arXiv subjects
Publications and source records attributed to B. Julia-Diaz.
The ability of the coupled-channels model (MSL) developed in recently in Ref. \cite{msl} to account simultaneously for the $πN$ scattering data and the $π$ photoproduction reactions on the nucleon is presented. An accurate description of $πN$ scattering has been obtained. A preliminary description of $π$ photoproduction is also discussed.
Recent developments on a dynamical coupled-channels model of hadronic and electromagnetic production of nucleon resonances are summarized.
As a first step to analyze the electromagnetic meson production reactions in the nucleon resonance region, the parameters of the hadronic interactions of a dynamical coupled-channel model, developed in {\it Physics Reports 439, 193 (2007)}, are determined by fitting the $πN$ scattering data. The channels included in the calculations are $πN$, $ηN$ and $ππN$ which has $πΔ$, $ρN$, and $σN$ resonant components. The non-resonant meson-baryon interactions of the model are derived from a set of Lagrangians by using a unitary transformation method. One or two bare excited nucleon states in each of $S$, $P$, $D$, and $F$ partial waves are included to generate the resonant amplitudes in the fits. The parameters of the model are first determined by fitting as much as possible the empirical $πN$ elastic scattering amplitudes of SAID up to 2 GeV. We then refine and confirm the resulting parameters by directly comparing the predicted differential cross section and target polarization asymmetry with the original data of the elastic $π^{\pm} p \to π^{\pm} p$ and charge-exchange $π^- p \to π^0 n$ processes. The predicted total cross sections of $πN$ reactions and $πN\to ηN$ reactions are also in good agreement with the data. Applications of the constructed model in analyzing the electromagnetic meson production data as well as the future developments are discussed.
The electromagnetic and axial form factors of the nucleon and its lowest positive parity excitations, the Delta(1232) and the N*(1440), are calculated with constituent-quark models that are specified by simple algebraic representations of the mass-operator eigenstates. Poincaré covariant current operators are generated by the dynamics from single-quark currents that are covariant under a kinematic subgroup. The dependence of the calculated form factors on the choice of kinematics and on the gross features of the wave functions is illustrated for instant-form, point-form, and front-form kinematics. A simple algebraic form of the orbital ground state wave function, which depends on two parameters, allows a fair description of all the form factors over the empirically accessible range, although with widely different choices of the parameters, which determine the range and shape of the orbital wave function. The neutron electric form factor requires additional features, for instance the presence of mixed symmetry S-state component with 1 -- 2 % probability in the ground state wave function. Instant and front form kinematics demand a spatially extended wave function, whereas in point form kinematics the form factors may be described with a quite compact wave function.
Within the dynamical model of Refs. [Phys. Rev. C54, 2660 (1996); C63, 055201 (2001)], we perform an analysis of recent data of pion electroproduction reactions at energies near the Delta(1232) resonance. We discuss possible interpretations of the extracted bare and dressed gamma N --> Delta form factors in terms of relativistic constituent quark models and Lattice QCD calculations. Possible future developments are discussed.
Differential cross-section and single polarization observables in the process gamma p --> K^+ Lambda are investigated within a constituent quark model and a dynamical coupled-channel formalism. The effects of two new nucleon resonances and of the K*(892)- and K1(1270)-exchanges are briefly presented.
The role of $q\bar q$ components in the nucleon and the N(1440) resonance is studied by explicit coupling of the lowest positive parity $qqqq\bar q$ state to the $qqq$ components in the harmonic oscillator quark model. The lowest energy $qqqq\bar q$ component, where the 4-quark subsystem has the flavor-spin symmetry $[4]_{FS}[22]_F[22]_S$, is close in energy to the lowest positive parity excitation of the nucleon in the $qqq$ quark model. The confining interaction leads to a strong mixing of the $qqqq\bar q$ system and the positive parity excited state of the $qqq$ system. This result is in line with the phenomenological indications for a two-component structure of the N(1440) resonance. The presence of substantial $q\bar q$ components in the N(1440) can bring about a reconciliation of the constituent quark model with the large empirical decay width of the N(1440).
The use of relativistic quark models with simple parametric wave functions for the understanding of the electromagnetic structure of nucleons together with their electromagnetic transition to resonances is discussed. The implications of relativity in the different ways it can be implemented in a simple model are studied together with the role played by mixed symmetry s-state and D-state deformations of the rest frame wave functions of the nucleon and Delta resonance.
A dynamical coupled-channel formalism for processes $πN \to KY$ and $γN \to KY$ is presented which provides a comprehensive investigation of recent data on the $γp \to K^+ Λ$ reaction. The non-resonant interactions within the subspace $KY\oplusπN$ are derived from effective Lagrangians, using a unitary transformation method. The calculations of photoproduction amplitudes are simplified by casting the coupled-channel equations into a form such that the empirical $γN \to πN$ amplitudes are input and only the parameters associated with the $KY$ channel are determined by performing $χ^2$-fits to all of the available data for $π^- p \to K^\circΛ, K^\circΣ^\circ$ and $γp \to K^+Λ$. Good agreement between our models and those data are obtained. In the fits to $πN \to KY$ channels, most of the parameters are constrained within $\pm 20%$ of the values given by the Particle Data Group and/or quark model predictions, while for $γp \to K^+ Λ$ parameters, ranges compatible with broken $SU(6)\otimes O(3)$ symmetry are imposed. The main reaction mechanisms in $K^+ Λ$ photoproduction are singled out and issues related to newly suggested resonances $S_{11}$, $P_{13}$, and $D_{13}$ are studied. Results illustrating the importance of using a coupled-channel treatment are reported. Meson cloud effects on the $γN \to N^*$ transitions are also discussed.
