Photo and electro production of mesons on the nucleon
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Photo- and Electro-Production of Mesons on the Nucleon. … powerful tools to explore:. Nucleon. Equipment. Electron scattering Longitudinally polarised beams Longitudinally and transversely polarised targets Polarisation transfer Photo-production Circularly and linearly polarised beams

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Photo- and Electro-Production of Mesons on the Nucleon

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Photo and electro production of mesons on the nucleon

Photo- and Electro-Productionof Mesons on the Nucleon

Erice School, 22/9/07


Powerful tools to explore

… powerful tools to explore:

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


Gdh sum rule

GDH sum rule

  • Proton: 205 b

  • Neutron: 233 b

Erice School, 22/9/07


Gdh @ mami

GDH @ MAMI

Erice School, 22/9/07


Gdh @ elsa

GDH @ ELSA

Erice School, 22/9/07


Proton excitation function

Proton excitation function

D13(1520)

D(1232)

F15(1680)

F35(1905)

Erice School, 22/9/07


Gdh integral

GDH integral

On proton

On neutron

 MAMI C

Erice School, 22/9/07


Generalised gdh integral

Generalised GDH integral

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


Hall b @ jlab

Hall B @ JLab

Photon Tagger



Erice School, 22/9/07


Proton g 1 g 1 x q 2 dx

Proton G1 = g1(x,Q2)dx

  • 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


G 1 d q 2 per nucleon

G1d(Q2) (per nucleon)

  • Like the proton: shows expected behavior at low and high Q2

  • Agreement with PT at the lowest points

Erice School, 22/9/07


Bjorken sum 1 p n q 2

Bjorken Sum: Γ1p-n(Q2)

  • 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


Resonance transition form factors

Resonance Transition Form Factors

  • 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


N transition form factor g m

NΔ Transition Form Factor GM*

  • Exclusive π0electro-production data

  • Meson contributions play an important role even at relatively high Q2.

Erice School, 22/9/07


N d multipole ratios r em r sm

ND Multipole Ratios REM, RSM

  • 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


Comparison with theory

Comparison with Theory

  • 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


Transitions to 2 nd resonance region

Transitions to 2nd resonance region

  • 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


P 11 1440 cqm comparison @ low q 2

P11(1440) CQM Comparison @ low Q2

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


P 11 1440 transition ff @ high q 2

P11(1440) Transition FF @ high Q2

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


S 11 1535 in p and n

S11(1535) in pη and N

New CLAS results

n+

p0

CLAS 2007

CLAS 2002

n+

, p0

previous results

preliminary

PDG 2006

CQM

PDG (2006): S11→ N (35-55)%

→ ηN (45-60)%

A1/2 from both pand N agree

Erice School, 22/9/07


Transition p d 13 1520

Transition γ*p  D13(1520)

A3/2

Previous pπ0

based data

preliminary

preliminary

A1/2

Q2, GeV2

Q2, GeV2

N, p+-

n+

n+

, p0

p0

nrCQM:

Erice School, 22/9/07


Helicity asymmetry for pd 13

Helicity Asymmetry for γ*pD13

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


Search for missing resonances

Search for “missing” resonances

  • (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


Missing resonances

“Missing” Resonances

Really missing, or just not observed yet ?

Erice School, 22/9/07


Photoproduction of k and k 0

Photoproduction of K+Λ and K+Σ0

P11

K exchange

P13

P13

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

Erice School, 22/9/07


Angular distributions

Angular distributions

Fit with P13(1900)

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

Erice School, 22/9/07


Pseudoscalar meson production

Pseudoscalar meson production

  • 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

Erice School, 22/9/07


Polarisation transfer p k

Polarisation transfer γp  K+Λ

K+

proton

proton

transferred polarization along x 

induced polarization along y 

transferred polarization along z 

density matrix , Pauli spin matrix  

Erice School, 22/9/07


Polarisation transfer p k1

Polarisation transfer γp  K+Λ

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

Erice School, 22/9/07


Beam target polarisation

Beam & target polarisation

Polarised HD-Target

Frozen Spin Target

P(D)

P(H)

FROST

Linearly polarised photons

Erice School, 22/9/07


Clas search for new resonances

CLAS search for new resonances

Erice School, 22/9/07


Polarised semi inclusive dis

Polarised semi-inclusive DIS

  • 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

Erice School, 22/9/07


Hermes @ desy

HERMES @ DESY

Erice School, 22/9/07


Flavour tagged q s

Flavour-tagged q’s

  • 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


Generalised parton distributions

Generalised Parton Distributions

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


Gpd s

GPD’s

Erice School, 22/9/07


Quark total angular momentum

Quark total angular momentum

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


Exclusive production information on polarised gpd s

Exclusive + production information on polarised GPD’s

  • 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

Erice School, 22/9/07


0 transverse target spin asymmetry

0transverse target-spin asymmetry

  • TTSA: access to GPD E

  • E related to Jq

[curves: Vinnikov et al. (2005)]

Erice School, 22/9/07


Exclusivity with hermes 4 detector

Exclusivity with HERMES 4 detector

Erice School, 22/9/07


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