Search

In this paper, we adopt an extended relativized quark model to investigate the triply-heavy tetrquarks systematically. The mass spectra are obtained by solving the four-body relativized Hamiltonian including the Coulomb potential, confining potential, spin-spin interactions, and relativistic corrections. We find that all of the triply-heavy tetraquarks lie above the corresponding meson-meson thresholds, and thus no stable one exists. In particular, besides the spin-spin interactions, the Coulomb and confining potentials also contribute to the mass splittings in the $cb\bar c \bar q$ and $cb\bar b \bar q$ systems. Moreover, the whole mass spectra for triply heavy tetraquarks show quite similar patterns, which preserve the light flavor SU(3) symmetry and heavy quark symmetry well. Through the fall-apart mechanism, the triply-heavy tetraquarks may easily decay into the heavy quarkonium plus heavy-light mesons, which are good candidates for investigation in future experiments., Comment: 15 pages, 4 figures. Minor modifications, accepted version

We investigate the hidden-charm pentaquarks as a Λc͞D⁽*⁾ – Σc⁽*⁾͞D⁽*⁾ hadronic molecule coupling to a compact five-quark state. The coupling to the compact state leads to an hadron short-range interaction generating an attraction. In addition, the one pion exchange potential (OPEP) as a long-range interaction is introduced by the Lagrangians satisfying the chiral and heavy quark spin symmetries. The OPEP has been known as a driving force to bind atomic nuclei, where the tensor term leading the coupled-channel effect generates a strong attraction. The mass degeneracy of heavy hadrons due to the heavy quark spin symmetry enhances the OPEP derived by the πD⁽*⁾͞D⁽*⁾ and πΣc⁽*⁾Σc⁽*⁾ couplings. Introducing those interactions, we consistently explain the masses and widths of the Pc states reported by LHCb in 2019. The short range interaction has a dominant role to determine the energy-level structures, while the OPEP tensor term does the decay width., Facultad de Ciencias Exactas

The last decade has seen a marked shift in how the internal structure of hadrons is understood. Modern experimental facilities, new theoretical techniques for the continuum bound-state problem and progress with lattice-regularised QCD have provided strong indications that soft quark+quark (diquark) correlations play a crucial role in hadron physics. For example, theory indicates that the appearance of such correlations is a necessary consequence of dynamical chiral symmetry breaking, viz. a corollary of emergent hadronic mass that is responsible for almost all visible mass in the universe; experiment has uncovered signals for such correlations in the flavour-separation of the proton's electromagnetic form factors; and phenomenology suggests that diquark correlations might be critical to the formation of exotic tetra- and penta-quark hadrons. A broad spectrum of such information is evaluated herein, with a view to consolidating the facts and therefrom moving toward a coherent, unified picture of hadron structure and the role that diquark correlations might play., 113 pages, 41 figures, 8 tables

Using a relativized diquark model Hamiltonian, we calculate the masses of $J^{PC}=0^{++}$ ground-state tetraquarks in the following systems: $b s \bar b \bar s$, $bb \bar n \bar n$ ($n=u, d$), $bb \bar s \bar s$, $cc\bar c \bar c$, $b b \bar b \bar b$, $b c\bar b \bar c$ and $b b \bar c \bar c$. We also compute extensive spectra for the fully-heavy quark flavour combinations. Finally, as a test of the diquark model approach, we compute the masses of fully-heavy baryons in the diquark model. Our results may be compared soon to the forthcoming experimental data for fully-heavy three-quark systems., Comment: Accepted on Eur. Phys. J. C

