arXiv subjects
Publications and source records attributed to M. J. Vicente Vacas.
The sigma meson mass and width are studied at finite baryonic density in the framework of a chiral unitary approach which successfully reproduces the meson meson phase shifts and generates the f0 and sigma resonances in vacuum.
In this talk I look into three different topics, addressing first the possibility that the $Θ^+$ is a bound state of $K πN$, exploiting the results of this study to find out the contribution of two meson and one baryon components in the baryon antidecuplet and in the third place I present results on a new resonant exotic baryonic state which appears as dynamically generated by the Weinberg Tomozawa $ΔK$ interaction.
We study the selfenergy of the $Θ^+$ pentaquark in nuclei associated with two types of interaction: the KN decay of the $Θ^+$ and two meson baryon decay channels of the $Θ^+$. Whereas the potential related to the first source is quite weak, the second kind of interaction produces a large and attractive potential that could lead to the existence of $Θ^+$ nuclear bound states.
The isoscalar and isovector scattering lengths (b_0,b_1) are determined using a unitarized coupled channel approach based on chiral Lagrangians. Using experimental values of pionic hydrogen and deuterium as well as low energy pi N scattering data, the free parameters of the model are calculated. Isospin violation is incorporated to a certain extent by working with physical particle masses. For the deuterium scattering length a_{pi-d} new significant corrections concerning real and imaginary parts are evaluated, putting new constraints from pi-d scattering on the values of (b_0,b_1). In particular, dispersion corrections, the influence of the Delta(1232) resonance, crossed terms and multiple scattering in a Faddeev approach are considered.
In this talk I look into three different topics, addressing first a method to determine the quantum numbers of the $Θ^+$, then exploiting the possibility that the $Θ^+$ is a bound state of $K πN$ and in the third place I present results on a new resonant exotic baryonic state which appears as dynamically generated by the Weinberg Tomozawa $ΔK$ interaction.
We study the properties of the kappa meson in the nuclear medium, starting from a chiral unitary model of S-wave, I=1/2 K-pi scattering, which describes the elastic K-pi phase shifts and generates a pole in the amplitude. Medium effects are considered by including pion and kaon selfenergies. We explore the changes in the kappa pole position and in the K-pi scattering amplitude at finite density, with two different models for the kaon selfenergy.
The photo-induced $K^*$ vector meson production is investigated for the study of the $Λ(1405)$ resonance. This reaction is particularly suited to the isolation of the second pole in the $Λ(1405)$ region which couples dominantly to the $\bar K N$ channel. We obtain the mass distribution of the $Λ(1405)$ which peaks at 1420 MeV, and differs from the nominal one. Combined with several other reactions, like the $π^- p \to K^0 πΣ$ which favours the first pole, this detailed study will reveal a novel structure of the $Λ(1405)$ state.
We investigate the photoproduction of $K^*$ vector meson for the study of the $Λ(1405)$ resonance. The invariant mass distribution of $πΣ$ shows a different shape from the nominal one, peaking at 1420 MeV. This is considered as a consequence of the double pole structure of $Λ(1405)$, predicted in the chiral unitary model. Combined with other reactions, such as $π^- p \to K^0 πΣ$, experimental confirmation of this fact will reveal a novel structure of the $Λ(1405)$ state.
We study the four-body decays $J/ψ\to N {\bar N}$ {\it meson meson} using a chiral unitary approach to account for the meson meson final state interaction (FSI). The calculation of the $J/ψ\to N \bar N π^+ π^-$ process properly reproduces the experimental data after taking the FSI of mesons and the contribution of intermediate $ρ$ meson into account. The isoscalar resonances $σ$, $f_0(980)$ and the isovector resonance $a_0(980)$ are generated through the FSI of the mesons in the channels $J/ψ\to N \bar N π^0 π^0$ and $J/ψ\to N \bar N π^0 η$, respectively. We also calculate the two mesons invariant mass distribution and the partial decay width of $J/ψ\to N \bar N K^+ K^-$ and $J/ψ\to N \bar N K^0 {\bar K}^0$, on which there is still no experimental data available.
Based on a prior determination of the $ϕ$ selfenergy in a nuclear medium we perform a theoretical study of inclusive $ϕ$ photoproduction in nuclei, looking at the $A$ dependence of the cross sections for different $ϕ$ momenta. We find sizeable reductions in the nuclear cross sections with respect to the elementary one, using a $ϕ$ selfenergy which implies a width about six times the free one at normal nuclear density. The calculations are done to match the set up for an ongoing experiment at {\it SPring8/Osaka} which should provide valuable information on the renormalization of the $ϕ$ properties in nuclei.
We discuss the mechanisms that lead to $Λ(1405)$ production in the $π^-p\to K^0πΣ$ reaction. The problem has gained renewed interest after different works converge to the conclusion that there are two resonances around the region of 1400 MeV, rather than one, and that they couple differently to the $πΣ$ and $\bar{K}N$ channels. We look at the dynamics of that reaction and find two mechanisms which eventually filter each one of the resonances, leading to very different shapes of the $πΣ$ invariant mass distributions. The combination of the two mechanisms leads to a shape of this distribution compatible with the experimental measurements.
In order to assess the relevance of chiral dynamics in the scalar sector we address two recent problems: radiative decay of the $ϕ$, for which there are quite recent data from Frascati, and the modification of the $σ$ properties in the nuclear medium seen through the $π^0 π^0$ photoproduction in nuclei.
