M. M. Giannini

A three-body force model for the baryon spectrum

Abstract 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.

STUDIES OF NUCLEON RESONANCE STRUCTURE IN EXCLUSIVE MESON ELECTROPRODUCTION

Studies of the structure of excited baryons are key factors to the N* program at Jefferson Lab (JLab). 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 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 electropro...

ELECTROMAGNETIC TRANSITION FORM FACTORS OF NEGATIVE PARITY NUCLEON RESONANCES

We have calculated the transition form factors for electromagnetic excitation of the negative parity resonances of the nucleon using different models previously proposed, and we discuss their results and limits by comparison with experimental data.

A three-body force model for the electromagnetic excitation of the nucleon

Abstract 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.

Electroproduction of nucleon resonances

Abstract.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 Q2 up to 4.0 (GeV/c)2 have been analyzed and the Q2 dependence of the helicity amplitudes have been extracted for a series of four star resonances. We compare single-Q2 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.

The pair current in the constituent quark model

Abstract The relativistic pair current is re-evaluated in the constituent quark model. Isovector charge and isoscalar current contributions change substantially compared to previous calculations. In particular, the previously predicted large differences in the change form factors of 3 He and 3 H are considerably reduced.

A note on the quadrupole (N → Δ) transition amplitude in quark models

Abstract Due to the usual strong truncation of the basis in dynamical quark model calculations, the charge-current operator for the quadrupole (N→ Δ ) transition violates gauge invariance. It is demonstrated explicitly for the nonrelativistic constituent quark model that, in the standard 2ℏ ω -basis, calculations of the quadrupole transition form factor differ by an order of magnitude, depending on whether one uses the charge or current for the evaluation. Since this effect is related to the behaviour of the two operators under hermitean conjugation and time reversal, it is expected to be of importance also for other wavefunctions as well as in relativistic models.

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.

Spectroscopy of pentaquark states

Abstract.We construct a complete classification of qqqq¯q pentaquark states in terms of the spin-flavour SU(6) representations. We find that only some definite SU(3) representations are allowed, that is singlets, octects, 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 is general and model independent and is useful both for model builders and experimentalists. The mass spectrum is obtained from a Gürsey-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 Θ+(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.

The physical meaning of scattering matrix singularities in coupled-channel formalisms⋆

Abstract.The physical meaning of bare and dressed scattering matrix singularities has been investigated. Special attention has been attributed to the role of the well-known invariance of the scattering matrix with respect to the field transformation of the effective Lagrangian. Examples of evaluating bare and dressed quantities in various models are given.