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We claim that a narrow peak in the cross section near 1685~MeV in the $\gamma n\to \eta n$ channel can be explained through a peculiar radial behaviour of the $p$-wave quark states with $j=1/2$ and $j=3/2$ in the low lying S11 resonances and the opening of the $K\Sigma$ threshold rather than by an exotic resonance. We explain the mechanism of its formation in the framework of a coupled channel formalism which incorporates quasi-bound quark-model states corresponding to the two low lying resonances in the S11 partial wave. A relation to the Single Quark Transition Model is pointed out., Comment: Contributed talk at the Mini-Workshop "Quarks, Hadrons, Matter", July 3-10, Bled, Slovenia, 6 pages LaTeX, published as Bled Workshops in Physics, Vol. 17, No. 1 (2016) p. 44

We review some results obtained by the Coimbra-Ljubljana collaboration that reveal the important role of the pion cloud in the low- and intermediate-energy nucleon and delta resonances., Comment: Talk presented at EEF70, Workshop on Unquenched Hadron Spectroscopy, 1-5 Sept. 2014, Coimbra

We apply a coupled-channel formalism incorporating quasi-bound quark-model states to calculate pion scattering into eta N, K Lambda and K Sigma channels, as well eta p, eta n, K+Lambda, and K0Sigma+ photo-production processes. The meson-baryon and photon-baryon vertices are determined in a SU(3) version of the Cloudy Bag Model. Our model predicts sizable amplitudes in the P11, P13, P33 and S11 partial waves in agreement with the latest MAID isobar model and the recent partial-wave analyses of the Bonn-Gatchina group. We are able to give a quark-model explanation for the apparent resonance at 1685 MeV in the eta n channel., Comment: 14 pages, 12 figures, to appear in EPJ A

The meson scattering and electroproduction amplitudes in the D13, D33 and D15 partial waves are calculated in a coupled-channel formalism incorporating quasi-bound quark-model states, extending our previous studies of the P11, P33 and S11 partial waves. The vertices of the baryon-meson interaction including the s- and d-wave pions and $\rho$-mesons, the s-wave $\eta$-meson, and the $s$- and p-wave $\sigma$-mesons are determined in the Cloudy Bag Model, with some changes of the parameters to reproduce the widths of the resonances. The helicity amplitudes and the electroproduction amplitudes exhibit consistent behavior in all channels but tend to be too weak compared to the experiment. We discuss possible origins of this discrepancy which arises also in the constituent quark model calculations., Comment: 11 pages, 11 figures, accepted for publication in EPJA

Pion scattering and electro-production amplitudes have been computed in a coupled-channel framework incorporating quasi-bound quark-model states, based on the Cloudy Bag model. All relevant low-lying nucleon resonances in the P33, P11, and S11 partial waves have been covered, including the Delta(1232), the N*(1440), N*(1535), and N*(1650). Consistent results have been obtained for elastic and inelastic scattering (two-pion, eta-N, and K-Lambda channels), as well as for electro-production. The meson cloud has been shown to play a major role, in particular in electro-magnetic observables in the P33 and P11 channels., Comment: 5 pages, 5 figures, contributed talk at the Hadron 2011 conference

We calculate the meson scattering and electro-production amplitudes in the S11 partial wave in a coupled-channel approach that incorporates quasi-bound quark-model states. Using the quark wave functions and the quark-meson interaction from the Cloudy Bag Model, we obtain a good overall agreement with the available experimental results for the partial widths of the N(1535) and the N(1650) resonances as well as for the pion, eta and kaon electro-production amplitudes. Our model is consistent with the N(1535) resonance being dominantly a genuine three-quark state rather than a quasi-bound state of mesons and baryons., Comment: 10 pages, 13 figures