We study the radiative $ϕ$ decay into $π^0 π^0 γ$ and $π^0 ηγ$ taking into account mechanisms in which there are two sequential vector-vector-pseudoscalar or axial-vector--vector--pseudoscalar steps followed by the coupling of a vector meson to the photon, considering the final state interaction of the two mesons. There are other mechanisms in which two kaons are produced through the same sequential mechanisms or from $ϕ$ decay into two kaons and then undergo final state interaction leading to the final pair of pions or $π^0 η$, this latter mechanism being the leading one. The results of the parameter free theory, together with the theoretical uncertainties, are compared with the latest experimental results of KLOE at Frascati.
The inclusive pion double charge exchange (DCX) on oxygen nuclei has been measured in the region where additional pion production is kinematically forbidden. The experiment was performed at ITEP PS at incident pi- kinetic energies T_0= 0.59, 0.75 and 1.1 GeV. The integrated forward differential cross section was found to decrease with energy slowly. At 1.1 GeV it exceeds the theoretical prediction within the conventional sequential single charge exchange mechanism with a neutral pion in the intermediate state (Glauber elastic rescattering) by about half an order of magnitude. The sequential mechanism with two pions in the intermediate state (Glauber inelastic rescatterings), which was proposed recently, seems to be able to explain the observed slow energy dependence and allows to predict the DCX cross section for higher energies.
The $s-$wave meson-baryon scattering is analyzed for the isospin-strangeness $I=1/2, S=0$ and $I=0,S=-1$ sectors, in a Bethe-Salpeter coupled channel formalism incorporating Chiral Symmetry. For both sectors, four channels have been considered: $πN$, $ηN$, $K Λ$, $K Σ$ and $πΣ$, $\bar K N$, $ηΛ$, $K Ξ$, respectively. The needed two particle irreducible matrix amplitudes are taken from lowest order Chiral Perturbation Theory in a relativistic formalism. There appear undetermined low energy constants, as a consequence of the renormalization of the amplitudes, which are obtained from fits to the available data: elastic $πN $ phase-shifts, $π^- p \to ηn$ and $π^- p \to K^0 Λ$ cross sections and to $πΣ\toπΣ$ mass-spectrum, the elastic $\bar K N \to \bar K N$ and $ \bar K N\to πΣ$ $t$--matrices and to the $ K^- p \to ηΛ$ cross section data. The position and residues of the complex poles in the second Riemann sheet of the scattering amplitude determine masses, widths and branching ratios of the $S_{11}-$ $N$(1535) and $-N$(1650) and $S_{01}-$ $Λ$(1405) and $-Λ$(1670) resonances, in reasonable agreement with experiment. A good overall description of data, from threshold up to around 2 GeV is achieved despite the fact that three-body channels have not been explicitly included.
The $ϕ$ meson spectrum, which in vacuum is dominated by its coupling to the $\bar{K} K$ system, is modified in nuclear matter. Following a model based on chiral SU(3) dynamics we calculate the $ϕ$ meson selfenergy in nuclear matter considering the $K$ and $\bar{K}$ in-medium properties. For the latter we use the results of previous calculations which account for $S-$ and $P-$wave kaon-nucleon interactions based on the lowest order meson-baryon chiral effective Lagrangian, and this leads to a dressing of the kaon propagators in the medium. In addition, a set of vertex corrections is evaluated to fulfill gauge invariance, which involves contact couplings of the $ϕ$ meson to $S-$wave and $P-$wave kaon-baryon vertices. Within this scheme the mass shift and decay width of the $ϕ$ meson in nuclear matter are studied.
The $s-$wave meson-baryon scattering is analyzed for the strangeness $S=-1$ and isospin I=0 sector in a Bethe-Salpeter coupled channel formalism incorporating Chiral Symmetry. Four channels have been considered: $πΣ$, $\bar K N$, $ηΛ$ and $K Ξ$. The required input to solve the Bethe-Salpeter equation is taken from lowest order Chiral Perturbation Theory in a relativistic formalism. There appear undetermined low energy constants, as a consequence of the renormalization of the amplitudes, which are obtained from fits to the $πΣ\toπΣ$ mass-spectrum, to the elastic $\bar K N \to \bar K N$ and $ \bar K N\to πΣ$ $t$--matrices and to the $ K^- p \to ηΛ$ cross section data. The position and residues of the complex poles in the second Riemann Sheet of the scattering amplitude determine masses, widths and branching ratios of the $S_{01}-$ $Λ$(1405) and $-Λ$(1670) resonances, in reasonable agreement with experiment. A good overall description of data, from $πΣ$ threshold up to 1.75 GeV, is achieved despite the fact that three-body channels have not been explicitly included.
We evaluate the two pion decay of the Roper resonance in a model where explicit re-scattering of the two final pions is accounted for by the use of unitarized chiral perturbation theory. Our model does not include an explicit $ε$ or $σ$ scalar-isoscalar meson decay mode, instead it generates it dynamically by means of the pion re-scattering. The two ways, explicit or dynamically generated, of introducing this decay channel have very different amplitudes. Nevertheless, through interference with the other terms of the model we are able to reproduce the same phenomenology as models with explicit consideration of the $ε$ meson.