GEp-2γ experiment (E04-019)

UPDATE

Mehdi MezianeThe College of William & Mary

- HALL C USER Meeting January 30, 2009 -

On behalf of the JLab-Gep Collaboration

OUTLINE

• INTRODUCTION

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

• TWO-PHOTON EXCHANGE (TPEX)

•THEORETICAL PREDICTIONS

• ELASTIC EVENTS SELECTION

• CONCLUSION

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.

ME GG /

Something beyond the Born approximation

Two-photon 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 ε dependence of to detect a ME GG /pepe possible two-photon exchange effect in the scattering

Measure separatly the two observables Pt/Pl and relative Pl

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) respectively ,~,,~ 2

2

2 QFQGM 22

2 Q,Q FGM

and 0,~ 2

3 QF

2

212

4with

2tan121

MQ

The virtual photon polarization is:

TWO-PHOTON EXCHANGE (TPEX)

q1 q2N

q1 q2

N

2~M

red

G

d

2~

~1

12 YG

G

M

E

2

~~

1M

E

GG

red

t dP

2

122Y

MG~

MG~

EG~

MG~

111

2

redl dP

21

2Y

Transverse polarization component

Reduced cross section

MG

FQY ~

~

111

, 322

with

TWO-PHOTON EXCHANGE (TPEX)

Longitudinal polarization component

Born Approx. Beyond Born Approx.

P.Blunden et al., Phys.Rev.C72: 034612 (2005)

D.Borisuyk, A.Kobushkin arXiv:0804.4128

THEORETICAL PREDICTIONS: HADRONIC CALCULATIONSOnly nucleon intermediate states are taken into account

The inclusion of Δ reduces the TPEX correction (Figure)

Higher nucleon resonances has smaller effect

P.Blunden et al., Phys.Rev.Lett.91: 142304 (2003)

S.Kondratyuk et al., Phys.Rev. C75:038201(2007)

Yu. Bystricky, E.A.Kuraev, E. Tomasi-Gustafsson Phys. Rev. C75, 015207 (2007)

Structure function method: 2γ effects small, higher orders change Rosenbluth slope (Figure)

Proton off-shell form factors make almost no difference

A.Afanasev et al., Phys.Rev.D72:013008 (2005) – GPD models: Gauss on Fig., smaller effect with Regge, or non-zero quark mass

Absolute correction to FF ratio Ge/Gm: • slow Q2 variation, strong effects at low ε• valid for high Q2 or high ε

THEORETICAL PREDICTIONS GPD CALCULATIONS

TPEX contributions are calculated assuming factorization of the soft nucleon-quark part, and the hard electron-quark interaction, where the TPEX takes place via the box diagram.

THEORETICAL PREDICTIONS

hadronic (elastic) : dominated by correction to GM

GPD (includes inelastic): dominated by Y2g and correction to GE

Both theories describe Rosenbluth data but have opposite predictions for GE/GM

GEP-2γ at JEFFERSON LAB HALL C

80μA beam current85% pol.20cm LH target

precision limited only by statistics(~ 1%), unlike Rosenbluth,very small p.t.p systematics:

Ay , h cancel out in the Pt/Pl ratioQ2 fixed, Pp fixed, spin precession fixed

e

e’ p

Key idea:• fixed Q2

• same spin transport • same analysing power.

Ee, GeV pp Ee’ θp, deg θe ε range <Q2>

1.867 2.068 0.527 14.13 106 .130-.160 2.49

2.839 2.068 1.507 30.76 45.3 .611-.647 2.49

3.549 2.068 2.207 35.39 32.9 .765-.786 2.49

Analyser 1

Analyser 2

HM

S D

C 1

HM

S D

C 2

FPP

DC

1,2

FPP

DC

3,4

DETECTORS

Focal Plane Polarimeter FPP

• 2 analyzers in serie• 2 sets of drift chamber pairs

Electromagnetic Calorimeter BigCal

1744 channels

KINEMATIC FORMULA

2sin21

2sin4

2

22

2

e

p

beam

ebeam

mE

EQ

pbeampbeam

pbeampbeampp EmE

EmEmp

222 cos

cos2

2

22

2 ppp

emm

mQ

p

10ppHMS

Proton momentum calculated from the electron angle:

with

Proton momentum calculated from the proton angle:

HMS proton momentum: and p0 = 2.0676 GeV/c

ELASTIC EVENTS SELECTION

Black: all events

pθp pθe (PHMS-pθe)/p0

Q2 = 2.49 GeV2

ε = 0.149 (smallest)Ebeam = 1.867 GeV

p0 = 2.0676 GeV/c

Red: background

pθp pθe (PHMS-pθe)/p0

Green: (θe - θp) and (φe- φp) cut

pθp pθe (PHMS-pθe)/p0

Blue: normalized background

pθp pθe (PHMS-pθe)/p0

σ = 0.28% σ = 0.22% σ = 0.18%

Bgd=3.2% Bgd=3.3% Bgd=3.2%

• Different shapes of the elastic peaksRatio elastic/inelastic changes with the kinematic variables

• Upper estimate of the background of 3.2% with ±3σ cuts

• Need a Monte-Carlo simulation to fit the background shape

PRELIMINARY RESULTS

The 2 theoretical curves have beennormalized by a different coefficientto fit the data.

Prove the validity of the Bornapproximation for data obtained by the polarization technique ata percent level.

No evidence of TPEX effect at apercent level PR

ELIMIN

ARY

CONCLUSION

• Fixed Q2 and pp

• Same analyzing power

•Same spin transport

• Upper estimate of the background of 3.2%

• Need a Monte-Carlo simulation to fit the background shape

• No evidence of effects beyond the Born approximation

• Prove the validity of the polarization method for the proton form factor measurements

Measure separatly two observables: relative Pl

and the ratio Pt/Pl