I will discuss two approaches for the analysis of pion photo-and electroproduction on nucleons in the resonance energy region: unitary isobar model and dispersion relations.

I will present also the results on the electroexcitation of nucleon resonances extracted from CLAS data.

Rapidly developing field of search for temporal and spatial variation of the fundamental constants now includes a very large number of laboratory, astronomical and geochemical measurements. Interpretation of these measurements and search for new research directions require QCD calculations of dependence of nuclear parameters on quark masses. We need to calculate this dependence for deuteron binding energy, positions of nuclear resonances, nuclear magnetic moments, strong interaction between nucleons and nuclear potentials.

Theories unifying gravity with other interactions suggest temporal and spatial variation of the fundamental "constants" in expanding Universe. I discuss effects of variation of the fine structure constant $\alpha=e^2/\hbar c$, strong interaction and quark mass. The measurements of these variations cover lifespan of the Universe from few minutes after Big Bang to the present time and give controversial results. There are some hints for the variation in Big Bang nucleosynthesis, quasar absorption spectra and Oklo natural nuclear reactor data.

A very promising method to search for the variation of the fundamental constants consists in comparison of different atomic clocks. A billion times enhancement of the variation effects happens in transition between accidentally degenerate atomic energy levels.

Various aspects of the phenomenological implemention of gauge invariance in meson production processes with real and virtual photons are discussed. A critical assessment of some often-used prescriptions is given.

Identifying a zero-range exchange of vector mesons as the driving force for the s-wave scattering of pseudo-scalar mesons off the baryon ground states, a rich spectrum of molecules is formed. We argue that chiral symmetry and large-$N_c$ considerations determine that part of the interaction which generates the spectrum. A bound state with exotic quantum numbers is predicted at mass 2.78 GeV. It couples strongly to the $(\bar D_s\,N),(\bar D \,\Lambda ),(\bar D\,\Sigma)$ channels. A further charm minus-one system is predicted at mass 2.84 GeV as a result of $(\bar D_s \Lambda), (\bar D\,\Xi)$ interactions. We suggest the existence of strongly bound crypto-exotic baryons, which contain a charm-anti-charm pair. Such states are narrow since they can decay only via OZI-violating processes. A narrow nucleon resonance is found at mass 3.52 GeV. It is a coupled-channel bound state of the $(\eta_c\,N), (\bar D\,\Sigma_c)$ system, which decays dominantly into the $(\eta' N)$ channel. Furthermore two isospin singlet hyperon states at mass 3.23 GeV and 3.58 GeV are observed as a consequence of coupled-channel interactions of the $(\bar D_s\,\Lambda_c), (\bar D\,\Xi_c)$ and $(\eta_c \,\Lambda),(\bar D\,\Xi_c')$ states. Most striking is the small width of about 1 MeV of the lower state. The upper state may be significantly broader due to a strong coupling to the $(\eta' \Lambda)$ state. The spectrum of crypto-exotic charm-zero states is completed with an isospin triplet state at 3.93 GeV and an isospin doublet state at 3.80 GeV. The dominant decay modes involve again the $\eta'$ meson. The two so far observed s-wave baryons with charm one are recovered. We argue that the $\Lambda_c(2880)$ is not a s-wave state. In addition to those states we predict the existence of about ten narrow s-wave baryon states with masses below 3 GeV. A triplet of crypto-exotic states decaying dominantly into channels with an $\eta'$ is obtained with masses 4.24 GeV and 4.44 GeV. In the charm-two sector we predict in addition to the chiral excitations of the ground states two triplets of bound states formed by channels involving open-charm mesons. The binding energy of the latter is larger than the one of the chiral excitations.

In order to formulate a relativistically covariant quark model that shares all the successful features apparent in non-relativistic constituent quark models one can invoke the Bethe-Salpeter equation and make the following assumptions: The full quark propagators are supposed to have the free form with an effective constituent mass and the interactions are assumed to be given by unretarded potentials, which in the present setup include a string-like linearly rising confinement potential with a suitable spin-dependence and a spin-flavour dependent quark interaction motivated by instanton effects.

