1. Genoa, 25/02/09
Electron scattering
in CLAS
(selected) review
M. Guidal, IPN Orsay

2. Form factors
Nucleon ground state
Nucleon resonances
Inclusive scattering
unpolarized
polarized
Semi-inclusive scattering
AUL, ALL, ALU for p SIDIS
Deep exclusive scattering
DVCS
Vector meson
Nuclear targets
Color tranparency

3. 
c Symmetry
Lattice
QCD
Constituent
Quarks p N
Large Nc 3 8
pQCD

4. Many thanks, for the slides, to:
M. Aghasyan (SIDIS),
H. Avakyan (SIDIS),
V. Burkert (FFs),
E. Christy (DIS),
W. Gohn (SIDIS),
K. Hafidi (Nuclear),
N. Guler (DIS),
S. Kuhn (DIS).
[N*,Spin08,ECT workshop,CLAS Coll.meetings,…]

5. Form factors
Nucleon ground state
Nucleon resonances
Inclusive scattering
unpolarized
polarized
Semi-inclusive scattering
AUL, ALL, ALU for p SIDIS
Deep exclusive scattering
DVCS
Vector meson
Nuclear targets
Color tranparency

6. Ground state and Transition Form Factors ,
N,N*
N=p,n
GE ,GM
GE ,GM ,GC
(or E1+ ,M1+ ,S1+)
N*=D(1232)
P11(1440)
D13(1520)
S11(1535) pN
pN,ppN
GE ,GM
(or A1/2 ,S1/2)
pN,ppN
GE ,GM ,GC
(or A1/2 ,A3/2 ,S1/2)
hN,pN
GE ,GM
(or A1/2 ,S1/2)
[Electric/Magnetic/Coulomb,Multipoles, Helicity amplitudes,…]

7. Neutron Magnetic Form Factor
CLAS
Hall B
Real time in-situ calibration of the neutron detection efficiency in the CLAS e.m. calorimeter EC measured online through ep→eπ+(n).
Results of the neutron magnetic
form factors from Q2 = 1- 5 GeV2 .
No large deviation observed from dipole form.
J. Lachniet et al. (CLAS) arXiv: (PRL)

8. NΔ Multipole Ratios REM, RSM
There is no sign for asymptotic pQCD behavior in REM (->+1) or RSM (->Cte).
REM < 0 at low Q2 favors oblate shape of Δ(1232) and prolate shape of the proton.
Dynamical models attribute the deformation to contributions of the pion cloud at low Q2.

9. P11(1440) amplitudes from Nπ and Nππ
Nππ (preliminary)
PDG
First observed zero crossing of a nucleon form factor!

10. Fits to diff. cross sections & structure functionsDR
UIM
DR w/o P11 DR
UIM
Q2=2.05GeV2
Q2=3.48GeV2

11. Transition amplitudes for D13(1520)
M. Dugger et al.,
PRC , 2007
Nππ (preliminary)
A3/2
A1/2

12. Transition amplitudes for D13(1520)
Ahel =
A21/2 – A23/2
A21/2 + A23/2
S1/2
CQM predictions:
A1/2 dominance with
increasing Q2.
Ahel
pπ+π- (prel.)
nπ+
nπ+
pπ0
pπ0

13. Transition amplitudes for S11(1535)
This state has traditionally been studied in the S11(1535)→pη channel, which is the prominent decay. For the study of S1/2 Nπ channel is important. S1/2 difficult to extract in pη channel.
A1/2 from nπ+ consistent with ph within uncertainties of b.r. 13

14. CLAS Form Factors M. Vanderhaeghen, L. Tiator p->D+ p->S11(1535)

15. Transverse Charge Densities
CLAS
Transverse Charge Densities
The charge
distribution of the
p-Roper+ transition
shows a dense
positive core of
0.5 fm radius,
and a negative
outer shell up to a
radius of 1 fm.
M. Vanderhaeghen, L. Tiator
p->D+
p->S11(1535)
p->D13(1535)