We investigate the hidden-charm pentaquarks as superpositions of $\Lambda_c \bar{D}^{(*)}$ and $\Sigma_c^{(*)} \bar{D}^{(*)}$ (isospin $I = 1/2$) meson-baryon channels coupled to a $uudc\bar{c}$ compact core by employing an interaction satisfying the heavy quark and chiral symmetries. Our model can consistently explain the masses and decay widths of $P_c^+(4312)$, $P_c^+(4440)$ and $P_c^+(4457)$ with the dominant components of $\Sigma_c \bar D$ and $\Sigma_c \bar D^\ast$ with spin parity assignments $J^P = 1/2^{-}, 3/2^{-}$ and $1/2^{-}$, respectively. We analyze basic properties of the $P_c$'s such as masses and decay widths, and find that the mass ordering is dominantly determined by the quark dynamics while the decay widths by the tensor force of the one-pion exchange., Comment: 6 pages, 2 figures

We investigate the hidden-charm and hidden-bottom pentaquarks as meson-baryon molecules coupled to the five-quark states. As a result of our calculation it emerges that in the charm sector, one needs to add the five-quark potential to the pion exchange potential in order to produce bound and resonant states, whereas, in the bottom sector, the pion exchange interaction alone is strong enough to produce states. Thus, from this investigation, it comes out that the hidden-bottom pentaquarks are more likely to be produced than their hidden-charm counterparts, and, for this reason, we suggest that the experimentalists should look for the hidden-bottom pentaquarks.

The observation of five $$\varOmega _c $$Ωc= ssc states by LHCb [Aaij et al. Phys. Rev. Lett. 118, 182001 (2017)] and the confirmation of four of them by Belle [Yelton et al. Phys. Rev. D 97, 051102 (2018)], may represent an important milestone in our understanding of the quark organization inside hadrons. By providing results for the spectrum of $$\varOmega _{ c}$$Ωc baryons and predictions for their $$\varXi _{ c}^{+}K^{-}$$Ξc+K- and $$\varXi _{ c}'^{+}K^{-}$$Ξc′+K- decay amplitudes within an harmonic oscillator based model, we suggest a possible solution to the $$\varOmega _{c}$$Ωc quantum number puzzle and we extend our mass and decay width predictions to the $$\varOmega _b$$Ωb states. Finally, we discuss why the set of $$\varOmega _{ c(b)}$$Ωc(b) baryons is the most suitable environment to test the validity of three-quark and quark–diquark effective degrees of freedom.

We compute masses of positive parity spin-$1/2$ and $3/2$ baryons composed of $u$, $d$, $s$, $c$ and $b$ quarks in a quark-diaquark picture. The mathematical foundation for this analysis is implemented through a symmetry-preserving Schwinger-Dyson equations treatment of a vector-vector contact interaction, which preserves key features of quantum chromodynamics, such as confinement, chiral symmetry breaking and low energy Goldberger-Treiman relations. This study requires a computation of diquark correlations containing these quarks which in turn are readily inferred from solving the Bethe-Salpeter equations of the corresponding mesons. Therefore, it serves as a unified formalism for a multitude of mesons and baryons. It builds on our previous works on the study of masses, decay constants and form factors of quarkonia and light mesons, employing the same model. We use two sets of parameters, one which remains exactly the same for both the light and heavy sector hadrons, and another where the coupling strength is allowed to evolve according to the available mass scales of quarks. Our results are in very good agreement with the existing experimental data as well as predictions of other theoretical approaches whenever comparison is possible., 15 pages, 8 figures

We describe the electroproduction ratios of baryon–meson states from nucleon, inferring from the sea quarks in the nucleon using an extension of the quark model that takes into account the sea. As a result we provide, with no adjustable parameters, the predictions of ratios of exclusive meson–baryon final states: ΛK+, Σ⁎K, ΣK, pπ0, and nπ+. These predictions are in agreement with the new JLab experimental data showing that sea quarks play an important role in the electroproduction. We also predicted further ratios of exclusive reactions that can be measured and tested in future experiments. In particular, we suggested new experiments on deuterium and tritium. Such measurements can provide crucial tests of different predictions concerning the structure of nucleon and its sea quarks helping to solve an outstanding problem. Finally, we compute the so called strangeness suppression factor, λs, that is the suppression of strange quark–antiquark pairs compared to nonstrange pairs, and we found that our finding with this simple extension of the quark model is in good agreement with the results of JLab and CERN experiments.

