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

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

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 ?

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 +

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

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

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

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: 20-25 nA Integrated Luminosity ≈ 40fb-1 .1 < xB < .8 Q2 up to 5 GeV2

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

p0 selection IC-IC After exclusivity cut

p0 selection IC-EC After exclusivity cut

p0 selection EC-EC After exclusivity cut

Neutron selection IC-IC

Neutron selection IC-EC

Neutron selection EC-EC

p+ p0 invariant mass r+ (0.775 GeV)

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,…)

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

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

r0 r+ 20

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

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

ds/dt (g*p  pr0) Fit by ebt

Angular distribution analysis, cos qcm r0004 : fraction of longitudinal r+ PRELIMINARY

SCHC : PRELIMINARY

Check of the SCHC : ds/dF

Longitudinal cross sections PRELIMINARY

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

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

“Hadronic approach”: JML model r+ r0

“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

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

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+ ?

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

dsL/dt (r0)

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 .