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Photo- and Electro-Production of Mesons on the Nucleon

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Photo- and Electro-Productionof Mesons on the Nucleon

Erice School, 22/9/07

Nucleon

Equipment

Electron scattering

Longitudinally polarised beams

Longitudinally and transversely polarised targets

Polarisation transfer

Photo-production

Circularly and linearly polarised beams

Longitudinally and transversely polarised targets

Polarisation transfer

4 detector

- Structure
- Sum rules
- Transition form factors
- Spin structure
- Semi-inclusive DIS
- Hard exclusive processes

- Spectroscopy
- In higher resonance regions
- Search for new resonances

- MAMI-A2, ELSA-GDH, JLab-CLAS, DESY-HERMES

Erice School, 22/9/07

- Proton: 205 b
- Neutron: 233 b

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Erice School, 22/9/07

Erice School, 22/9/07

D13(1520)

D(1232)

F15(1680)

F35(1905)

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On proton

On neutron

MAMI C

Erice School, 22/9/07

DIS

pQCD

G1

operator product expansion

quark models

Lattice QCD?

ChPT

1

Q2 (GeV2)

GDH sum rule

Elastic contribution not included

The spin-dependent virtual photon Compton amplitudes S1(n,Q2) and S2(n,Q2) are calculable and can be related to measurable structure functions via dispersion relations

calculablemeasurable

Variation with Q2:

Parton

Hadron

Erice School, 22/9/07

Photon Tagger

Erice School, 22/9/07

- Shows expected trend toward DIS result at high Q2
- At low Q2 we observe a negative slope as expected from GDH Sum Rule.
- Agreement with PT at the lowest points.

Erice School, 22/9/07

- Like the proton: shows expected behavior at low and high Q2
- Agreement with PT at the lowest points

Erice School, 22/9/07

- Agreement with PT up to Q2 = 0.25 (GeV/c)2.
- NNLO pQCDin reasonable agreement with the data
- Higher twists are small, even down to Q2 = 0.75 (GeV/c)2

MENU2007, FZ Juelich, 14/9/07

- Improve understanding of nucleon structure in confinement region
- γNΔ or γNN* transition amplitudes at various distance scales (Q2) wave functions of excited states
- Diff. cross sections & polarisation observables in full angular range partial wave analysis
- Isospin amplitudes of final state decomposition resonance flavour

e’

, K

γv

e

N*,△*

N’,△’, L

N

A1/2, A3/2, S1/2

Erice School, 22/9/07

- Exclusive π0electro-production data
- Meson contributions play an important role even at relatively high Q2.

Erice School, 22/9/07

- Precision measurement of baryon form factors:REM, RSM < 0.01
- REM remains small and negative at -2% to -3.5% up to 6 GeV2
- NoREM sign change or asymptotic behavior. Helicity conservation:REM→ +100(%)!
- RSM negative and increases in magnitude. Helicity conservation:RSM→ constant!

K. Joo, et al., PRL88 (2002); J. Kelly et al., PRL95 (2005); M. Ungaro, et al., PRL97 (2006)

Erice School, 22/9/07

- Quenched Lattice QCD
- E1+/M1+: Good agreement within large errors.
- S1+/M1+: Undershoots data at low Q2
- Linear chiral extrapolations may not be adeqate or dynamical quarks required

- Dynamical Models
- Pion cloud model:reasonable description of quadrupole ratios over large Q2 range.

Erice School, 22/9/07

- Poorly understood in nrCQMs
- Alternative models:
- Light front kinematics (relativity)
- Hybrid baryon with gluonic excitation |q3G>
- Quark core with large meson cloud |q3m>
- Nucleon-sigma molecule |N>
- Dynamically generated resonance

P11(1440)

Hard form factor (slow fall-off with Q2)

Not a quark resonance, but KΣ dynamical system?

S11(1535)

Change of helicity structure with increasing Q2 from λ=3/2

to λ=1/2 dominance, predicted in nrCQMs, pQCD

D13(1520)

Measure Q2 dependence of Transition FF

Erice School, 22/9/07

nrCQM

nrCQM

LC CQM

LC CQM

- Non-relativistic CQ Models do not reproduce sign of A1/2 at Q2=0, and show no zero-crossing.
- Relativistic CQ Models (LC) give correct sign and show zero-crossing, but miss strength at Q2=0, possibly due to missing meson cloud contributions.