We describe the relativistic interacting quark-diquark model formalism and its application to the calculation of strange and nonstrange baryon spectra. The results are compared to the existing experimental data. We also discuss the application of the model to the calculation of other baryon observables, like baryon magnetic moments, open-flavor strong decays and baryon masses with self-energy corrections.

The hidden-charm and hidden-bottom pentaquarks are investigated as meson-baryon molecules coupled to the five-quark states. As a result of our calculation, we find that, in the charm sector, one needs the five-quark potential in addition to the pion exchange potential in order to produce bound and resonant states, whereas, in the bottom sector, the pion exchange interaction is strong enough to produce states. Thus, from this investigation, it emerges that the hidden-bottom pentaquarks are more likely to form than their hidden-charm counterparts; for this reason, we suggest that the experimentalists should look for pentaquark states in the bottom sector.

We study baryo-charmonium [ηc- and J/ψ-N⁎, ηc(2S)-, ψ(2S)- and χc(1P)-N] and baryo-bottomonium [ηb(2S)-, ϒ(2S)- and χb(1P)-N] bound states, where N is the nucleon and N⁎ a nucleon resonance. In the baryo-quarkonium model, the five qqqQQ¯ quarks are arranged in terms of a heavy quarkonium core, QQ¯, embedded in light baryonic matter, qqq, with q=u or d. The interaction between the QQ¯ core and the light baryon can be written in terms of the QCD multipole expansion. The spectrum of baryo-charmonium states is calculated and the results compared with the existing experimental data. In particular, we can interpret the recently discovered Pc(4380) and Pc(4450) pentaquarks as ψ(2S)-N and χc2(1P)-N bound states, respectively. We observe that in the baryo-bottomonium sector the binding energies are, on average, slightly larger than those of baryo-charmonia. Because of this, the hidden-bottom pentaquarks are more likely to form than their hidden-charm counterparts. We thus suggest the experimentalists to look for five-quark states in the hidden-bottom sector in the 10.4–10.9 GeV energy region.

In this paper, we investigate the hidden-charm pentaquarks as $\bar{D}^{(\ast)}\Lambda_{\rm c}$ and $\bar{D}^{(\ast)}\Sigma^{(\ast)}_{\rm c}$ molecules coupled to the five-quark states. Furthermore, we extend our calculations to the hidden-bottom sector. The coupling to the five-quark states is treated as the short range potential, where the relative strength for the meson-baryon channels is determined by the structure of the five-quark states. We found that resonant and/or bound states appear in both the charm and bottom sectors. The five-quark state potential turned out to be attractive and, for this reason, it plays an important role to produce these states. In the charm sector, we need the five-quark potential in addition to the pion exchange potential in producing bound and resonant states, whereas, in the bottom sector, the pion exchange interaction is strong enough to produce states. Thus, from this investigation, it emerges that the hidden-bottom pentaquarks are more likely to form than their hidden-charm counterparts; for this reason, we suggest that the experimentalists should look for states in the bottom sector., Comment: 58 pages, 13 figures, and 10 tables, published version

We provide results for the open-flavor strong decays of strange and nonstrange baryons into a baryon-vector/pseudoscalar meson pair. The decay amplitudes are computed in the ${^{3}P}_{0}$ pair-creation model, where $s\overline{s}$ pair-creation suppression is included for the first time in the baryon sector, in combination with the $U(7)$ and hypercentral models. The effects of this $s\overline{s}$ suppression mechanism cannot be reabsorbed in a redefinition of the model parameters or in a different choice of the ${^{3}P}_{0}$ model vertex factor. Our results for the decay amplitudes are compared with the existing experimental data and previous ${^{3}P}_{0}$ and elementary meson emission model calculations. In this respect, we show that distinct quark models differ in the number of missing resonances they predict and also in the quantum numbers of states. Therefore, future experimental results will be important in order to disentangle different models of baryon structure. Finally, in the appendixes, we provide some details of our calculations, including the derivation of all relevant flavor couplings with strangeness suppression. This derivation may be helpful to calculate the open-flavor decay amplitudes starting from other models of baryons.