A coupled channel model with $γN$, $KY$ and $πN$ channels has been used to analyze the recent data of $γp \to K^+ Λ$. The non-resonant interactions within the subspace $KY \oplus πN$ are derived from effective Lagrangians using a unitary transformation method. The direct photoproduction reaction is obtained from a chiral constituent quark model with $SU(6)\otimes O(3)$ breaking. Missing baryon resonances issues are briefly discussed.
The electromagnetic form factors, charge radii and decay constants of pion, K and K*(892) are calculated using the three forms of relativistic kinematics: instant form, point form and (light) front form. Simple representations of the mass operator together with single quark currents are employed with all the forms. Making use of previously fixed parameters, together with the constituent quark mass for the strange quark, a reasonable reproduction of the available data for form factors, charge radii and decay constants of pion, rho, K and K*(892) is obtained in front form. With instant form a similar description, but with a systematic underestimation of the vector meson decay constants is obtained using two different sets of parameters, one for pion and rho and another one for K and K*(892). Point form produces a poor description of the data.
Several realistic phenomenological nucleon-nucleon interaction models are employed to investigate the possibility of bound deuteron-like states of such heavy flavor hyperons and nucleons, for which the interaction between the light flavor quark components is expected to be the most significant interaction. The results indicate that deuteron-like bound states are likely to form between nucleons and the $Ξ_c^{'}$ and $Ξ_{cc}$ charm hyperons as well as between $Ξ$ hyperons and double-charm hyperons. Bound states between two $Σ_c$ hyperons are also likely. In the case of beauty hyperons the corresponding states are likely to be deeply bound.
The $Δ-N$ electromagnetic transition form factors are calculated in the Poincaré covariant quark model in three forms of relativistic kinematics. Addition of $D-$state components to pure $S-$state model wave functions, chosen so as to reproduce the empirical elastic electromagnetic nucleon form factors with single constituent currents, brings the calculated $R_{EM}$ ratio for the $Δ(1232)\to Nγ$ transition closer to the empirical values in instant and point form kinematics. The calculated $R_{SM}$ ratio is insensitive to the $D-$state component. In front form kinematics the substantial violation of the angular condition for the spin 3/2 resonance transition amplitude in the impulse approximation prevents a unique determination of $R_{EM}$ and $R_{SM}$, both of which are very sensitive to $D-$state components. In no form of kinematics do $D-$state deformations of the rest frame baryon wave functions alone suffice for a description of the empirical values of these ratios.
The pion decay constants of the lowest orbitally excited states of the nucleon and the $Δ(1232)$ along with the corresponding axial transition form factors are calculated with Poincaré covariant constituent-quark models with instant, point and front forms of relativistic kinematics. The model wave functions are chosen such that the calculated electromagnetic and axial form factors of the nucleon represent the empirical values in all three forms of kinematics, when calculated with single-constituent currents. The pion decay widths calculated with the three forms of kinematics are smaller than the empirical values. Front and instant form kinematics provide a similar description, with a slight preference for front form, while the point form values are significantly smaller in the case of the lowest positive parity resonances.
The electromagnetic form factors of the $π$ and the $ρ$ are obtained using the three forms of relativistic kinematics, instant form, point form and (light) front form. Simple representations of the mass operator together with single quark currents are employed with all the forms. The Poincaré covariant current operators are generated by the dynamics from single-quark currents that are covariant under the kinematic subgroup. Front and instant forms allow to reproduce the available data for the pion form factor. On the other hand point form is not able to reproduce qualitatively the experimental data with reasonable values for the wave function parameters. For the $ρ$ electromagnetic form factors, instant and front forms provide a consistent picture. The obtained results do not depend appreciably on the wave function used.
The ground states of double-charm hyperons form a spin 1/2 isospin 1/2 multiplet analogous to that of nucleons. Their main strong interaction may be inferred directly from the corresponding nucleon-nucleon interaction by multiplication of the interaction components by the appropriate fractional difference between interaction strengths for pairs of light flavor quarks and pairs of triplets, e.g. nucleons, of light flavor quarks. By construction of the interaction between the recently discovered double-charm hyperons by this method from several realistic nucleon-nucleon interaction models it is shown that double-charm hyperons are likely to form bound (or possibly meta-stable) states akin to the deuteron in the spin triplet state. Double beauty baryons would form corresponding deeply bound states. Nucleons and double charm (beauty) hyperons will also form bound states. The existence of hypernuclei with double-charm and double-beauty hyperons, which are stable against the strong decay, is very likely.
It is shown that the phenomenological description of the baryon magnetic moments in the quark model carries over to the Poincaré covariant extension of the model. This applies to all the three common forms of relativistic kinematics with structureless constituent currents, which are covariant under the corresponding kinematic subgroups. In instant and front form kinematics the calculated magnetic moments depend strongly on the constituent masses, while in point form kinematics the magnetic moments are fairly insensitive to both the quark masses and the wave function model. The baryon charge radii and magnetic moments are determined in the different forms of kinematics for the light-flavor, strange and charm hyperons. The wave function model is determined by a fit to the electromagnetic form factor of the proton.
By means of a NN to NN*(1440) transition potential derived in a parameter-free way from a quark-model based NN potential, we determine simultaneously the $πNN^*(1440)$ and $σNN^*(1440)$ coupling constants. We also present a study of the target Roper excitation diagram contributing to the $p(d,d')$ reaction.