Search

Our recently proposed model of the $\Delta(1600)$ resonance, in which the dominant component is a quasi-bound state of the $\Delta(1232)$ and the pion, is confronted with a similar model of the $N^*(1440)$ resonance as its counterpart in the P11 partial wave. We stress an essentially different mechanism responsible for generating the two resonances., Comment: Contributed talk at the Mini-Workshop "Electroweak Processes of Hadrons", Bled, Slovenia, July 15-19, 2019, 4 pages, 3 figures

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

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., 5 pages, 5 figures, contributed talk at the Hadron 2011 conference

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., 30 pages, 11 figures, version including discussion of meson correlators

Different approaches to the Roper resonance are reviewed, revealing the multiple facet of this state and the need for some crucial experiments., Dajemo pregled nekoliko mogućih tumačenja Roperove rezonancije, te ukazujemo na složenost tog stanja, kao i na potrebu za novim mjerenjima.

Recent RHIC results on eta' multiplicity in heavy-ion collisions are of great importance because they clearly signal a partial restoration of U_A(1) symmetry at high temperatures, and thus provide an unambiguous signature of the formation of a new state of matter. To explain these experimental results of STAR and PHENIX collaborations, a minimal generalization of the Witten-Veneziano relation to finite temperatures was proposed.The present paper provides a detailed, pedagogical discussion and explanation thereof. After explaining why these results show that the zero-temperature Witten-Veneziano relation cannot be straightforwardly extended to temperatures T too close to the chiral restoration temperature T_Ch and beyond, we find the quantity which should replace, at T>0, the Yang-Mills topological susceptibility appearing in the T=0 Witten-Veneziano relation, in order to avoid the conflict with experiment at T>0. This is illustrated through concrete T-dependences of pseudoscalar meson masses in a chirally well-behaved, Dyson-Schwinger approach, but our results and conclusions are of a more general nature and, essentially, model-independent.

The virtual bremsstrahlung process, in addition to the real photon bremsstrahlung, provides an elaborate tool for studying the nucleon-nucleon interaction. The additional degree of freedom which exists in the virtual bremsstrahlung production, a longitudinal polarization of the virtual photon, allows transversal, longitudinal, and interference terms to be studied. The squared transition amplitude for p + p → p + p + e+ + e− can be written [1, 2]: $$\begin{array}{*{20}{c}} {|A{{|}^{2}} \propto \frac{1}{{M_{\gamma }^{2}}}\left\{ {{{W}_{T}}\left( {1 - \frac{{2{{l}^{2}}}}{{M_{\gamma }^{2}}}{{{\sin }}^{2}}{{\theta }_{l}}} \right)} \right. + {{W}_{L}}\left( {1 - \frac{{4{{l}^{2}}}}{{k_{0}^{2}}}{{{\cos }}^{2}}{{\theta }_{l}})} \right)} \\ { + \frac{{2{{l}^{2}}{{{\sin }}^{2}}{{\theta }_{l}}}}{{M_{\gamma }^{2}}}({{W}_{{TT}}}\cos 2{{\phi }_{l}} + W_{{TT}}^{\prime }\sin 2{{\phi }_{l}}} \\ {\left. { + \frac{{2{{l}^{2}}\sin 2{{\theta }_{l}}}}{{{{k}_{0}}{{M}_{\gamma }}}}({{W}_{{LT}}}\cos {{\phi }_{l}} + W_{{LT}}^{\prime }\sin {{\phi }_{l}})} \right\},} \\ \end{array}$$ (1) where 1 is a relative momentum of dilepton pair and θ ι , φ ι are polar and azimuthal angle of 1 relative to a reaction plane. By measuring the momenta of the outgoing dilepton pair, all kinematic factors in Eq. (1) are determined and in principle the response functions (transversal, longitudinal, and interference terms) W i , i = T, L, TT, TT′, LT, LT′ can be extracted.

It is pedagogically how a very successful description of the eta-eta' mass matrix can be achieved in the consistently coupled Schwinger-Dyson and Bethe-Salpeter approach in spite of the limitations of the ladder approximation. This description is in agreement with both phenomenology and lattice results.

The gamma -> 3 pi form factor was calculated in a simple--minded constituent model with a constant quark mass parameter, as well as in the Schwinger-Dyson approach. The comparison of these and various other theoretical results on this anomalous process, as well as the scarce already available data (hopefully to be supplemented by more accurate CEBAF data), seem to favor Schwinger--Dyson modeling which would yield relatively small low--momentum values of the constituent (dynamically dressed) quark mass function.