Mixing of pseudoscalar-baryon and vector-baryon in meson-baryon interaction and the generation of resonances

Resumen

During this Thesis we have used Chiral Perturbation Theory and its extensions with the Hidden Gauge Formalism to face many problems in hadron physics. Mainly, we have studied the meson-baryon interaction to describe hadronic resonances and their properties, such as mass, width, partial decay widths, radiative decay and helicity amplitudes. The study of the vector meson - baryon interaction has been extended mixing pseudoscalar mesons with vector mesons in an approach that allows a better understanding of the nature of those hadronic states. We have used this formalism to calculate the transition diagrams to obtain the amplitudes in coupled channels that are included in the $V$ potential matrix. Then we have used the Bethe-Salpeter equation to calculate the unitarized T-matrix using the $G$ function, dimensional regularized with a subtraction constant. We look for poles in the complex plane changing to the second Riemann sheet and then we calculate the couplings for each channel that are determined as the residues of the poles. This procedure has been used to study $N^*$ states with $J^P=1/2^-$ and $3/2^-$ with much success, and we have shown that the mixing of the interaction of pseudoscalar meson - baryon with vector meson has been crucial when one tries to reproduce the experimental widths and branching ratios. Other problems involving meson - meson interaction with the few body approach or the finite volume have also been studied.