The resulting Salpeter equation was solved to calculate the complete mass spectrum of light-flavoured hadrons up to 3 GeV [1,2]. We found that the resulting spectra do depend on the spinorial structure of the confinement potential, of which we studied two versions both for mesons and for baryons. Constituent quark masses and confinement parameters were fixed to account for Regge trajectories and those of the instanton-induced interaction (acting on (pseudo)scalar mesons and flavour-antisymmetric quark pairs only) to ground-state hyperfine splittings, the rest of the spectrum then being a genuine prediction. In this manner we can describe virtually all major features in the experimental hadron spectra, in particular the splitting and mixing of (pseudo)scalar mesons as well as the conspicuous low position of the Roper-resonance, and the approximate parity doublets apparent in the N-and Λ spectra.

Form factors and decay properties of resonances were calculated in this covariant framework in the Mandelstam formalism on the basis of vertex functions derived from the Salpeter amplitudes. Since all model parameters have been fixed from the mass spectra the calculation of electromagnetic properties constitutes a genuine prediction. In particular we find a good description of static electroweak baryon properties, for which we developed a novel formula on the basis of the Salpeter amplitudes directly. We also find a fair description of the magnetic and axial nucleon form factors and the magnetic N-Δ-transition form factor up to moderate (3-4 GeV²) momentum transfers. The relevance of the instanton induced interaction for the momentum dependence of the electro-weak form factors will be discussed. Results on other transition form factors and helicity amplitudes [3] will be presented. An estimate of strong two-body decay widths of baryon resonances can also be obtained parameter free by calculating in lowest order the contribution of single quark-loops.

First results on the decay widths of some low-lying baryon resonances will be discussed.

References.

1. R. Ricken, M. Koll, D. Merten, B.Ch. Metsch, H.-R. Petry, Eur. Phys. J. A 9 221 (2000); M. Koll, R. Ricken, D. Merten, B.Ch. Metsch, H.-R. Petry, Eur. Phys. J. A 9 73 (2000).
2. U. Löring, K. Kretzschmar, B.Ch.Metsch, H.-R.Petry, Eur. Phys. J. A 10 309 (2001); U. Löring, B.Ch. Metsch, H.R. Petry, Eur. Phys. J. A 10 395 (2001); ibid. 447.
3. D. Merten, U. Löring, B.Ch.Metsch, H.-R.Petry, Eur. Phys. J. A 18 193 (2003); T. Van Cauteren, D. Merten, T. Corthals, S. Janssen, B. Metsch, H.-R. Petry, J. Ryckebusch, Eur. Phys. J. A 20 283 (2004).

We present a phenomenological model [1,2] for description of double charged pion production by real and virtual photons off protons in the nucleon resonance region. This approach allows to determine N* photocouplings as well as various isobar channel contributions from combined fit of all measured observables. Double pion channel is valuable source of information on N* structure, being particularly sensitive to the high lying states (M > 1.65 GeV), which preferably decay through 2π emission. This channel is also promising for "missing" baryon state search. Signals from possible new baryon state 3/2⁺(1720) were found in analysis of CLAS double charged pion electroproduction data [3].

Recent studies of this exclusive channel in combined analysis of CLAS photo- [4] and electroproduction [3] data revealed several new non-resonant mechanisms: a) π⁺D₁₃⁰(1520), π⁺F₁₅⁰(1685), π^-P₃₃⁺⁺(1600) isobar channels; b) 2π production in meson/baryon exchange processes; c) complementary contact interaction in πΔ channels. Considerable improvement in real and virtual photon data fit was achieved after implementation of these new mechanisms. The quality of CLAS data allowed to establish all relevant background mechanisms in double charged pion production in N* excitation region, implementing particular meson-baryon diagrams and fitting theirs parameters to the measured observables. Reliability of background description was checked and confirmed in combined analysis of CLAS πN and ππN electroproduction data.

Recent CLAS ππN data were analyzed within a framework of improved dynamical model with a goal to extract the photocouplings for most nucleon excitations in mass range < 2.0 GeV. First data on N* photocoupling at various photon virtualities Q² < 1.5 GeV², obtained from analysis of double charged pion exclusive channel, are reported and discussed.

References.

1. V. D. Burkert, et. al., Phys of Atom. Nucl. 67, 1918 (2004).
2. V. Mokeev, et. al. in: Proc. of the Workshop on the Physics of Excited Nucleons, Grenoble, France 24-27 March 2004, World Scientific, 2005, ed. by J.-P. Bocquet, V.Kuznetsov, D.Rebreyend., 317.
3. M. Ripani, et. al., Phys. Rev. Lett. 91, 022002 (2003).
4. M. Bellis, et. al. in: Proc of the Workshop on the Physics of Excited Nucleons, Grenoble, France 24-27 March 2004, World Scientific, 2005, ed. by J.-P. Bocquet, V.Kuznetsov, D.Rebreyend., 139.