16. Conclusions
NΔ(1232) amplitudes are well determined at Q2 up to 6 GeV2.
No sign of transition to asymptotic QCD behavior
Roper P11(1440) amplitudes determined up to 4.5 GeV2 using two different analysis approaches (DR, UIM), and two channels
Sign change of A1/2 seen in Nπ and Nππ
S11(1535) amplitudes measured in nπ+ channel, for the first time
Hard A 1/2 form factor confirmed
First measurement of S1/2.
D13(1520) in nπ+ and pπ+π-
Helicity switch from A3/2 dominance to A1/2 dominance at Q2>0.6 GeV2

17. Form factors
Nucleon ground state
Nucleon resonances
Inclusive scattering
unpolarized
polarized
Semi-inclusive scattering
AUL, ALL, ALU for p SIDIS
Deep exclusive scattering
DVCS
Vector meson
Nuclear targets
Color tranparency

18. Need proton and neutron targets to pin down u/d PDFs from DIS
At Leading order
x[4/9u(x) + 1/9d(x)]
x[4/9d(x) + 1/9u(x)]
At large x proton dominated by u(x) and neutron by d(x)‏
due to charge weighting.
PDF’s least well known
at large x
Also, proton + neutron data
provides way to
separate valence cleanly
(Gluon comparable in size to dv at x~0.3)
u(x)‏
d(x)‏ 18

19. Deuterium + proton data does not allow clean determination of neutron for x  1 due to nuclear corrections 19

20. CLAS with the RTPC and one electron event.
to CLAS n p
To BoNuS RTPC
φ, z from pads
r from time
beam
Helium/DME at 80/20 ratio
dE/dx from charge
along track (particle ID)

21. Spectator Tagging
E = GeV e p n
<Q2> = 1.19 (GeV/c)2 * 21

22. Structure function ratio
PRELIMINARY
Good agreement with
previous data in smaller
x region.
Full acceptance
correction method
forthcoming.

23. Double polarized inclusive electron scattering
Long. Polarized
Electron qe e Pe Pt y
Long. Polarized
Nucleon
Cross section can be expressed in terms of virtual photon asymmetries A1 and A2. A1 A2 N+ N- p e
Difference in the cross sections for two cases is expressed as “double spin asymmetry”.

24. Q2 evolution of the GDH integral
Both Bjorken and GDH sum rules are fundamental sum rules
Small Q2
GDH sum rule
Experiments at Mainz, Bonn
Chiral perturbation theory
ChPT
GDH sum rule
DIS
pQCD
operator product expansion
quark models
Lattice QCD?
Q2 (GeV2) 1 G1
Intermediate Q2
Extended GDH sum rule
Several different models
Experiments at JLAB(CLAS/Hall A/Hall C)
A good test of “at what distance scale pQCD corrections and higher twist expansions will break down and physics of confinement dominate”. ?
Large Q2
Bjorken Sum rule
Experiments at CERN,SLAC,DESY
Higher order QCD expansion
EG1B has good precision data with wide Q2 coverage to answer some of these questions.
Dramatic change of sign of G1 from DIS-regime to the value at the real photon point.
At low Q2 , g1(x,Q2) is dominated by resonance excitations.

25. A1 Deuteron
Δ(1232)P33 A1
PRELIMINARY
N( )D13/S13
W(GeV)

26. Γ1 Deuteron, Comparison to world data

27. Γ1 Deuteron, Comparison to world data

28. Γ1 Deuteron, Comparison to world data
PRELIMINARY

29. Γ1 Proton, Comparison to world data

30. Γ1 Proton, Comparison to world data
PRELIMINARY

31. Γ1 Proton, Comparison to world data
PRELIMINARY

32. CLAS – Saturation of the strong coupling
Bjorken sum:
Γ1 (p-n) =∫(g1p-g1n)dx
Fit to Bjorken sum:
Γ1(p-n) = gA/6[1-αs(Q)/π]
The saturation of s at large distances is a necessary condition for the application of the AdS5/CFT correspondence that can be used to carry out non-perturbative QCD calculations (Brodsky, de Teramond).

