Study of conventional and non-conventional scalar and vector mesons

Enormous progress in physics enriched our knowledge about the particles which build matter. Among them there are mesons, to which this thesis is entirely devoted. The overwhelming majority of mesons is made of `conventional' $q\bar{q}$ pairs, but nowadays there is mounting evidence for `non-conventional' mesons, such as tetraquarks, glueballs, hybrids, and molecules. The main aim of this thesis is to understand the nature of some scalar and vector mesonic states which still remain puzzling. In this thesis we investigate two nonets of $q\bar{q}$ excited vector mesons. Within an effective QFT model we evaluate various decay channels of these states and compare the results with experimental data from PDG. Moreover, we make predictions for a not-yet observed $s\bar{s}$ state in $1$ $^3D_1$ nonet. Some non-conventional mesons can be understood by the mechanism of dynamical generation. We show that the inclusion of a single conventional $q\bar{q}$ seed in the relativistic QFT Lagrangian may cause the appearance of an additional associated state as a dynamically generated companion pole. Along this line, we show that light scalar kaon $K^*_0(700)$ emerges as a companion pole of the $q\bar{q}$ meson $K_0^*(1430)$. Moreover, we show that the enigmatic resonance $X(3872)$ in the charmonium sector can be interpreted as the (virtual) companion pole of the $c\bar{c}$ state $\chi_{c1}(2P)$. Then, we study the $c\bar{c}$ meson $\psi(4040)$. We show that an additional companion pole appears on the complex plane. At first sight one may identify it with the enhancement $Y(4008)$ observed by the Belle Collaboration. A detailed analysis reveals that different mechanism is responsible for the creation of $Y(4008)$. Hence, the bump associated with $Y(4008)$ should not be interpreted as an independent resonance, but rather as a manifestation of the strong coupling of $\psi(4040)$ to the $D^*D$ channel.