A. Vassallo

Heavy quarkonium hybrids from Coulomb gauge QCD

Using the nonrelativisitc reduction of Coulomb gauge QCD we compute a spectrum of the low mass hybrid mesons containing a heavy quark-antiquark pair. The gluon degrees of freedom are treated in the mean field approximation calibrated to the gluelump spectrum. We discuss the role of the non-Abelian nature of the QCD Coulomb interaction in the ordering of the spin-parity levels.

A new application of the Gürsey and Radicati mass formula

Abstract.We study the spin- and flavour-dependent SU(6) violations in the baryon spectrum by means of a Gürsey-Radicati mass formula. The average energy of each SU(6) multiplet is described using the SU(6)-invariant interaction given by a hypercentral potential containing a linear and a hyper-Coulomb term. We show that the nonstrange- and strange-baryon masses are, in general, fairly well reproduced and moreover that the Gürsey-Radicati formula holds in a satisfactory way also for the excited states up to 2 GeV.

Gluelump spectrum from Coulomb gauge QCD

We compute the energy spectrum of gluelumps defined as gluonic excitations bound to a localized, static octet source. We are able to reproduce the nontrivial ordering of the spin-parity levels and show how this is related to the non-Abelian part of the Coulomb interaction between color charges.

High Q{sup 2} behavior of the electromagnetic form factors in the relativistic hypercentral constituent quark model

The ratio R{sub p} between the electric and magnetic proton form factors has been recently measured at Jefferson Lab up to Q{sup 2}=8.5 GeV{sup 2}. We have extended the calculation of the nucleon form factors with the hypercentral constituent quark model and compared them with the data on R{sub p} and on the Q{sup 2} behavior of the ratio Q{sup 2}F{sub 2}/F{sub 1}. In both cases, the theoretical curves agree with the experimental points.

Relativistic quark-diquark model of baryons with a spin-isospin transition interaction: Non-strange baryon spectrum and nucleon magnetic moments

Abstract.The relativistic interacting quark-diquark model of baryons, recently developed, is here extended introducing in the mass operator a spin-isospin transition interaction. This refined version of the model is used to calculate the non-strange baryon spectrum. The results are compared to the present experimental data. A preliminary calculation of the magnetic moments of the proton and neutron is also presented.

Gluon chain formation in presence of static charges

The gluon chain model of Greensite and Thorn [1–4] identifies the chromoelectric flux tube that exists between static quark charges with a string of quasi particles, constituent gluons. Through lattice simulations and phenomenological analyses it is well established that the instantaneous, Coulomb potential between static charges is confining [5–8]. Even though it does not correspond to a physical observable, the static potential does provide physical insight into the possible origins of the confinement mechanism as illustrated by the GribovZwanzinger model [9, 10] and other, e.g. variational models [11–15]. Lattice simulations indicate that the corresponding string tension is larger (by a factor of 2 to 3) as compared to the string tension extracted from time-dependent large Wilson loops. This is consistent with expectations of variational analysis. At fixed quark-antiquark separation the Coulomb potential corresponds to the energy of a quark-antiquark pair in a vacuum state that is unmodified by the presence of the pair while the energy extracted from the Wilson loop corresponds to the energy of the exact QCD eigenstate in which the quark-antiquark (QQ̄) pair polarizes the vacuum [16]. The gluon chain model is a particular realization of the latter, i.e., the exact pair state. Confinement originates from the condensation of chromomagnetic charges [17–20]. Formation of the gluon chain should therefore also provide insights into the interplay between constituent gluons and magnetic domains in the vacuum. In the Hamiltonian formulation the true QQ̄ state is generated by the evolution operator limβ→∞ exp(−βH) from the unperturbed vacuum. This is because in a physical gauge the Hamiltonian H contains all gluon interactions which also couple to the classical, external quark-antiquark color source. In this paper we investigate if/how the gluon chain emerges from the evolution operator. We follow a canonical formulation of QCD in the Coulomb gauge since it contains only physical degrees of freedom, and these can be directly related to quasi particles. The gluon field is decomposed into normal modes representing particle excitations, and a physical state is represented as a superposition of multi-gluon states. Furthermore the normal mode expansion can be performed with respect to a non-vanishing classical background. Such a background is introduced to (phenomenologically) parametrize topologically disconnected sectors of the vacuum. In terms of the path integral representation these sectors correspond to large field configurations, i.e., field domains that cannot be smoothly connected to the null field configuration [21]. The paper is organized as follows. In the next section we review the structure of the Hamiltonian, introduce the particle basis, and discuss the role of the individual interaction terms in the formation of the chain. In Sec. III we propose a simplified computational scheme for studying the formation of the chain state and discuss numerical results. A summary and outlook are given in Section IV.

Electromagnetic form factors in the hypercentral CQM

Abstract.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 GeV2. Futhermore, the model has been used for predictions concerning the electromagnetic transverse and longitudinal transition form factors giving a good description of the medium Q2 behaviour. We show that the discrepancies in the reproduction of the helicity amplitudes at low Q2 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.

Electromagnetic N-Delta(1232) Transitions within the Point-Form of Relativistic Quantum Mechanics

The electromagnetic N − Δ(1232) transition amplitudes are calculated using the point-form of relativistic quantum mechanics. The relativistic effects incorporated in the electromagnetic matrix elements give a good description of the transition amplitudes to the Δ(1232) resonance, reproducing well the Q2 behaviour of the data, apart from the low Q2 one.

Elastic nucleon form factors

The relativized hypercentral Constituent Quark Model is used for the calculation of the elastic electromagnetic form factors of the nucleon. The results are compared with the recent measurements at Jlab.

Electromagnetic form-factors in the hypercentral CQM

Abstract.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 GeV2. Futhermore, the model has been used for predictions concerning the electromagnetic transverse and longitudinal transition form factors giving a good description of the medium Q2 behaviour. We show that the discrepancies in the reproduction of the helicity amplitudes at low Q2 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.