Theoretical Aspects of physics with electromagnetic probes

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Theoretical Aspects of physics with
electromagnetic probes
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Theoretical Aspects of Physics with
electromagnetic Probes
Klaus Goeke, University of Bochum, Germany

• Goals in hadronic physics
• Probes in hadronic physics
• New probes Hard exclusive Reactions
• Perspectives
• Problems
• Visions

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Theoretical Aspects of Physics with
electromagnetic Probes
Klaus Goeke, University of Bochum, Germany

• Goals in hadronic physics
• Probes in hadronic physics
• New probes Hard exclusive Reactions
• Perspectives
• Problems
• Visions

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Structure of the Nucleon
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Structure of the Nucleon
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Goals of hadronic physics

• Determine the parameters of Quantum Chromo
  Dynamics (QCD)
• Understand the origin and dynamics of confinement
• Understand the origin and dynamics of spontaneous
  chiral symmetry breaking
• Understand the quark and gluon structure of
  hadrons based on QCD
• Understand the relation between parton degrees of
  freedom in the infinite momentum frame and the
  structure of hadrons in the rest frame.
• Develope quantitatively reliable models and
  approximations to QCD
• Explore the role of quarks and gluons in nucleons
  and nuclei and matter under extreme conditions
• Explore the role of hadronic corrections in
  precision measurements of electroweak processes

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Goals of hadronic physics

• Determine the parameters of Quantum Chromo
  Dynamics (QCD)
• Understand the origin and dynamics of confinement
• Understand the origin and dynamics of spontaneous
  chiral symmetry breaking
• Understand the quark and gluon structure of
  hadrons based on QCD
• Understand the relation between parton degrees of
  freedom in the infinite momentum frame and the
  structure of hadrons in the rest frame.
• Develope quantitatively reliable models and
  approximations to QCD
• Explore the role of quarks and gluons in nucleons
  and nuclei and matter under extreme conditions
• Explore the role of hadronic corrections in
  precision measurements of electroweak processes

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Quark and Hadron Dynamics

• Hadron projectiles
• Symmetries
• Threshold behaviours
• Confinement
• Chiral Symmetry
• High count rates
• New phenomena
• Exotic systems
• Decay channels

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Hadron-Hadron Collisions

• Hadronic projectile
• Interaction process
• Hadronic target

Neither
are well understood
Problem Theoretical description
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Hadronic physics in Europe
Hadronic probes MUST be accomanpied with
leptonic probes
Low energy meson-baryon picture of nucleon and
hadrons
High energy partonic picture of nucleon and
hadrons
Hard exclusive reactions Allow to connect both
pictures and are extremely versatile
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Leptonic probes in hadron physics
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Leptonic probes in hadronic physics

• Leptons
• Leptonic projectiles provide
• Probing operator with weak (small, slight)
  interaction
• Probing operator well defined in terms of QCD
• Only few projectiles in nature Electron,
  Neutrino
• But New projectiles with similar and even
  better properties

• New projectiles
• Hard exclusive reactions induced by leptons
• Hard exclusive reactions create a short-distance
  probe
• QCD has weak (small, slight) interaction at small
  distances
• Observables factorize into hard part (calculable
  by pQCD) and soft part.
• Soft part Response of the nucleon to hard
  external probe
• Problem Low count rate

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Virtual vs. Deeply Virtual Compton Scattering
Large virtuality Q Process happens at one quark

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DVCS Generalized parton distribution

• Hard reaction process (large Q)
• Perturbative QCD
• Handbag diagramms
• Other suppressed by 1/Q

• Soft response of the system to the external probe
  
• Generalized parton distribution
• Non-perturbative QCD
• Well defined in many body terms ? 1-quark
  properties.

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General Probe Hard exclusive Reaction
Hard probe defines the process
Link Only the two quark lines
Universal (process independent) generalized
parton distribution 1-quark properties of
target baryon
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Factorization Hard exclusive Reaction
Hard process provides well defined QCD probe
(handbag)
Response to the QCD-probe Generalized parton
distribution
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Factorization Properties of the probe
Hard process provides well defined QCD probe
(handbag)
Structure of probing operator
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Factorization Properties of the target
Response to the QCD-probe Generalized parton
distribution
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Factorization Properties of the target
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Single particle density matrix
Generalized parton distribution
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Sum rules and limiting cases of GPDs
GPDs connect two regimes of hadronic physics
Theory Good QCD-models required
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HEMP Generalized parton distribution
Meson transition amplitude
Non-diagonal in flavour and momentum
Outgoing meson is a filter selects quantum
numbers of probing operator
Theory Good QCD-models required
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HEMP Mesons as helicity filter
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HEMP Mesons as flavour filter
Possibility to access directly strange component
of the nucleon in an new way
Parity violating electron scattering
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Quark-Gluon hybrids
Production of hybrids (meson-gluon,C1) via two
mechanisms
Possibility to measure gluon content of hybrids
in a new way
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Baryon spectroscopy with light-cone probes

• New probe for production of excited baryons
• Probe defined along the light cone
• Perhaps new resonances are produced
• Perhaps new view on missing resonances

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Chiral Symmetry Pion pole
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Chiral Symmetry Electro-production of pions

Cross section
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Chiral symmetry Hard electroproduction of pions
transverse spin asymmetry
Spin asymmetry of 60
Transverse spin asymmetry for longitudinal
electroproduction of pions at different t-values
Pion pole relevant
Transversely polarized proton, unpolarized
electrons, longitudinal photon, phi?-phi.
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Hard electroproduction of strangeness
Kaon pole relevant
Spin asymmetry of 60
Transverse spin asymmetry for longitudinal
electroproduction of kaons at different t-values
Transversely polarized proton, unpolarized
electrons, longitudinal photon, phi?-phi
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Angular Momentum of Quarks
Contribution of the angular momentum of quarks
with flavour q to the total angular momentum of
the nucleon (X.Ji)
To be measured in DVCS
DVCS-probe ? energy momentum tensor
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Rho-production Quark-spin
Transverse spin asymmetry in rho-production
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Rho-0 production Different Q,t
Transverse spin asymmetry in rho-production
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Energy Momentum Tensor New Formfactors
Lorentz decomposition
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Energy momentum tensor r-space
Breit-Frame
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Shear and pressure
D-term Can be extracted from DVCS charge
asymmetry
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D-Term DVCS Charge asymmetry
Without D-Term Prediction
D-term with wrong sign
Charge asymmetry vs.
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HERMES DVCS - SSA
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Transverse beam spin asymmetry
Transversely polarized electron beam
? unpolarized proton
target
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DDVCS virtual ? virtual

• Single-Spin-Asymmetries SSA
• DVCS interference of BH (real) and Imag(DVCS)
  measured
• BH enlarges cross section
• But H(x,x,t) measured
• DDVCS H(x,x,t) measured
• Or H(x,x,t) measured for xltx
• xltx ERBL-region

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Summary Hadrons ? Electron Scattering ? Theory

• Hard exclusive leptonic reactions provide many
  new and versatile weakly interacting probes
• Nucleon response clearly defined in terms of QCD
• Generalized parton distributions ? 1-body density
  matrix
• Filter for quantum numbers, gluons, quarks
• Chiral symmetry pion and kaon pole, ChPT
• Strange quarks, finite masses, asymmetries
• Distribution of quark spins (intr.orb.), flavour
  decomposition
• Forces and pressures inside nucleon and nuclei
• Independent excitation of spectrum of baryons
• Quark-Correlations by higher twist effects

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The End ...
Thank you !