33. Form factors
Nucleon ground state
Nucleon resonances
Inclusive scattering
unpolarized
polarized
Semi-inclusive scattering
AUL, ALL, ALU for p SIDIS
Deep exclusive scattering
DVCS
Vector meson
Nuclear targets
Color tranparency

35. SIDIS (g*p→pX) cross section at leading twist (Ji et al.)
Unpolarized target
Longitudinally pol. target
Transversely pol. target e p
Boer-Mulders
1998
Kotzinian-Mulders
1996
Collins-1993
structure functions = pdf × fragm × hard × soft (all universal)

36. Longitudinal Target SSA measurements at CLAS
p1sinf+p2sin2f
CLAS PRELIMINARY
W2>4 GeV2
Q2>1.1 GeV2
y<0.85
0.4<z<0.7
MX>1.4 GeV
p1=-0.042±0.015
p2=-0.052±0.016
p1=0.082±0.018
p2=0.012±0.019
p1= 0.059±0.010
p2=-0.041±0.010
Large sinf for all pions and no indication of sin2f for p0 ’s suggests Sivers dominance
PT<1 GeV
0.12<x<0.48 36

37. ALL PT-dependence 0.4<z<0.7
m02=0.25GeV2
mD2=0.2GeV2
M.Anselmino et al hep-ph/
p+ A1 suggests broader kT distributions for f1 compared to g1
p- A1 may require non-Gaussian kT-dependence for different helicities and/or flavors

38. ALU
ALU

39. ALU
Results p0

40. Form factors
Nucleon ground state
Nucleon resonances
Inclusive scattering
unpolarized
polarized
Semi-inclusive scattering
AUL, ALL, ALU for p SIDIS
Deep exclusive scattering
DVCS
Vector meson
Nuclear targets
Color tranparency

41. DVCS@CLAS epa epg e’ g p 420 PbWO4 crystals : ~10x10 mm2, l=160 mm
JLab/ITEP/
Orsay/Saclay
collaboration
420 PbWO4 crystals : ~10x10 mm2, l=160 mm
Read-out : APDs +preamps

42. JLab Hall B Collaboration,
DVCS beam spin
asymmetry
VGG (1999)
JLab Hall B Collaboration,
PRL 100:162002,2008

43. Unpolarized Cross Section
0.09<-t<0.2 GeV2
0.2<-t<0.4 GeV2
0.4<-t<0.6 GeV2
POTENTIAL
0.6<-t<1 GeV2
1<-t<1.5 GeV2
1.5<-t<2 GeV2

44. r w f

45. Background Subtraction (normalized spectra)
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.

46. sL (g*Lp  prL0)
ep  epr 0
EPJA39 (2009)
VGG GPD model
GK GPD model

47. sL (g*Lp  prL0)
Regge/Laget

48. ds/dt (g*p  pr0)
Large tmin ! (1.6 GeV2)
Fit by ebt

49. p+p0 invariant mass =√( p p+ + p p0)2
N p+p0
Counts/20 MeV
100% e1-dvcs statistics
p+p0 invariant mass =√( p p+ + p p0)2

50. Form factors
Nucleon ground state
Nucleon resonances
Inclusive scattering
unpolarized
polarized
Semi-inclusive scattering
AUL, ALL, ALU for p SIDIS
Deep exclusive scattering
DVCS
Vector meson
Nuclear targets
Color tranparency

