r0, w, r+ exclusive electroproduction on the proton at CLAS

,

Regge theory: Exchange of families of mesons in the t-channel

Regge theory: Exchange of families of mesons in the t-channel

M(s,t) ~ sa(t) where a(t) (trajectory) is the relation between the spin and the (squared) mass of a family of particles

M->sa(t)

stot~1/s x Im(M(s,t=0))->sa(0)-1 [optical theorem]

stot ds/dt

s t

ds/dt~1/s2 x |M(s,t)|2->s2a(t)-2 ->[ea(t)lns(s)]

,

,

Q2>>

,

Q2>>Q2>>

Some signatures of the (asymptotic) « hard » processes:

sL/sT~Q2

/ / /(r w f J/ )Y ~9/1/2/8

sL~1/Q6 sT~1/Q8

s~|xG(x)|2

Q2 dependence:

W (or xB) dependence: (for gluon handbag)

Ratio of yields: (for gluon handbag)

Saturation with hard scale of aP(0), b, …

SCHC : checks with SDMEs

H1, ZEUS

,

Q2>>Q2>>

H1, ZEUSCLAS

HERMESCOMPASS

+ « older »data from:E665, NMC,Cornell,…

H1, ZEUS

,

Q2>>Q2>>

H1, ZEUSCLAS

HERMESCOMPASS

+ « older »data from:E665, NMC,Cornell,…

Ratios

/ / /(r w f J/ ) Y ~ 9/1/2/8

|r0>=1/sqrt(2){|uu>-|dd>}

| >=w 1/sqrt(2){|uu>+|dd>}~{2/3-(-1/3)}~{2/3+(-1/3)}

Ratio /r w=9

For W>5 GeV, good indications that the “hard”/pQCD regime is reached/dominant for m2=(Q2+MV

2)/4 ~ 3-5 GeV2.

Data are relatively well described by GPD/handbag approaches

Great, one can hope to learn about the gluon (transverse)spatial distribution in the nucleon, the gluon orbital momentum contribution to the nucleon spin, etc…

H1, ZEUS

,

Q2>>Q2>>

H1, ZEUSCLAS

HERMESCOMPASS

Exclusive r0, , w f & r+ electroproduction on the proton @ CLAS6

S. Morrow et al., Eur.Phys.J.A39:5-31,2009 (r0@5.75GeV)

J. Santoro et al., Phys.Rev.C78:025210,2008 (f@5.75GeV)

L. Morand et al., Eur.Phys.J.A24:445-458,2005 (w@5.75GeV)

C. Hadjidakis et al., Phys.Lett.B605:256-264,2005 (r0@4.2 GeV) K. Lukashin et al., Phys.Rev.C63:065205,2001 (f@4.2 GeV) } e1-b

(1999)

} e1-6(2001-2002)

A. Fradi, Orsay Univ. PhD thesis (r+@5.75 GeV) } e1-dvcs(2005)

F.-X. Girod

Exclusive r 0 electroproduction

e1-6 experiment (Ee =5.75 GeV)(October 2001 – January 2002)

ep ep p+(p-)

Mm(epX)

Mm(epp+ X)

e

p

p+

(p-)

1) Ross-Stodolsky B-W for r0(770), f0(980) and f2(1270)with variable skewedness parameter,2) D++(1232) p+p- inv.mass spectrum and p+p- phase space.

Background Subtraction

IM(pp+)

IM(pp-)

V. Mokeev 2p e-prod model

Working up to W=1.8 GeV

up to Q2~1.5 GeV2

J-M. Laget 2p e-prod model

+f0 + f2

s +f0 + f2

(*)+ *pD pN

all (incl.r0)

Good description of CLAS,SLAC data

Laget model: Photoproduction

Laget model: Electroproduction

CLAS data

Laget model

At the time of the analysis (~2008), radiative corrections were treated as averaged over Mp+p-, i.e. only as a function

of (xB, Q2)

Relatively small effect on r0 cross sections asthe “distortion” of the Mp+p- spectrum was “absorbed”

in the effective p+p- continuum background subtraction(also ~25% syst. uncertainty was included)

Mp+p- (GeV)

However, current more refined re-analysis of these data (B. Garillon, aimed at f0 and f2 analysis, see talk on Saturday)

show that there is a Mp+p--dependence

r0 f0 f2

1) Ross-Stodolsky B-W for r0(770), f0(980) and f2(1270)with variable skewedness parameter,2) D++(1232) p+p- inv.mass spectrum and p+p- phase space.

