arXiv subjects
Publications and source records attributed to M. M. Giannini.
The results for the elastic nucleon form factors and the electromagnetic transition amplitudes to the Delta(1232) resonance, obtained with the Hypercentral Constituent Quark Model with the inclusion of a meson cloud correction are briefly presented. The pion cloud effects are explicitly discussed.
We briefly report on results about the electromagnetic form factors of the nucleon obtained with different models and then we concentrate our attention on recent results obtained with the hypercentral constituent quark model (hCQM).
We discuss the properties of the pentaquark in a collective stringlike model with a nonplanar configuration of the four quarks and the antiquark. In an application to the mass spectrum of exotic Theta baryons, we find that the ground state pentaquark has angular momentum and parity J(p)=1/2(-) and a small magnetic moment of 0.382 nuclear magnetons. The decay width is suppressed by the spatial overlap with the decay products.
We propose a collective stringlike model of pentaquarks with the geometry of an equilateral tetrahedron in which the four quarks are located at the four corners and the antiquark in its center. The nonplanar equilibrium configuration is a consequence of the permutation symmetry of the four quarks. In an application to the spectrum of exotic Theta baryons, we find that the ground state pentaquark has angular momentum and parity J(p)=1/2(-) and a small magnetic moment of 0.382 nuclear magnetons. The decay width is suppressed by the spatial overlap with the decay products.
We construct a complete classification of pentaquark states in terms of the spin-flavour SU(6) representations. We find that only some definite SU(3) representations are allowed, singlets, octets, decuplets, anti-decuplets, 27-plets and 35-plets. The latter three contain exotic states, which cannot be constructed from three quarks only. This complete classification scheme is general and model independent and is useful both for model builders and experimentalists. The mass spectrum is obtained from a Gursey-Radicati type mass formula, whose coefficients have been determined previously by a study of qqq baryons. The ground state pentaquark which is identified with the recently observed Theta(1540) state, is predicted to be an isosinglet anti-decuplet state. Its parity depends on the interplay between the spin-flavour and orbital contributions to the mass operator.
We analyze the magnetic moment of the exotic pentaquarks of the flavor antidecuplet in the constituent quark model for the case in which the ground state is in an orbital L(p)=0(+) or a L(p)=1(-) state. We derive sum rules for the magnetic moments. The magnetic moment of the Theta(1540) is found to be 0.38, 0.09 and 1.05 mu_N for J(p)=1/2(-), 1/2(+) and 3/2(+), respectively, which is compared with the results obtained in other approaches.
We study the generalized sum rules and polarizabilities of the nucleon in the framework of the hypercentral constituent quark model. We include in the calculation all the well known $3^*$ and $4^*$ resonances and consider all the generalized sum rules for which there are data available. To test the model dependence of the calculation, we compare our results to the results obtained in the harmonic oscillator CQM. We furthermore confront our results to the model-independent sum rules values and to the predictions of the phenomenological MAID model. The CQM calculations provide a good description of most of the presented generalized sum rules in the intermediate $Q^2$ region (above $\sim0.2$ GeV$^2$) while they encounter difficulties in describing these observables at low $Q^2$, where the effects of the pion cloud, not included in the present calculation, are expected to be important.
The sum rules serve a powerful tool to study the nucleon structure by providing a bridge between the statical properties of the nucleon (such as electrical charge, and magnetic moment) and the dynamical properties (e.g. the transition amplitudes to excited states) in a wide range of energy and momentum transfer $Q^2$. We study the generalized sum rules of the nucleon in the framework of the constituent quark model. We use two different CQM, the one with the hypercentral potential \cite{hccqm1}, \cite{hccqm2}, \cite{hccqm3}, and with the harmonic oscillator potential \cite{KI}, both with only few parameters fixed to the baryonic spectrum. We confront our results to the model independent sum rules and to the predictions of the phenomenological MAID \cite{MAID} model and find that in all the cases considered, in the intermediate $Q^2$ range (0.2-1.5 GeV$^2$), both CQM models provide a good description of the sum rules on the neutron.
We report on the recent results of the hypercentral Constituent Quark Model (hCQM). The model contains a spin independent three-quark interaction which is inspired by Lattice QCD calculations and reproduces the average energy values of the SU(6) multiplets. The splittings are obtained with a SU(6)-breaking interaction, which can include also an isospin dependent term. Concerning Constituent Quark models, we have shown for the first time that the decreasing of the ratio of the elastic form factors of the proton is due to relativistic effects using relativistic corrections to the e.m. current and boosts. Now the elastic nucleon form factors have been recalculated, using a relativistic version of the hCQM and a relativistic quark current showing a very detailed reproduction of all the four form factor existing data over the complete range of 0-4 $GeV^2$. Futhermore, the model has been used for predictions concerning the electromagnetic transverse and longitudinal transition form factors giving a good description of the medium $Q^2$ behaviour. We show that the discrepancies in the reproduction of the helicity amplitudes at low $Q^2$ are due to pion loops. We have calculated the helicity amplitudes for all the 3 and 4 star resonances opening the possibility of application to the evaluation of cross sections.
We discuss the spectroscopy of pentaquarks. The quantum numbers of the ground state depend on the interplay between spin-flavor and orbital contributions to the energy. The magnetic moments of the lowest pentaquark state with negative and positive parity are found to be 0.382 μ_N and 0.089 μ_n, respectively.