53. PRELIMINARY

54. IM(pp+)

55. IM(pp-)

56. N ∆ electroproduction experiments after 1999
Reaction
Observable W Q2
Author, Conference, Publication
LAB
p(e,e’p)π0
σ0 σTT σLT σLTP
1.221
0.060
S. Stave, EPJA, 30, 471 (2006)
MAMI
1.232
0.121
H. Schmieden, EPJA, 28, 91 (2006)
Th. Pospischil, PRL 86, 2959 (2001)
0.127
C. Mertz, PRL 86, 2963 (2001)
C. Kunz, PLB 564, 21 (2003)
N. Sparveris, PRL 94, (2005)
BATES
0.200
N. Sparveris, SOH Workshop (2006)
N. Sparveris, nucl-ex/611033
ALT ALTP
P. Bartsch, PRL 88, (2002)
D. Elsner, EPJA, 27, 91 (2006)
p(e,e’π+)n
C. Smith, SOH Workshop (2006)
JLAB / CLAS
σ0 σTT σLT
K. Joo, PRL 88, (2001)
σLTP
0.40,0.65
K. Joo, PRC 68, (2003)
K. Joo, PRC 70, (2004)
K. Joo, PRC 72, (2005)
H. Egiyan, PRC 73, (2006)
16 response functions
1.0
J. Kelly, PRL 95, (2005)
JLAB / Hall A
M. Ungaro, PRL 97, (2006)
2.8, 4.0
V. Frolov, PRL 82 , 45 (1999)
JLAB / Hall C

57. 2nd and 3rd nucleon resonance regions
(PDG 2006)
State
ηNπ
ηNη
ηNππ
P11(1440)
D13(1520)
0.0023
S11(1535)
< 0.1
D33(1700)
P13(1720)
0.04
> 0.7
Analysis tools:
Unitary isobar model (UIM), starting from MAID.
Dispersion relations (DR), for 1-pion analysis.
Isobar model (JM06) for 2-pion analysis with leading contributions as observed in the data. Fit to 9 independent one-dimensional projections of 5-dim. cross sections.

58. UIM & DR Fit at low & high Q2
# data points > 50,000 , Ee = 1.515, 1.645, 5.75 GeV
Observable
Number of
Data points
dσ/dΩ(π0)
0.40
0.65
3 530
3 818
dσ/dΩ(π+)
2 308
1 716
33 000
Ae(π0)
956
805
Ae(π+)
918
812
3 300
dσ/dΩ(η)
0.375
0.750
172
412
Low Q2 results:
I. Aznauryan et al., PRC71, , 2005; PRC 72, , 2005;
High Q2 results on Roper: I. Aznauryan et al., arXiv:  [nucl-ex].

59. Fits to diff. cross sections & structure functionsDR
UIM
DR w/o P11 DR
UIM
Q2=2.05GeV2
Q2=3.48GeV2

60. Legendre moments for σT + εσL
Q2 = 2.05 GeV2
~ const.
~cosθ
~(1 + bcos2θ)
DR UIM
DR w/o P11(1440)

61. Multipole amplitudes for γ*p→ π+n
Q2 =0
Q2 =2.05 GeV2
At Q2= , resonance behavior is seen in these amplitudes more clearly than at Q2 =0
DR and UIM give close results for real parts of multipole amplitudes Im
Re_UIM
Re_DR

62. Γ3 and Γ5 Proton

63. Comparison with Theory
Quenched Lattice QCD
- E1+/M1+: Good agreement within large errors.
- S1+/M1+: Undershoots data at low Q2.
- Linear chiral extrapolations may be naïve and/or dynamical quarks required
Dynamical Models
- Pion cloud model allows reasonable description of quadrupole ratios over large Q2 range.
What are we learning from E/M, S/M?
Shape of pion cloud?
Deformation of N, △ quark core?
LQCD consistent with observed rise in magnitude with Q2 of RSM
Need to isolate the first term (within model) or go to high Q2 to study quark core.

64. N-Δ(1232) Quadrupole Transition
SU(6): E1+=S1+=0
The Delta excitation has been studied experimentally for 3 decades, and the dominant magnetic dipole transition is well established, mediated by a simple spin flip. Electric or scalar quadrupole transitions, which are identical 0 in a symmetry SU(6) model, would indicate a deformation of the Delta and the nucleon.
Dynamically, such contributions may arise through interaction with the pion cloud, or through the one gluon exchange.
helicity conservation requires that the E/M asymptotically approaches +1. Qualitatively one expects such a behavior. The sign of the this ratio is related to the shape of the deformation of the Delta
in such a way. 64

65. y z x x
M.G., Polyakov, Radyushkin, Vanderhaeghen (2005) b
(fm)