We present the last applications of the unquenched quark model (UQM) to the description of flavour asymmetry and strangeness in the proton when baryon-meson components are included. In the meson sector, the UQM is used to calculate the charmonium and bottomonium spectra with self-energy corrections due to the coupling to the meson-meson continuum., arXiv admin note: substantial text overlap with arXiv:1506.05015

The relativistic interacting quark-diquark model of baryons, recently developed, is here extended to introduce a spin-isospin transition interaction into the mass operator. The refined version of the model is used to calculate the non strange baryon spectrum. The results are compared to the present experimental data.

The relativistic interacting quark-diquark model, constructed in the framework of point form dynamics, is extended to strange baryons. The strange and nonstrange baryon spectra are calculated and compared with the experimental data., Comment: arXiv admin note: text overlap with arXiv:1410.0590

We briefly discuss the relativistic interacting quark-diquark model formalism and its application to the calculation of strange and nonstrange baryon spectra. The results are compared to the existing experimental data., Talk presented at the NSTAR 2015 Conference, May 25-28 2015, Osaka (Japan)

The ratio between the anomalous magnetic moments of proton and neutron has recently been suggested to be connected to the ratio of proton momentum fractions carried by valence quarks. This relation has been obtained within a parametrization of the Generalized Parton Distributions (GPD) \cite{gpv}, but it is completely independent of such a parametrization.\\ It will be shown that using different CQMs this relation holds within a few percent accuracy. This agreement is based on what all the CQMs have in common: the effective degrees of freedom of the three constituent quarks and the underlying SU(6) symmetry.\\ On the other hand, the experimental value of the ratio is not reproduced by CQMs. This means that the SU(6)-breaking mechanism contained in the phenomenological partonic distributions does not correspond to the SU(6) breaking mechanism implemented in the CQMs we have analyzed \cite{noi03}.\\ We will also show how this relation can be used in order to understand in which way to implement an $SU(6)$-breaking mechanism and to test models., Comment: Workshop Symmetries in Nuclear Structure, Erice March 23-30, 2003, in honor of the 60th birthday of Franco Iachello Published in Symmetries in nuclear structure, Science and culture Series, Physics 24, 378-383, (A. Vitturi, R. F. Casten eds.) World Scientific, Singapore 2004, 378-383

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., Comment: 12 pages, 3 figures

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., Comment: 10 pages, 8 figures, Prepared for ERICE International School of Nuclear Physics, 24th Course, Erice-Sicily: 16-24 September 2002

The formalism for a new generation of unquenched quark models for baryons in which the effects of quark–antiquark pairs are taken into account in an explicit form via a microscopic, QCD-inspired, quark–antiquark creation mechanism, is presented. As an application the quark–antiquark contribution to the spin of the proton and the Σ+ is discussed. Finally the strangeness radius of the Σ+ in the closure limit is calculated.

