Unquenched Meson Spectroscopy

Quantum chromodynamics (QCD), the quantum field theory of strong interactions, is highly nonpertubative in the low-energy sector, where confinement dominates and resonance phenomena are observed. Therefore, phenomenological unquenched models based on the old ideas of the $\mathcal{S}$-matrix theory give a fundamental contribution to understand the complex pattern of masses, widths and shapes of experimentally observed meson resonances. In the present thesis we employ the Resonance-Spectrum-Expansion coupled-channel model to study two enigmatic meson states, the isoscalar vector $\phi(2170)$ and the charmonium-like axial-vector $X(3872)$. The same model is applied to describe the peculiar pattern of masses and widths of the open-charm axial-vectors - pseudovectors $D_1(2420)$ and $D_1(2430)$, and $D_{s1}(2460)$ and $D_{s1}(2536)$. Furthermore, a simplified Schr\"{o}dinger model is used to study the dominant wave-function components of $X(3872)$ near its resonance mass. Both models successfully describe the whole variety of special features observed in experiment, which are not so easily explained in QCD-inspired quenched models.