GEp-2γ experiment

and the new JLab Hall-C Focal Plane

Polarimeter

Mehdi MezianeThe College of William & Mary

- APS Meeting April 14, 2008 -

On behalf of the JLab-Gep Collaboration

OUTLINE

• INTRODUCTION

• TWO PHOTONS EXCHANGE (TPEX)

• THE NEW FOCAL PLANE POLARIMETER IN HALL-C

• EXPECTED UNCERTAINTIES

• SUMMARY

Two methods, two different results

ROSENBLUTH AND POLARIZATION TRANSFER MEASURMENTS OF OF THE PROTON

• Recoil polarization and Rosenbluth

ratios are clearly

different in the Born approximation.

• Difference increase systematically with Q2.

pp ME GG /

Something beyond the Born approximation

Two photons exchange

• Either cross section or recoil polarization measurements to extract the proton form factor

THE GEP-2γ (04-019) EXPERIMENT AT JLAB HALL-C

• In the Born approximation we don’t have such dependence

• We carried out the experiment last year:

Q2 = 2.5 GeV2 for 3 values of ε: 0.15, 0.63 and 0.78

Requires <0.01 statistics for a ratio of 0.7

Systematics cancelled out because Q2 and pp fixed

• We look for a kinematical dependence of to detect a

pepe possible two photons exchange effect in the scatteringpp ME GG /

T matrix: )'()()'(2

2

pukukuQe

T p

MP

FGM

2~~

23~

MKP

F pu

MG~

2~F 3

~Fwith complexand,

In the Born approximation:

are form factors (real) ,~,,~ 2

22 QFQGM 22

2 , QFQGM and 0~3 F

The virtual photon polarization is:

TWO PHOTONS EXCHANGE (TPEX)

2~M

red

G

d

2~~

112 Y

GG

M

E

2

~~

1M

E

GG

red

t dP

2

122Y

MG~

MG~

EG~

MG~

1112

red

l dP

21

2Y

Polarization components:

The reduced cross section is:

MG

FQY ~

~

111

, 322

with

2Y 2YIn redd lP tPand the TPEX contribution enters as while in only as

For a non-zero contribution to the cross section at small the absolute

value of the TPEX contribution will change by a factor 1/ for tP

TWO PHOTONS EXCHANGE (TPEX)

BASIC PRINCIPLES OF THE EXPERIMENT

Measure the polarization of the recoil proton in the scatteringpepe

• In the Born approximation:

Two non-zero polarization components:2

Ml GP MEt GGP and

• Measurement of the ratio

By measuring the asymmetry distribution f after (re)scattering of the recoil proton in the analyser:

fpplyPhAf sin cossin fpp

tyPhA

ly

ty

PhA

PhA

M

E

GG

Analyser 1

Analyser 2

HM

S D

C 1

HM

S D

C 2

FPP

DC

1,2

FPP

DC

3,4

Focal Plane Polarimeter (FPP) in Hall-C

• 2 Chambers with 3 planes eachper polarimeter

u x vConsists in 2 polarimeters in series to increase the efficiency

12 planes of detectionActive area 134*166 (cm2)

Measure coordinates u, x, v

1164 instrumented wires

Need 3 tracking procedures (2 polarimeters and HMS)

• Gas mixture Argon50%/Ethane50%

• Operating HV 2350V (field and cathode wires)

Focal Plane Polarimeter (FPP) in Hall-C

CH2 Analysers

• 145cm *111cm *60cm

• Layered, split (L/R), 1 ton each

Open like a sliding door for straight through tracking

• Independently supported on interleaved frame

Focal Plane Polarimeter (FPP) in Hall-C

EXPECTED UNCERTAINTIES

Fixed Q2, fixed proton momentum

Same analysing power and HMS settings (spin transport) for the 3 kinematics

We measured the transferredpolarization component ratio Pt/Pl and relative Pl separately in ep elastic for a fixed Q2 of 2.5GeV2 at three different kinematics (epsilons) by changingthe beam energy.

Statistical errors onlyStatistics < 0.01 for a ratio of 0.7

LONGITUDINAL POLARIZATION TRANSFERT

The ε dependence of the form factor ratio mainlycomes from Pt

As Q2 is constant

Relative polarization have small systematicuncertainties ≈ 1% (Möller measurements)

Statistical (expected) and systematic errors

A. Afanasev et al.P.G. Blundel et al.

SUMMARY

• Absolute value of TPEX contribution changes by a factor 1/ for Pt

• A new detector: the HALL C focal plan polarimeter

• The form factor ratio ε dependence comes from Pt

• Statistics < 0.01 for a ratio of 0.7