Check at higher Q2

Erice School, 22/9/07

Nπ, pπ+π-

nπ+

DR

UIM

pπ0

nπ+

pπ0

- Analysis with
- Unitary Isobar Model (UIM)
- Fixed-t Dispersion Relations (DR)

First zero-crossing observed for a nucleon form factor!

Erice School, 22/9/07

New CLAS results

n+

p0

CLAS 2007

CLAS 2002

pη

n+

, p0

previous results

pη

preliminary

PDG 2006

CQM

PDG (2006): S11→ N (35-55)%

→ ηN (45-60)%

A1/2 from both pand N agree

Erice School, 22/9/07

A3/2

Previous pπ0

based data

preliminary

preliminary

A1/2

Q2, GeV2

Q2, GeV2

N, p+-

n+

n+

, p0

p0

nrCQM:

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A21/2 – A23/2

Ahel =

A21/2 + A23/2

Ahel

D13(1520)

CQMs and pQCD

Ahel→ +1 at Q2 → ∞

Helicity structure of transition changes rapidly with Q2 from helicity 3/2(Ahel= -1) to helicity 1/2 (Ahel= +1) dominance!

Erice School, 22/9/07

- (Nearly) complete, measurements in p→N, ηN, K+Y and n→N, K0Y by using various combinations of beam, target and recoil polarisations
- Differential cross sections with unpolarised, circularly polarised and linearly polarised photon beams
- longitudinally or transversely polarised proton and neutron (deuteron) targets
- recoil polarisation for hyperons

- Other reactions: p → ρN, ωp, N with linearly polarised beams or targets

Erice School, 22/9/07

Really missing, or just not observed yet ?

Erice School, 22/9/07

P11

K exchange

P13

P13

Coupled channel fit: Bonn-Gatchina group, Anisovich et al., 2007

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Fit with P13(1900)

KΛ

KΣ

Coupled channel fit: Bonn-Gatchina group, Anisovich et al, 2007

Erice School, 22/9/07

- Bilinear combinations of 4 complex amplitudes:
- 16 observables

- Required:8 observables
- differential cross section d/d
- 3 single polarization observables: P, T,
- 4 double polarization observables

- CLAS:
- linearly + circularly polarised photons
- longitudinal + transverse target polarisation
- recoil polarisation in hyperon production

I. S. Barker, A. Donnachie, J. K. StorrowNucl. Phys. B 95(1975)347

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K+

proton

proton

transferred polarization along x

induced polarization along y

transferred polarization along z

density matrix , Pauli spin matrix

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w/o P13(1900)

with P13(1900)

Quark-Diquark Model

(E. Santopinto, 2005)

Includes *** / **** states

Coupled channel 2007 fit: Bonn-Gatchina group, Anisovichet al.

Fit favours existence of second P13state

Evidence against quark-diquark model

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Polarised HD-Target

Frozen Spin Target

P(D)

P(H)

FROST

Linearly polarised photons

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- Flavour separation of
- Quark & antiquark helicity distributions by
- Detecting hadron h in coincidence with scattered lepton

- Flavour content of final state hadrons related to
- Flavour of struck quark via fragmentation functions D.
- In LO QCD:
- Spin-independent purity matrix P from Monte-Carlo

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Erice School, 22/9/07

- First 5-flavour fit to q(x)
- Input:

- Sizeable positive polarisation of u quarks
- Negative polarisation of d quarks
- No significant polarisation
- In 0.023 < x < 0.6 :

Erice School, 22/9/07

Nucleon matrix element of quark energy-momentum tensor

Formfactors M2(t): Mass distribution inside the nucleon in transverse space

J(t): Angular momentum distribution

d1(t): Distribution of forces and pressure

These form factors are moments of GPD’s, e.g.

Ji’s sum rule:

Erice School, 22/9/07

Erice School, 22/9/07

HERMES 2002-2004 data:

- Unpolarised beam, transversely polarised target
- Best VGG fit for0.2< Ju< 0.4(assuming u-quark dominance)

DVCS

Erice School, 22/9/07

- no L/T separation, but T suppressed as 1/Q2
- LO VGG X-sections too low
- LO VGG X-sections + power correction to space-like pion form factor overestimate data

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- TTSA: access to GPD E
- E related to Jq

[curves: Vinnikov et al. (2005)]

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Erice School, 22/9/07