Progress in determining the baryon spectrum using Monte Carlo lattice calculations with large sets of extended operators will be outlined.

We investigate how the exotic pentaquark Θ(1540) baryon production can be identified in the γ N → K K N reactions, focusing on the influence of the background (non-Θ production) mechanisms.

By imposing the SU(3) symmetry and using various quark model predictions, we are able to fix the coupling constants for evaluating the so-called Drell diagrams, the KK production through the intermediate vector meson and tensor meson photoproduction, and the mechanisms involving intermediate Λ(1116), Λ(1405), Λ(1520), Σ(1193), Σ(1385), and Δ(1232) states. The vector meson photoproduction part is calculated from a phenomenological model which describes well the experimental data at low energies. The charged tensor meson production is calculated from a one-pion-exchange model which describes well the total cross section data of γ p → a₂⁺(1320) n. We point out that the neutral tensor meson production can not be due to π⁰-exchange because of C parity. The neutral tensor meson production is estimated by considering the vector meson exchange and found to be too weak to generate any peak at the position near Θ(1540).

For Θ(1540) production, we assume that it is an isoscalar and hence can only be produced in γ n → K⁺K^-n and γ p → K⁰K⁰p$ reactions, not in γ p → K⁺K^-p and γ n → K⁰K⁰ n. The total cross section data of γ p &rarr K⁺K^- p is thus used to fix the form factors which regularize the background amplitudes so that the signal of Θ(1540) in γ n → K⁺K^-n and γ p → K⁰K⁰p cross sections can be predicted. We find that the predicted K⁺K^- and K⁺ n invariant mass distributions of the γ n → K⁺ K^- n reaction can qualitatively reproduce the shapes of the JLab data. However, the predicted Θ(1540) peak can not be identified unambiguously with the data. High statistics experiments are needed to resolve the problem.

We also find that an even-parity Θ is more likely to be detected, while it will be difficult to identify an odd-parity Θ, even if it exists, from the background continuum, if its coupling constants are small as in the present quark model predictions.

In this talk I will show how the ChPT sort of low-energy expansions can be extended into the Delta-resonance region. This novel EFT framework is applied to the pion photo- and electro-production in the Delta region with the aim of a model-independent extraction of the gamma-N-Delta form factors and Delta's magnetic moment. These results will be contrasted with the state-of-the-art calculations of the dynamical models and of the lattice QCD studies.

Three recent experiments on parity violation in electron-proton scattering are consistent with a small positive value for the strangeness magnetic moment of the proton $\mu_s$. The original expection had been for a negative value, mainly due to kaon loop fluctuations of the proton. A complete analysis of all positive parity $uuds\bar s$ configurations of the proton reveals that a positive value for $\mu_s$ obtains if the $uuds$ subsystem is orbitally excited, with exception for the energetically unfavored case where the total spin of the $uuds$ subsystem is 2. This is consistent with the assumption that there is a strong spin-dependent hyperfine interaction between the quarks in the proton. It is shown that the presence of a small analogous orbitally excited $uudd$ configuration in the $\Delta(1232)$ can explain the systematic underprediction of the width of the $\Delta(1232)$ in the valence quark model.

A survey of recent meson photo- and electroproduction experiments at Jefferson Lab will be presented. Cross section data in the resonance region for the (π, 2π, K, η, ρ, ω) decay channels together with an increasing variety of beam, target and recoil polarization observables will be compared to phenomenological models.

We summarize recent soft-QCD model results for electromagnetic elastic and transition form factors of mesons, and for the masses and form factors of di-quark correlations that are expected to be important for the description of the nucleon and its low-lying excitations. The approach to modeling is fully covariant, contains essentially one infrared parameter, and is based upon truncation of the QCD Dyson-Schwinger equations of motion.

The theoretical ingredients of the Dubna-Mainz-Taipei dynamical model for the pion EM production will be presented and discussed. Comparisons with other models will be made wherever possible. The predictions of the model from threshold to the resonance region will be compared to the experiments.