We derive a method to calculate the multi-channel K matrix applicable to a broad class of models in which mesons linearly couple to the quark core. The method is used to calculate pion scattering amplitudes in the energy region of low-lying P11 and P33 resonances. A good agreement with experimental data is achieved if in addition to the elastic channel we include the $\pi\Delta$ and $\sigma N$ ($\sigma\Delta$) channels where the $\sigma$-meson models the correlated two-pion decay. We solve the integral equation for the K matrix in the approximation of separable kernels; it yields a sizable increase of the widths of the $\Delta(1232)$ and the N(1440) resonances compared to the bare quark values., Comment: 17 pages, 7 figures

Chiral solitons with baryon number one are investigated in the Spectral Quark Model. In this model the quark propagator is a superposition of complex-mass propagators weighted with a suitable spectral function. This technique is a method of regularizing the effective quark theory in a way preserving many desired features crucial in analysis of solitons. We review the model in the vacuum sector, stressing the feature of the absence of poles in the quark propagator. We also investigate in detail the analytic structure of meson two-point functions. We provide an appropriate prescription for constructing valence states in the spectral approach. The valence state in the baryonic soliton is identified with a saddle point of the Dirac eigenvalue treated as a function of the spectral mass. Due to this feature the valence quarks never become unbound nor dive into the negative spectrum, hence providing stable solitons as absolute minima of the action. This is a manifestation of the absence of poles in the quark propagator. Self-consistent mean field hedgehog solutions are found numerically and some of their properties are determined and compared to previous chiral soliton models. Our analysis constitutes an involved example of a treatment of a relativistic complex-mass system., Comment: 30 pages, 11 figures, version including discussion of meson correlators

We study the approximation of chiral quark models with simpler models, obtained via gradient expansion. The resulting Lagrangian of the type of the linear sigma-model contains, at the lowest level, an additional term with two derivatives. We investigate the dynamical consequences of this term and its relevance to the phenomenology of the soliton models of the nucleon. It is found that the inclusion of the new term allows for a more efficient approximation of the underlying quark theory, especially in those cases where dynamics allows for a large deviation of the chiral fields from the chiral circle, such as in quark models with non-local regulators., Comment: minor misprints corrected, to appear in Nucl. Phys. A

Properties of hedgehog solitons in a chiral quark model with nonlocal regulators are described. We discuss the formation of the hedgehog soliton, the quantization of the baryon number, the energetic stability, the gauging and construction of Noether currents with help of path-ordered P-exponents, and the evaluation of observables. The issue of nonlocality is thoroughly discussed, with a focus on contributions to observables related to the Noether currents. It is shown that with typical model parameters the solitons are not far from the weak nonlocality limit. The methods developed are applicable to solitons in models with separable nonlocal four-fermion interactions., Comment: 42 pages, 8 figures, a new section presenting the results with the chiral-circle constraint

We describe how the non-local regularization can be implemented in the calculation of solitons in the Nambu Jona-Lasinio model as well as in the equivalent linear sigma-model. We investigate different forms of regulators and show that the 3-momentum cut-off leads to serious conceptual difficulties., Comment: 11 pages, 6 Postscript figures, plain LaTeX, talk presented at the Mini-Workshop on ``Hadrons as Solitons'', Bled, Slovenia, 6-17 July 1999

A chiral quark model is described which is regularized in terms of Lorentz invariant non-local interactions. The model is regularized to all loop orders and it ensures the proper quantization of the baryon number. It sustains bound hedgehog solitons which, after suitable centre of mass corrections, can adequately describe the nucleon., Comment: 10 pages, 3 Postscript figures, uses aipproc.cls, talk presented by G. Ripka at the International Workshop on Hadron Physics ``Effective Theories of Low Energy QCD'', Coimbra, Portugal, September 10-15, 1999

Hedgehog solitons are found in a chiral quark model with non-local interactions. The solitons are stable without the chiral-circle constraint for the meson fields, as was assumed in previous Nambu-Jona--Lasinio model with local interactions., Comment: 8 pages