The unitary isobar model MAID has been extended and used for a partial wave analysis of pion photo- and electroproduction in the resonance region W < 2 GeV. Older data from the world data base and more recent experimental results from Mainz, Bates, Bonn and JLab for Q^2 up to 4.0 (GeV/c)^2 have been analyzed and the Q^2 dependence of the helicity amplitudes have been extracted for a series of four star resonances. We compare single-Q^2 analyses with a superglobal fit in a new parametrization of Maid2003 together with predictions of the hypercentral constituent quark model. As a result we find that the helicity amplitudes and transition form factors of constituent quark models should be compared with the analysis of bare resonances, where the pion cloud contributions have been subtracted.
We report on the recent results of the hypercentral Constituent Quark Model (hCQM). The model contains a spin independent three-quark interaction which is inspired by Lattice QCD calculations and reproduces the average energy values of the SU(6) multiplets. The splittings within each multiplet are obtained with a SU(6)-breaking interaction, which can include also an isospin dependent term. All the 3- and 4-stars resonances are well reproduced. Moreover, as all the Constituent Quark models, the hCQM predicts ``missing'' resonances ({\em e.g.} extra $S11$ and $P13$ states) which can be of some help for the experimental identification of new resonances. The model provides also a good description of the medium $Q^2$-behavior of the electromagnetic transition form factors. In particular the calculated helicity amplitude $A_{1/2}$ for the $S_{11}(1535)$ resonance agrees very well with the recent CLAS data. More recently, the elastic nucleon form factors have been calculated using a relativistic version of the hCQM and a relativistic quark current.
The hypercentral constituent quark model contains a spin independent three-quark interaction inspired by lattice QCD calculations which reproduces the average energy of SU(6) multiplets. The splittings are obtained with a residual generalized SU(6)-breaking interaction including an isospin dependent term. The long standing problem of the Roper is absent and all the 3- and 4-star states are well reproduced. The model has been used in a systematic way for transverse and longitudinal electromagnetic transition form factor of the 3- and 4- star and also for the missing resonances. The prediction of the electromagnetic helicity amplitudes agrees quite well with the data except for low $Q^2$, showing that it can supply a realistic set of quark wave functions. In particular we report the calculated helicity amplitude $A_{1/2}$ for the $S11(1535)$, which is in agreement with the TJNAF data.
We report on the recent results of the hypercentral Constituent Quark Model (hCQM). The model contains a spin independent three-quark interaction which is inspired by Lattice QCD calculations and reproduces the average energy values of the SU(6) multiplets. The splittings are obtained with a SU(6)-breaking interaction, which can include also an isospin dependent term. The model has been used for predictions concerning the electromagnetic transition form factors giving a good description of the medium $Q^2$-behaviour. In particular the calculated helicity amplitude $A_{1/2}$ for the $S_{11}(1535)$ resonance agrees very well with the recent CLAS data. Furthermore, we have shown for the first time that the decreasing of the ratio of the elastic form factors of the proton is due to relativistic effects. Finally, the elastic nucleon form factors have been calculated using a relativistic version of the hCQM and a relativistic quark current.
The formulation of the Gerasimov-Drell-Hearn sum rule is revisited, showing its connection with other sum rules occurring in electron scattering and discussing the problem of its saturation, both from the theoretical and experimental point of v iew. The generalisation to other nuclear targets is also reported.
We report on the recent results of the hypercentral constituent quark model. The model contains a spin independent three-quark interaction which is inspired by QCD lattice calculations and reproduces the average energy values of the SU(6) multiplets. The splittings are obtained with a residual generalized SU(6)-breaking interaction, including an isospin dependent term. The long standing problem of the Roper resonance is absent and all the 3- and 4-star states are well reproduced. The model has also been used for predictions concerning the electromagnetic transition form factors giving a good description of the medium $Q^2$ -behaviour. In particular the calculated $S_{11} A_{1/2}$ helicity amplitude agrees very well with the recent CLAS data. Finally the ratio of the elastic form factors of the proton, calculated including kinematic relativistic corrections, exhibits a substantial decreasing with $Q^2$ in agreement with the recent TJNAF experiment.
The ratio between the anomalous magnetic moments of proton and neutron has been recently parametrized by the ratio of proton momentum fractions $M_{2}^{q_{val}}$. This ratio is evaluated using different constituent quark models, starting from the CQM density distributions and calculating the next-to leading order distributions. We show that this momentum fractions $M_{2}^{q_{val}}$ ratio is a sensitive test for SU(6)-breaking effects and therefore it is useful to distinguish among different CQMs. We investigate also the possibility of getting constraints on the formulation of quark structure models.
The constituent quark model based on a hypercentral approach takes into account three-body force effects and standard two-body potential contributions. The quark potential contains a hypercentral interaction, to which a hyperfine term is added. While the hypercentral potential supplies good values for the centroid energies of the resonance multiplets and a realistic set of quark wave functions, the hyperfine splittings are sometimes not sufficient to account for the observed masses. In this work we have introduced an improved form of the hyperfine interaction and an isospin dependent quark potential. The resulting description of the baryon spectrum is very good, also for the Roper resonance, specially thanks to the flavour dependent interaction.