Searcharxiv⌕ Search

arXiv subjects

A. Vassallo

Publications and source records attributed to A. Vassallo.

At least 19 recordsLinked to original sources
arXiv2015

Electromagnetic Form Factors and the Hypercentral CQM

New results about the electromagnetic form factors of the nucleon are obtained with a semirelativistic version of the hypercentral constituent quark model (hCQM) and a relativistic current. The complex structure of the constituent quarks is taken into account implicitly by means of phenomenological constituent quark form factors. We obtain a detailed reproduction of the experimental data up to $5 GeV^2$, moreover our findings about constituent quark root mean square radii are of the same order than the recent ones obtained analyzing the proton structure functions.

nucl-th↗
arXiv2015

SU(6)-breaking symmetry and the ratio of proton momentum distributions

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.

nucl-th↗
arXiv2015

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.

nucl-th↗
arXiv2015

High $Q^2$ behavior of the electromagnetic form factors in the relativistic hypercentral CQM

The ratio $R_p$ between the electric and magnetic proton form factors has been recently measured at JLab up to $Q^2=8.5 ~GeV^2$. We have extended the calculation of the nucleon form factors with the hypercentral Constituent Quark Model and compared them with the data on $R_p$ and on the $Q^2$ behavior of the ratio $Q^2 F_2/F_1$. In both cases the theoretical curves agree with the experimental points.

nucl-th↗
arXiv2015

Relativistic quark-diquark model of baryons. Non strange spectrum and nucleon electromagnetic form factors

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.

hep-ph↗
arXiv2015

Analysis Tools for Next-Generation Hadron Spectroscopy Experiments

The series of workshops on New Partial-Wave Analysis Tools for Next-Generation Hadron Spectroscopy Experiments was initiated with the ATHOS 2012 meeting, which took place in Camogli, Italy, June 20-22, 2012. It was followed by ATHOS 2013 in Kloster Seeon near Munich, Germany, May 21-24, 2013. The third, ATHOS3, meeting is planned for April 13-17, 2015 at The George Washington University Virginia Science and Technology Campus, USA. The workshops focus on the development of amplitude analysis tools for meson and baryon spectroscopy, and complement other programs in hadron spectroscopy organized in the recent past including the INT-JLab Workshop on Hadron Spectroscopy in Seattle in 2009, the International Workshop on Amplitude Analysis in Hadron Spectroscopy at the ECT*-Trento in 2011, the School on Amplitude Analysis in Modern Physics in Bad Honnef in 2011, the Jefferson Lab Advanced Study Institute Summer School in 2012, and the School on Concepts of Modern Amplitude Analysis Techniques in Flecken-Zechlin near Berlin in September 2013. The aim of this document is to summarize the discussions that took place at the ATHOS 2012 and ATHOS 2013 meetings. We do not attempt a comprehensive review of the field of amplitude analysis, but offer a collection of thoughts that we hope may lay the ground for such a document.

hep-ph↗
arXiv2012

Gluon chain formation in presence of static charges

We consider the origins of the gluon chain model. The model serves as a realization of the dynamics of the chromoelectric flux between static quark-antiquark sources. The derivation is based on the large-N_C limit of the Coulomb gauge Hamiltonian in the presence of a background field introduced to model magnetic confinement.

hep-ph↗
arXiv2005

A New Application of the Gursey and Radicati Mass Formula

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 hypercoulomb term. We show that the non strange 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. The coefficients of the Gürsey Radicati SU(6) breaking part obtained by the fit of the three-quark spectrum can be used to evaluate in first approximation the splitting within multiplets also for exotic baryon systems.

nucl-th↗
arXiv2005

Electromagnetic Form Factors and the hCQM

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).

hep-ph↗
arXiv2004

Electromagnetic Form Factors in the hypercentral CQM

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.

nucl-th↗
arXiv2003

Electroproduction of nucleon resonances

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.

nucl-th↗
arXiv2003

Extra S11 and P13 in the Hypercentral Constituent Quark Model

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.

nucl-th↗
arXiv2003

The Hypercentral Constituent Quark Model

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.

nucl-th↗
arXiv2003

An Overview of the Hypercentral Constituent Quark Model

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.

nucl-th↗
arXiv2003

Helicity Amplitudes In the Hypercentral Constituent Quark Model

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.

nucl-th↗
arXiv2002

Quark model predictions for the SU(6)-breaking ratio of the proton momentum distributions

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.

nucl-th↗