Proton Form Factor ratio GEp/GMp with polarization method

--on behalf of Jefferson lab GEp3 collaboration

Wei Luo

Lanzhou University, China

April 14 2008

APS April Meeting 2008 　 St. Louis Missouri

Outline

• 1. Introduction• 2. Experiment Setup• 3. Current status Data analysis Detector issues• 4. Summary

1. Introduction• JLab Experiment E04-108: GEp-III and E04-019: GEp-2γ • These two experiments use the same polarization method to measure

proton form factor ratio in ep elastic scattering process.• Experiment GEp-2 γ finished Oct. 2007-Jan. 2008. GEp-III is running.

pepe '

One photon exchange:

2tan

2

)( 'eee

l

t

Mp

Ep

M

EE

P

P

G

G

)()(

2tan)()1(

)(2222

222'

QGQG

QG

M

EEP

MpEp

eMp

eel

)()(

2tan)1(2

2222 QGQG

GGP

MpEp

eMpEp

t

0nP

What’s the explanation of the discrepancy between polarization and Rosenbluth data? 2γ?

What will the proton f.f. be at higher Q2?

?

?

2. Experiment setup• Two New detectors in Jlab Hall C: Focal Pane Polarimeter( In HMS hut),

E-M Calorimeter BigCal. • Coincidence between HMS(proton) and BigCal(electron)

beam

electronproton

electron

HMS

(FPP)

Kinematics tableEbeam

(Gev)

Q2

(Gev2) ε Tp

(Gev)

Θp

(°)

DBigCal

(cm)

θe

(°)

Ee’

(Gev)

1.873 2.506 0.146 1.332 14.495 493.2 105.16 0.541

2.847 2.506 0.631 1.332 30.995 1200.0 44.90 1.515

3.680 2.506 0.796 1.332 36.105 1102.3 30.60 2.348

4.052 5.200 0.374 2.781 17.94 605.0 60.3 1.271

5.714 8.520 0.239 4.540 11.681 435.0 68.6 1.174

5.714 7.100 0.464 3.784 17.853 760.0 47.3 1.930

Red GEp—IIIBlue GEp—2γ

Now here!

FPP2 set of CH2 Analyzer + 3 layer drift chamber, active size: 166cm(v) x 134cm(h).Reconstruct 3 tracks: HMS track, FPP1 track, FPP2 trackTwo Analyzers improved efficiency

• TF1-0 type lead glass, 12-stage Russian FEU-84 PMT, coupled by 1cm soft optical ‘cookie’.

• Protvino part: 1024 bars of 3.8x3.8x45cm3 lead glass. Cover 1.479m2

• RCS part: 720 bars of 4.0x4.0x40cm3 lead glass. Cover 1.152m2

• BigCal goal: identify elastic event, reduce high background rate

BigCal Calorimeter

2.631m2

3. Current StatusData analysis: Elastic event identification: HMS spectrometer; HMS-BigCal correlation. Example: Ebeam=1.837GeV Q2=2.506 ε=0.146

Position correlation

x-diff

θp

δp

Y-diff

Missing momentum

momentum Gev/c

Asymmetry at Focal Plane

),(),( －＋ NN

......]2sin2cos

sin))((cos))((1[2

1),(

00

00

dc

PAbPAaf fppyy

fppxy

is the azimuthal distribution with two beam helicity state; a0 ,b0 ,c0 ,d0 ……are the instrumental asymmetry.

Sum of two distribution:

),(),(

),(),(

NN

NNDifference of two distribution:

),( f

Preliminary Preliminary

φ φ

NN

NN

NN

NN

• Red: beam energy 2.847Gev• Blue: beam energy 3.548Gev• Elastic asymmetry Inelastic asymmetry

Elastic peak

Inelastic cut

P miss

Counts

Inelastic dominated by π0 production and Compton scattering:

Inelastic Focal Plane asymmetry

Elastic peak

Inelastic cut

Counts

P miss

pp 'pp 0

Counts

• Detector issues:

• FPP• The alignment of FPP to HMS track. Optimization of FPP DC can significantly reduce false asymmetry.

• BigCal• Radiation damage made BigCal lead glass “dark” and the gain of signal drop, the energy and position resolution

get worse.• About 2 weeks low power UV light curing during the shutdown of accelerator (without take out phototubes).• The lead glass recovered to about 70% of the beginning of the experiment.

Before experiment After experiment UV curing

Note: Low power UV light curing, did not take out phototube from back side.

4. Summary• Detectors works well as designed• GEp-2 γ finished and data analysis is going on

• GEp-III one kinematics data point is done.

• Focal plane asymmetry at beam energy: 2.847Gev shows difference between elastic and inelastic, resonance state near this energy region maybe the explanation of this discrepancy.