Virtual Compton Scattering at Jefferson Lab : preliminary results at Q$^2$ = 1. and 1.9 GeV$^2$

(1)

HAL Id: in2p3-00010661

http://hal.in2p3.fr/in2p3-00010661

Submitted on 5 Dec 2001

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Virtual Compton Scattering at Jefferson Lab :

preliminary results at Q

2

_{= 1. and 1.9 GeV}

2

S. Jaminion

To cite this version:

S. Jaminion. Virtual Compton Scattering at Jefferson Lab : preliminary results at Q2 _{= 1. and 1.9}

GeV2_{. Electromagnetic Interactions with Nucleons and Nuclei, Oct 2001, Santorini, Greece. pp.1-22.}

(2)

1 Virtual Compton Scattering at JLab :

Preliminary results Q

2 = 1. and 1.9 GeV

2 Stephanie Jaminion

LPC, Universite Blaise Pascal

IN2P3-CNRS

E93050 collaboration

for the hall A Collaboration of Jefferson Lab.

SANTORINI 2001

Electromagnetic Interactions with Nucleons

and Nuclei

2-7 october 2001

(3)

2 Virtual Compton Scattering

below

ππππ

0 threshold

hadronic system

(

)

2

2 and

'

Q

q

k

q

=

−

=

−

p

γ

∗

→

Real Compton Scattering

α

,

β

(

Q

2 =

0 )

Virtual Compton Scattering :

α

,

β

(

_Q

2 ≠

₀

)

+ new observables

γ∗

(q )

p(p ) p’(p’)

e(k )

e’(k’)

γ

_(q’)

Generalized Polarizabilities

of the nucleon

(γ

p

γ

p)

(4)

3 Generalized Polarizabilities

Photon electroproduction is the coherent sum of 2 processes

VCS : is emitted by

nucleon : Born term

entirely calculable

(proton electromagnetic form factors : )

excited state : Non Born term

: UNKNOWN

NB

B

BH

ep

T

γ

=

+

calculable unknown (GPs)

γ

P M P E

G

and

photon electroproduction amplitude :

(5)

4 P Guichon : develop amplitude in polynomial of q’

_cm

Low Energy Theorem

[

_BH

_B

]

( )

2

5

5 '

'

_cm

B

BH

ep

q

d

σ

γ

=

σ

+

Φ

M

₀

−

M

₀

+

O

(P. Guichon and M. Vanderhaegen, Prog.Part. Nucl. Phys. 41 (1998) 125)

(F.E. Low, Phys. Rev. 96 (1954) 1428)

(q’

_cm

= outgoing photon momentum in center mass)

(

γ

*

p

)

phase space factor

(

−

BH

+

B

)

0

0 M

M

is the

lowest order term

containing an effect of

Generalized Polarizabilities (know combination)

6 GPs are needed to describe T

ep

γ

(D.Drechsel et al, Phys. Rev. C55 (1997) 424)

5 GPs in unpolarized case, and we extract two structure

(6)

5 Photon electroproduction cross section

Santorini2001 jaminion@clermont.in2p3.fr

:

Bosted parametrisation

(P.E. Bosted, Phys. Rev. C51 (1995) 409)

p

M

G

p

E

G

: ratio measured at Jefferson Lab

:

)

(

)

(

2 2

Q

G

Q

G

P M P E P

µ

10 -3 10 -2 10 -1 1 10 -150 -100 -50 0 50 100 150

D. Drechsel,

M. Gorchtein,

A. Metz,

B. Pasquini,

M. Vanderhaegen

Dispersion Relation formalism

BH+B cross section

(M.K. Jones, et al, Phys. Rev. Lett. 84 (2000) 1398)

[

−

°

+

°

]

∈

260 ,

100 cm

(7)

(8)

7 Jefferson Lab : Hall A

(9)

8 Missing mass for events in coincidence

electron and proton are detected in coincidence

(10)

(11)

(12)

11 Cross section

a

n

d

rati

o at

Q

2 =

1.9 GeV

2 B

H

+B

or

n (GP

s=0

)

Q2=1.9 Gev2. d 5σ (EP → EP γ )/dk , d Ωe,lab d Ωγγ ,cm 10 -3 10 -2 10 -1 -250 -200 -150 -100 -50 0 5 0 100 10 -3 10 -2 10 -1 -250 -200 -150 -100 -50 0 5 0 100 10 -3 10 -2 10 -1 -250 -200 -150 -100 -50 0 5 0 100

Q2=1.9 GeV2. (Exp-BHB)/BHB (BHB=BetheHeitler+Born)

(13)

(14)

(15)

14

1 1.5 2 2.5 3 3.5 4 4.5 5 0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 1.2

all off-plane angles

(16)

15

Santorini2001 j aminion @ cle rmont.in2p3.f r

[]

(

)

2 B

BH

5

5 '

'

cm

B

BH

ep

q

d

O

M

0

0 +

−

Φ

+

=

+

σ

γ

()

cm

LT

cm

TT

cm

LL

B

BH

q

P

v

q

P

q

P

v

M

+ 



 



−

=

−

+

ε

1

0

0 kinematic

coefficients

M

0 -M

0 BH+B

:

contains

the Generalized Polarizabilities

0 M

∆

Generali

zed P

o

la

rizabili

ti

es

Photon electroproduction

cross section five fold differential

(17)

(18)

17 at

Q

2 =1.0 GeV

2 at

Q

2 =1.9 GeV

2 Generalized Polarizabilities extraction

(19)

(20)

19 Conclusion

we

have to

d

eterm

ine the

system

atic errors

other approach using data below

and

above

π

0 threshold

toge

th

er

with

dispers

ion re

la

tion form

alism

first

extraction of Generalized Polarizabilities

(21)

(22)

(23)

22 beam

proton from VCS

proton from

(ep) elastic

photon from

VCS

photon from

(ep) elastic

*

γ

e’