Background Subtraction

sr (g*p pr0) vs W

ds/dt (g*p pr0)

Fit by ebt

Large tmin ! (1.6 GeV2)

Angular distribution analysis, cos qcm

Relying on SCHC (exp. check to the ~25% level)

Longitudinal cross section sL (g*Lp prL

0)

Interpretation “a la Regge” : Laget model

g*p pr0

g*p pw

g*p pfFree parameters:*Hadronic coupling constants: gMNN

*Mass scales of EM FFs: (1+Q2/L2)-2

Regge/Laget

sL (g*Lp prL0)

Pomeron s,f2

VGG GPD model

+

GK GPD model

+

H, H, E, E (x,ξ,t)~ ~

x+ξ x-ξ

tγ, π, ρ, ω…

-2ξ

xξ-ξ +

1-1 0

Quark

distribution q q Distribution

amplitudeAntiquark

distribution

“ERBL” region “DGLAP” region

W~1/x

ERBL DGLAP

DDs + “(fitted) meson exchange”

DDs w/o “meson exchange” (VGG)

“(fitted) meson exchange”

Comparison of cross sections is model-dependent:kperp dependence ansatz, model for GPDs,…

sL/sT~Q2sL~1/Q6 sT~1/Q8Q2 dependence:

Saturation with hard scale of b SCHC : checks with SDMEs

/ / /(r w f J/ )Y ~9/1/2/8

s~|xG(x)|2W (or xB) dependence:

Ratio of yields:

Some signatures available for gluon handbag are not relevant for quark handbag:

Model-independent features:

CLAS@6 GeV S. Morrow et al., Eur.Phys.J.A39:5-31,2009 (r0@5.75GeV)

b (from ebt) as a function of Q2 (be careful of correlation with W)

sL/sT as a function of Q2 (be careful of correlation with W)

Exclusive r0 electroproduction at CLAS12 CLAS12 proposal PR12-11-103

Q2 rangeat 6 GeV

Q2 rangeat 12 GeV

r0 decay angular distribution (polar angle qcm)CLAS12 proposal PR12-11-103

purely statistical error bars

statistical error bars+10% systematic

Exclusive w electroproduction

ep->ep ( w p+p-

[p0])

cos(qcm) distribution

fcm distribution

Laget Regge model

for g*p pw

Cross section s(g*p p )w

Laget sT+esL

Laget esL VGG esL

(H&E)

Issue with GPD approach ifp0 exchange dominant :

p0->E

E subleading in handbag for VM production

(GK includes p0 exchange with kperp effects)

~

~while

DESY data

CORNELL data

Cross section s(g*p p )w Comparison with GPD calculation (VGG)

Exclusive r + electroproduction

(A. Fradi’s thesis)

Slow but steady progress towards publication !

p+

e

(p-)

(p)

gg

One event in CLAS

e p e’ [n] r+ e’ [n] p+p0 e’ [n] p+ g g

Channel selection

IC

EC

Invariant mass IM(p+p0)

Invariant mass IM(np0)

Invariant mass IM(np+)

Invariant mass IM(p+p0)

PRELIMIN

ARY

Total cross section (s Q2,xB) r+

Laget Regge “hadronic” approach

+ r+p+

r+

n

r+

n

GPD “partonic” approach

r+L

H , E

ρ0 euHu - edHd

euEu - edEd

weuHu + edHd

euEu + edEd

ρ+ Hu - Hd

Eu - Ed

“Hadronic approach”: Laget model

r+

r0

“Hadronic approach”

Laget model does not reproduce the drop of ds/dt for t →0

PRELIMINARY

(*)

“Partonic approach”: GPDs

r0 VGG GPD model

GK GPD model

r+

GPDs

p n

g*L

r+

ee’

GPDs model agrees fairlywith the data at low xB (high W)

GPDs model missesthe data at high xB (low W)

(*)

“Partonic approach”: GPDs

Only H in this calculation

(*)

Hint of GPD E dominance

The GPD E reproduces the drop of ds/dt for t →0

L. Morand et al., Eur.Phys.J.A24:445-458,2005 (w@5.75GeV)

C. Hadjidakis et al., Phys.Lett.B605:256-264,2005 (r0@4.2 GeV)

K. Lukashin, Phys.Rev.C63:065205,2001 (f@4.2 GeV)

J. Santoro et al., Phys.Rev.C78:025210,2008 (f@5.75GeV)

S. Morrow et al., Eur.Phys.J.A39:5-31,2009 (r0@5.75GeV)

w

r0

f

r+

A. Fradi, Orsay Univ. PhD thesis, 2009 (r+@5.75GeV)

(preliminary)

Comparison of t-slope for r0, w, f channels at 6 GeV

VMs (r0, ,w f) the only exclusive process [with DVCS] measured over a W range of 2 orders of magnitude (sL,T, ds/dt, SDMEs,…)

At low energy (W<5 GeV), success of “hard” approach forthe f channel ( nucleon gluon imaging) but large failurefor the r0, , w r+ channels.This is not understood. Why the GPD/handbag approach sould set at much larger Q2 for valence quarks ? Are the widely used GPD parametrisations in the valence region completely wrong ?

JLab12 PR12-11-103 proposal: broader phase space, checkwhen Q2 independence settles for a variety of observables

At high energy (W>5 GeV), transition from “soft” to “hard”(m2 scale) physics relatively well understood (further workneeded for precision understanding/extractions) handbag interpretation and nucleon imaging

W~1/x

GPDs/handbag

GPDs/handbag ???