0, , + exclusive electroproduction on the proton at clas on the proton at clas
TRANSCRIPT
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 ([email protected])
J. Santoro et al., Phys.Rev.C78:025210,2008 ([email protected])
L. Morand et al., Eur.Phys.J.A24:445-458,2005 ([email protected])
C. Hadjidakis et al., Phys.Lett.B605:256-264,2005 ([email protected] GeV) K. Lukashin et al., Phys.Rev.C63:065205,2001 ([email protected] GeV) } e1-b
(1999)
} e1-6(2001-2002)
A. Fradi, Orsay Univ. PhD thesis ([email protected] 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 ([email protected])
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 ([email protected])
C. Hadjidakis et al., Phys.Lett.B605:256-264,2005 ([email protected] GeV)
K. Lukashin, Phys.Rev.C63:065205,2001 ([email protected] GeV)
J. Santoro et al., Phys.Rev.C78:025210,2008 ([email protected])
S. Morrow et al., Eur.Phys.J.A39:5-31,2009 ([email protected])
w
r0
f
r+
A. Fradi, Orsay Univ. PhD thesis, 2009 ([email protected])
(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 ???