We discuss the nature of the ${\ensuremath{\chi}}_{b}(3P)$ and $X(3872)$ mesons. Are the ${\ensuremath{\chi}}_{b}(3P)$'s standard $b\overline{b}$ mesons or $b\overline{b}$ states with a significative continuum component? Is the $X(3872)$ a $c\overline{c}$ state with continuum coupling effects or a meson-meson molecule? To do that, we compare quark model and unquenched quark model results for the mass barycenter and splittings of the ${\ensuremath{\chi}}_{b}(3P)$ multiplet. Future and more precise experimental results will discriminate between the two interpretations. In the case of the $X(3872)$, we interpret it as a $c\overline{c}$ core plus higher Fock components due to the coupling to the meson-meson continuum, and thus we think that it is compatible with the meson ${\ensuremath{\chi}}_{c1}(2P)$, with ${J}^{PC}={1}^{++}$. The ${J}^{PC}={1}^{++}$ quantum numbers are in agreement with the experimental results found by the LHCb collaboration. In our view, the $X(3872)$'s mass is lower than the quark model's predictions because of self-energy shifts. We also provide an estimation of the open charm/bottom strong decay modes of the $X(3872)$ and ${\ensuremath{\chi}}_{b}(3P)$ mesons, such as $X(3872)\ensuremath{\rightarrow}D{\overline{D}}^{*}$ and ${\ensuremath{\chi}}_{b2}(3P)\ensuremath{\rightarrow}B\overline{B}$, and radiative transitions.

In this contribution, we discuss the spin and flavor content of the proton in the framework of the unquenched quark model, and address the role of valence and sea quarks in the nucleon., Comment: 4 pages, 2 tables, to appear in Journal of Physics: Conference Series

The formalism for a new generation of unquenched quark models for baryons is presented. The effects of quark-antiquark pairs are taken into account in an explicit form via a microscopic, QCD-inspired, quark-antiquark creation mechanism. As an application, the quark-antiquark contribution to the spin of the proton is discussed.

We present an exactly solvable model of baryon spectroscopy which has O (7) dynamic symmetry and is based on a hyperCoulomb three-body quark potential. The corresponding form factors, which are calculated analytically, show a power-law behavior as a function of momentum transfer Q 2 . Because of the O (7) symmetry there is a degeneracy between the Roper resonance and the negative parity excited states, in qualitative agreement with the observed spectrum. A linear confinement term is also added. The SU (6) symmetry is broken by a spin-spin interaction and by a small tensor term.

We have performed a calculation of the helicity amplitudes for the electromagnetic excitation of nucleon resonances based on a three-body force model for baryon structure. All parameters have been previously fixed studying the baryon spectrum and therefore this is a parameter free calculation. We have taken into account the hyperfine mixing in all the baryon states and we present our results in comparison with those given by other approaches proposed in the literature. A brief discussion on open problems is also given.

Studies of the structure of excited baryons are key to the N* program at Jefferson Lab. Within the first year of data taking with the Hall B CLAS12 detector following the 12 GeV upgrade, a dedicated experiment will aim to extract the N* electrocouplings at high photon virtualities Q2. This experiment will allow exploration of the structure of N* resonances at the highest photon virtualities ever yet achieved, with a kinematic reach up to Q2 = 12 GeV2. This high-Q2 reach will make it possible to probe the excited nucleon structures at distance scales ranging from where effective degrees of freedom, such as constituent quarks, are dominant through the transition to where nearly massless bare-quark degrees of freedom are relevant. In this document, we present a detailed description of the physics that can be addressed through N* structure studies in exclusive meson electroproduction. The discussion includes recent advances in reaction theory for extracting N* electrocouplings from meson electroproduction off protons, along with QCD-based approaches to the theoretical interpretation of these fundamental quantities. This program will afford access to the dynamics of the non-perturbative strong interaction responsible for resonance formation, and will be crucial in understanding the nature of confinement and dynamical chiral symmetry breaking in baryons, and how excited nucleons emerge from QCD., 91 pages, 52 figures

We present a theoretical approach to the internal baryon structure based on three-body forces among quarks. Using three-body potentials, one can both include the current two-body potential models (in an effective way) and investigate new dynamical mechanisms. The spin-independent part of the interaction is assumed to be hypercentral and treated using the hyperspherical harmonic formalism, leading to a level scheme reproducing qualitatively the observed pattern of the spectrum. We discuss some results obtained using a potential with a (mostly linear) confinement part and some inverse power terms; however the potential can be easily generalized in order to allow a systematic analysis. Adding the hyperfine interaction, which is diagonalized directly in the basis states, the spectrum can be fairly reproduced. The compatibility of the spectrum and the change form factor is briefly discussed.

