part (parameterized via GPDs – handbag approximations)

DVCS arXiv:hep-ex/0605012 v3 3Jul 2006

Deeply Virtual Compton Scattering

Virtual Compton Scattering in the Bjorken regime

Virtual Compton Scattering : Electroproduction of photons from nucleons

The cleanest way of gathering information on nucleon structure

The simplest experiment for studying GPDs (W > 2GeV, Q2 > 1 (GeV/c)2)

Feynman diagrams for DVCS and Bethe-Heitler processes contributing to the amplitude of scattering ep ep

The experiment measures the interference of DVCS and Bethe-Heitler (BH) process

Beam Spin Asymmetry

where : beam polarization (70%) : number of events at positive beam helicity : number of events at negative beam helicity

42

2

4 4

2 2

1,1 0,12

| |

Im( )

1 Im sin Im sin 2

DVCS BH

B

DVCS BH

B B

dT T

dQ dx dtd

d dT T

dQ dx dtd dQ dx dtd

a M b M OQ

4 4

4 4

( )1

( )e

N Nd dA

d d P N N

ePN

N

ep ep

ep ep

The data points are fitted with the function The fitted parameters are

In the Bjorken regime should vanish, leaving only the contri

bution from transverse photons.

Theoretical calculations are from fixed values of

( ) sin sin 2 .A

0.202 0.028 0.013 ,

0.024 0.028 0.009 .

stat sys

stat sys

2 2

2

1.25 ( / ) ,

0.19,

0.19

B

Q GeV c

x

t GeV

dependence of the beam spin asymmetry A. The dark shaded region is the range of the fitted function defined by the statistical errors of parameters and , the light shaded region includes systematic uncertainties added linearly to the statistical uncertainties.

( )A

Longitudinal Target-Spin Asymmetry AuL measured for

e p e'p ϒ with 5.72 GeV electron beams

Theoretical calculations in good agreement with the magnitude and kinematic dependence of target-spin asymmetry, which is sensitive to GPDs and H

Leading term Aulsin increase

s with increasing , in agreement with model prediction

where, , H, and E are sums over quark flavor of the corresponding GPDs with argument ,F1 and F2 are the known Dirac and Pauli form factors of the proton, and M is the rest mass of the proton. In the range of this experiment the asymmetry is dominated by both H and , while and E are kinematically suppressed.

12GeV Upgrade

•Time Resolution : 60 ps• Plastic : Bicron BC-408•PMT Assembly : Hamamatsu H2431-50

•High Magnetic Field : 5 Tesla•High Energy : 12 GeV•High Luminosity : 1035 /cm2sec

50 pieces Barrel Detector Design, Development and Test

Micro Channel Plate PMT

Barrel Plastic Scintillator Prototype

Summary

Results provide strong constraints on isobar-based effective Lagrangian models, or approaches

employing fundamental parton degrees of freedom, such as LQCD, GPDs, LCSR, pQCD

12 GeV Upgrade Planned

Greater theoretical progress will be necessary before good quantitative agreement with the experimental high-Q2 data is obtained.