1. Exclusive electroproduction of the r+ on the proton at CLAS
e(5.75 GeV) p e n r+  e’ n p+p0
AHMED FRADI
Institut de Physique Nucléaire d’Orsay (France)
CLAS Collaboration meeting
Genoa, 27/02/2009

2. Outline:
Physics motivations
Data analysis: r+ cross sections
Theoretical interpretation
Conclusion

3. Meson electroproduction
Regge “hadronic”approach:
*Exclusive reactions.
*Small t.
*Small Q2.
*Exchanged particles:
Mesons, baryons.
GPDs
“partonic” approach:
*Exclusive reactions.
*Small t.
*High Q2.
*Exchanged particles:
Quarks, gluons.
Question1: Up to which Q2 “hadronic” approach is valid ?
Question2: From which Q2 “partonic approach” is dominant ?

4. Regge “hadronic” approach
Small Q2
Meson
Baryon
Beyond the resonance region (W> 2 GeV) , the reaction amplitude is dominated by the meson trajectory exchanges in the t-channel.
Exp: r+ electroproduction: r+ n p+ r+ n +

5. Generalized Parton Distributions (GPDs)
Ji, Radyushkin, Collins, Strikman, Frankfurt,…(1996)
Large Q2
For mesons the factorisation applies only for longitudinal photons
Small -t :-t << Q2 e’
γ, π, ρ, ω… Q2
g*T t e
g*L
F(z)
1-z
-2x z
x+x
x-x ~ ~
H,E,H,E(x,x,t)
-1<x<1
0<x<1
t=(p-p’)2
t=D2
p’(=p+D) p

6. GPDs extraction ~ ~
Hq, Eq, Hq , Eq are defined for each quark flavor q (u,d,s) ~
DVCS : H, E, H et E ~ ~ p0
euHu - edHd h
euHu + edHd ~
H , E
Pseudo-scalar mesons ~ ~
Vector meson
H , E ρ0
euHu - edHd ω
euHu + edHd ρ+
Hu - Hd
Eu - Ed

7. Data analysis: r+ cross sections
Outline:
Physics motivations
Data analysis: r+ cross sections
Theoretical interpretation
Conclusion 7

8. e p e n r+  e’ n p+p0 n e p+ g g
The e1 – dvcs experiment (March - May 2005)
e p e n r+  e’ n p+p0 n e p+ g g
Beam energy = 5.75 GeV
Current: nA
Integrated Luminosity ≈ 40fb-1
.1 < xB < .8
Q2 up to 5 GeV2

9. e p e’ [n] r+  e’ [n] p+p0 e’ [n] p+ g g
Channel selection
e p e’ [n] r+  e’ [n] p+p0 e’ [n] p+ g g p+ e
(p-)
(p) g EC
An event in CLAS IC

10. p0 selection
IC-IC
After exclusivity cut

11. p0 selection
IC-EC
After exclusivity cut

12. p0 selection
EC-EC
After exclusivity cut

13. Neutron selection
IC-IC

14. Neutron selection
IC-EC

15. Neutron selection
EC-EC

16. p+ p0 invariant mass
r+ (0.775 GeV)

17. Acceptance calculation
200 million events generated with a realistic generator and simulated with CLAS simulator
Acc(Q2,xB,t,…) = rec(Q2,xB,t,…) / gen(Q2,xB,t,…)

18. Kinematical variables: Data, MC r+ + MC phase spacexB Q2 W -t F
p+p0 invariant mass
Cosp+
 p+

19. Background subtraction
Ross-Stodolsky B-W for r+(770)
with variable skewedness parameter.
p+p0 phase space.

20. r0 r+ 20

21. Total cross section s (Q2,xB) r+
PRELIMINARY

22. ds/dt (Q2,xB,t) r+
PRELIMINARY
Fit by ebt

23. ds/dt (g*p  pr0)
Fit by ebt

24. Angular distribution analysis, cos qcm
r : fraction of longitudinal r+
PRELIMINARY

25. SCHC :
PRELIMINARY

26. Check of the SCHC :
ds/dF

27. Longitudinal cross sections
PRELIMINARY

28. Data analysis: r+ cross sections
Outline:
Physics motivations
Data analysis: r+ cross sections
Theoretical interpretation
Conclusion 28

29. “Hadronic approach”: JML model
JML model agrees
with the data at low xB (high W) and moderate Q2

30. “Hadronic approach”:
JML model r+ r0

31. “Partonic approach”: GPDs
(*)
GPDsmodel agrees fairly
with the data at low xB (high W)
GPDs model misses
the data at high xB (low W)
(*) Guidal, Polyakov, Radyushkin,
Vanderhaegen, PRD 72 (2005) 31

32. “Partonic approach”:
GPDs
(*) r+ r0
VGG GPD model
GK GPD model 32

33. r0 “meson exchange” Can we add the “meson exchange” term for r+ ?
DDs + “meson exchange”
DDs w/o “meson exchange” (VGG)
“meson exchange”
Can we add the “meson exchange” term for r+ ?

34. 2 days ago: (Peter Kroll)
mb/GeV2 ?
Hint:
dsL/dt (r+) ?

35. dsL/dt (r0)

36. Conclusion
In this analysis, we explore for the first time the r+ cross sections.
An extraction of longitudinal r+ cross sections was done.
Theoretical interpretation:
* “Hadronic degrees of freedom”: meson trajectory exchanges
in the t-channel (JML). Good agreement up to Q2  3.5 GeV .
* “Partonic degrees of freedom”: quark handbag diagram and GPDs.
VGG misses the data for small W: the handbag diagram is not
dominant or a BIG part is missing in the model: q q correlation .