A simple hybrid quark-diquark model for the baryons is constructed as a partial solution to the well-known ``missing resonances'' problem. In this model, the quark-diquark approach is merged with the usual constituent three-quark model. The underlying idea is that the quark-diquark approach describes the excited states while the three-quark model the ground states. The spectrum is calculated through a mass formula, a generalization of the G\"ursey-Radicati formula, built to reproduce the rotational and vibrational Regge trajectories. Using the quark-diquark scheme, we were also able to describe the spin-flavor degrees of freedom in the framework of an algebraic model. Moreover, we have constructed a complete classifications of the possible quark-diquark states only based on group theory and thus it can be useful both for other model builders and for experimentalists.

The diquark is a system of two strongly correlated quarks. Obviously no free diquarks can exist, since they carry color. However they can theoretically form inside hadrons. While the actual possibility that two quarks can form stable bounds inside hadrons is still discussed, the use of a diquark effective degree of freedom can be very useful because it reduces the total number of states predicted and simplify the interactions inside hadrons. Here we present a possible application of the diquark to the tetraquark meson in the diquark-antidiquark limit. In this limit the tetraquark meson is composed of a diquark and an antidiquark. The complete set of states are constructed and a simple algebraic mass formula is applied in order to calculate the spectrum.

We briefly describe our relativistic quark-diquark model, developed within the framework of point form dynamics, which is the relativistic extension of the interacting quark-diquark model. In order to do that we have to show the main properties and quantum numbers of the effective degree of freedom of constituent diquark. Our results for the nonstrange baryon spectrum and for the nucleon electromagnetic form factors are discussed., Talks presented at the conference "Beauty in Physics: Theory and Experiment : Meeting in honor of Francesco Iachello on the occasion of his 70th birthday", 14-18 May 2012. Cocoyoc, Mexico

We investigate the photoexcitation of nucleon resonances in nuclei taking into account the quark structure of the nucleon. We calculate the total strength of the {ital M}1, {ital E}1, and {ital E}2 excitation in a two-nucleon system, considered as two three-quark clusters with a properly antisymmetrized wave function. The calculations show that the resonance excitation in the deuteron is not modified with respect to the free nucleon case. In contrast, when the nucleon relative distance assumes typical nuclear values, quark exchange leads to a {similar_to}11% damping of the {ital D}{sub 13} resonance and {similar_to}23% of the {ital F}{sub 15}, but it leaves the {Delta} excitation practically unaffected, in agreement with the recent photoabsorption measurements.

A relativistic interacting quark-diquark model of baryons with direct and exchange interaction has been used to calculate the nucleon electromagnetic form factors. The results have been compared with experimental data.

A relativistic quark-diquark mass operator with direct and exchange interaction has been constructed in the framework of point form dynamics. The nonstrange baryon spectrum has been calculated and compared with experimental data.

We present an unquenched quark model for baryons in which the effects of quark‐antiquark pair creation (uū, dd and ss) are taken into account in an explicit form via a microscopic, QCD‐inspired, quark‐antiquark creation mechanism. As an application we discuss the flavor content of octet baryons.

We study the structure of the spin of the proton and the Λ hyperon in an unquenched quark model which incorporates the effects of quark‐antiquark pair creation via a microscopic, QCD‐inspired, quark‐antiquark creation mechanism. It is shown that the inclusion of the qq pairs leads to a sizeable contribution of the orbital angular momentum to the spin of the proton and the Λ hyperon.

We present an unquenched quark model for baryons in which the effects of quark‐antiquark pair creation (uū, dd and ss) are taken into account in an explicit form via a microscopic, QCD‐inspired, quark‐antiquark creation mechanism. It is shown that, while the inclusion of the qq pairs does not affect the baryon magnetic moments, it leads to a sizeable contribution of the orbital angular momentum to the spin of the proton.

