Deep Vector Meson Electroproduction

Transcription

1 Deep Vector Meson Electroproduction M.Guidal & S. Morrow IPN Orsay & SPhN/Saclay

2 1/ Data analysis of the ep epρ 0 reaction at CLAS with the 5.75 GeV beam (e1( e1-6 experiment) σ L (γ* L p pρ L0 ) 2/ Interpretation in terms of GPDs (& Regge : JML model) Role of mesons in GPDs

3 e1-6 experiment (5.75 GeV) (CLAS detector) (October 2001 January 2002)

4 ep ep π + (π - ) Mm(epπ + X) π + e Mm(epX) (π - ) p

5 Background Subtraction 1) Ross-Stodolsky B-W for ρ 0 (770), f 0 (980) and f 2 (1270) with variable skewedness parameter, 2) Δ ++ (1232) π + π - inv.mass spectrum and π + π - phase space.

6 A1 σ ρ (γ * p pρ 0 ) vs W

7 Slide 6 A1 Administrateur, 11/4/2005

8 dσ/dt (γ * p pρ 0 ) Large t min! (1.6 GeV 2 ) Fit by e bt

9 Angular distribution analysis, cos θ cm Relying on (and exp. checking) SCHC W (cos θ HS )= 3 8 [ (1 r 04 00)+(3r ) cos 2 θ HS ] R = σ L σ T = 1 ɛ r r World s data for R

10 Longitudinal cross section σ L (γ * Lp pρ L0 )

11 e γ L q z e Φ Interpretation in terms of GPDs? M q Δ L A L = 2ie 9 ( 1 0 dz Φ(z) ) 4παS (Q 2 {[ ) +1 1 dx z Q 1 x ξ + iɛ + 1 x + ξ iɛ ] F (x, ξ, t) } p x + ξ F(x, ξ,t) x ξ p +Δ F (x, ξ, t) =H N M (x, ξ, t) N(p )γ nn(p)+e N M (x, ξ, t) N(p )iσ κλ n κδ λ 2m N N(p) N t =Δ 2 N LO (w/o kperp effect) LO (with kperp effect) Soft overlap (partial) Handbag diagram calculation needs k perp effects to account for preasymptotic effects

12 M. Vanderhaeghen, P. Guichon, M.G. (PRD 60:094017,1999) GPDs parametrization based on DDs (VGG model)

13 VGG GPDs model Quarks GPDs (DDs) Gluons GPDs 22/05/2007 Michel Guidal, Steven Morrow 12

14 -2ξ x+ξ t x-ξ γ, π, ρ, ω ~ ~ H, H, E, E (x,ξ,t) ERBL region DGLAP region -1 -ξ 0 ξ +1 Antiquark distribution q q Distribution amplitude ERBL Quark distribution DGLAP x W~1/ξ

15 Out of ERBL region (W>5 GeV),, decent agreement with data which thus seem to be interpretable in terms of (k perp modified) LO handbag diagram and GPDs (k perp In ERBL region, either : BIG part is missing in GPDs parametrization (unconstrained meson exchange contributions in ξ<x< <x<ξ region?) H(x,0,0) = q(x) H(x,ξ,t)dx = F(t) ERBL region is basically unknown and unconstrained, escapes all normalisation constraints Add (in addition to DDs) (and fit) Gegenbauer (odd) polynomial to H (or/and E) GPD(s) -Very Poor man s way of introducing qq correlations in the ERBL domain of GPDs-

16 Various GPDs contributions Quarks GPDs (DDs) Gluons GPDs Quarks GPDs (DDs + meson exchange ) 22/05/2007 Michel Guidal, Steven Morrow 15

17 Out of ERBL region (W>5 GeV),, decent agreement with data which thus seem to be interpretable in terms of (k perp modified) LO handbag diagram and GPDs (k perp In ERBL region, either : BIG part is missing in GPDs parametrization (unconstrained meson exchange contributions in ξ<x< <x<ξ region?) H(x,0,0) = q(x) H(x,ξ,t)dx = F(t) ERBL region is basically unknown and unconstrained, Escapes all normalisation constraints Add (in addition to DDs) (and fit) Gegenbauer (odd) polynomial to H (or/and E) GPD(s) Or, NO such important contribution and keep standard (DDs based) GPDs but falls into classical problems of onset of factorization (CZ vs asympt.. DA, Feynman mechanism, )

18 dσ L /dt (γ * p pρ 0 ) Quarks GPDs (DDs) Quarks GPDs (DDs + meson exchange )

19 Do t-channel meson exchange belong to GPDs? Do GPDs reflect nucleon structure or meson structure? At ξ=0, DGLAP regime no correlations, diagonal configuration, probability interpretation, nucleon structure/imaging

20 xp δ z y Image valid at ξ=0 z b x x Guidal, Polyakov, Radyushkin, VdH (2005) b (fm)

21 Do t-channel meson exchange belong to GPDs? Do GPD reflect nucleon structure or meson structure? At ξ=0, DGLAP regime no correlations, diagonal configuration, probability interpretation, nucleon structure/imaging At ξ=0, ERBL regime correlations, non-diagonal configuration, no probablity interpretation, nucleon interaction potential

22 Interpretation a la Regge : Laget model γ*p pρ 0 γ*p pω γ*p pφ

23 σ L (γ* L p pρ L0 ) Regge/Laget Quarks GPDs (DDs + meson exchange )

24 dσ L /dt (γ * p pρ 0 ) Regge/Laget Quarks GPDs (DDs + meson exchange )

25 Largest set ever of data for VM (ρ 0 ω,φ),ω,φ production in the valence region (σ L,T, dσ/dt, ) Out of ERBL region (W>5 GeV),, decent agreement with data which thus seem to be interpretable in terms of (k perp modified) LO handbag diagram and GPDs (k perp In ERBL region, either : Summary BIG part is missing in GPDs parametrization (unconstrained meson exchange contributions in ξ<x< <x<ξ region?) Hints from JLab DVCS data that something is missing in GPD parametrisation?- Or, NO such important contribution and keep standard (DDs based) GPDs but falls into classical problems of onset of factorization, (CZ vs asympt.. DA, Feynman mechanism, ) Also, large tmin at large x ; important higher twists effects?-?

26 Cross section σ L (γ * p pω) Theory GPDs- JLab/CLAS: σ T +εσ L (μb), 2.7 GeV 2 < t < t 0 Calcul VGG: εσ L, t(10 o ) < t < t < x B < < x B < < x B < < x B < < x B < < x B < < x B < < x B < Q 2 (GeV 2 ) Q 2 (GeV 2 ) Q 2 (GeV 2 ) Q 2 (GeV 2 )