The axial form factor of the nucleon is studied in a two-component model consisting of a three-quark intrinsic structure surrounded by a meson cloud. The experimental data in the space-like region are well reproduced with a minimal number of parameters. The results are similar to those obtained from a dipole fit up to 1 GeV$^2$, but they surprisingly show a zero between 2 and 3 GeV$^2$. Such behavior, predicted by the present model completely disagrees with the dipole parametrization. The theoretical expression for the form factor is then analytically extrapolated to the time-like region, giving the first predictions in this regime. This kinematical region is especially interesting for present and future colliders., 12 pages 2 figures Final version to appear in Phys. Rev. C

We present the formalism for a new generation of unquenched quark models for baryons in which the effects of quark-antiquark pairs (uū, d \( \bar d \) and s \( \bar s \)) are taken into account in an explicit form via a microscopic, QCD-inspired, quark-antiquark creation mechanism. The present approach is an extension of the flux-tube breaking model of Geiger and Isgur in which now the contributions of quark-antiquark pairs can be studied for any inital baryon and for any flavor of the q \( \bar q \) pair. It is shown that the inclusion of q \( \bar q \) pairs leads to a large contribution of orbital angular momentum to the proton spin.

We present the formalism for a new generation of unquenched quark models for baryons in which the effects of quark‐antiquark pairs are taken into account in an explicit form via a microscopic, QCD‐inspired, quark‐antiquark creation mechanism. The present approach is an extension of the fiux‐tube breaking model of Geiger and Isgur in which now the contribution of quark‐antiquark pairs can be studied for any inital baryon, for any fiavor of the qq pair (not only ss but also uū and dd) and for arbitrary hadron wave functions. The method is illustrated with an application to the spin of the proton and the flavor asymmetry of the nucleon sea.

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., 5 pages, 7 figures, Invited talk at the ICTP 4th International Conference on Perspectives in Hadronic Physics, Trieste, Italy, 12-16 May 2003. Accepted by Eur. Phys. J. A

A simple quark-diquark model of baryons with direct and exchange interactions is constructed. Spectrum and form factors are calculated and compared with experimental data. Advantages and disadvantages of the model are discussed., 13 pages, 3 eps-figures, accepted by Phys.Rev. C Rapid Communications

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., 6 pages, 2 figures, invited talk at Nuclear Physics, Large and Small, April 19-22, 2004, Hacienda Cocoyoc, Mexico, to be published in AIP Conference Proceedings. Reference added

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., 26 pages, 4 figures, 11 tables, revised version with 2 extra tables, an updated list of references and expanded discussion of the results. Accepted for publication in Eur. Phys. J. A

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., Comment: 7 pages,3 figures, Talk given at NStar 2002 workshop on the physics of excited nucleons, Pittsburgh, Pennsylvania, October 9-12, 2002

We have calculated the ratio between the charge and magnetic form factors of the proton, making use of the hypercentral constituent quark model with relativistic corrections. The parameters of the model have been fixed in order to reproduce the baryon spectrum. The predicted values of this ratio are compared with the recent experimental results.

The low-lying states of baryons are thought to be mainly composed of three-quarks configurations. Although for light (u,d) quarks a construction of the quark-quark interaction is very difficult, it has been suggested that a good fraction of this interaction is of three-body character [1], and that, in addition, it can be well approximated by a hypercentral potential, i.e. an interaction that is invariant under rotations in the six dimensional space spanned by the intrinsic coordinates of the three-body problem. This suggestion arose from a study of the flux tube configurations of Fig. 1 for heavy-quark baryons. However, in general, the fact that QCD is a non-abelian gauge theory leads to gluon-gluon couplings which, in turn, produce three-body forces, as shown in Fig. 2. One is thus led to the conclusion that three-body forces can be a very important part of the quark interactions and this is particularly so for baryons composed of light quarks where the coupling constant αs is large.