We study meson-baryon scattering with strangeness \char21{}1 in unitary chiral perturbation theory. Ten coupled channels are considered in our work, namely, ${$\pi${}}^{0}$\Lambda${}$, ${$\pi${}}^{0}{$\Sigma${}}^{0}$, ${$\pi${}}^{$-${}}{$\Sigma${}}^{+}$, ${$\pi${}}^{+}{$\Sigma${}}^{$-${}}$, ${K}^{$-${}}p$, ${\overline{K}}^{0}n$, $$\eta${}$\Lambda${}$, $$\eta${}{$\Sigma${}}^{0}$, ${K}^{0}{$\Xi${}}^{0}$, and ${K}^{+}{$\Xi${}}^{$-${}}$. A large amount of experimental data are analyzed, including the recent precise measurement by the SIDDHARTA Collaboration of the energy shift and width of the $1s$ state of kaonic hydrogen. This leads to a strong constraint on the free parameters in our theory and of the resulting meson-baryon scattering amplitudes. We also analyze the uncertainty that stems by using several different strategies to perform the fits to data. It is found that large uncertainties in the subthreshold extrapolation of the ${K}^{$-${}}p$ scattering amplitude arise by either employing only one common weak pseudoscalar decay constant or distinguishing between ${f}_{$\pi${}}$, ${f}_{K}$, and ${f}_{$\eta${}}$. However, in both cases a good reproduction of experimental data is obtained. We also discuss the pole content of the resulting $S$-wave amplitudes, particularly in connection with the two-pole structure of the $$\Lambda${}(1405)$ resonance.

Bound, antibound and resonance states are associated to poles in the on-shell partial wave amplitudes. We show here that from the residues of the pole a rank 1 projection operator associated with any of these states can be extracted, in terms of which a sum rule related to the composition of the state can be derived. Although typically it involves complex coefficients for the compositeness and elementariness, except for the bound state case, we demonstrate that one can formulate a meaningful compositeness relation with only positive coefficients for resonances whose associated Laurent series in the variable $s$ converges in a region of the physical axis around $\mathrm{Re}{s}_P$, with $s_P$ the pole position of the resonance. It is also shown that this result can be considered as an analytical extrapolation in $s_P$ of the clear narrow resonance case. We exemplify this formalism to study the two-body components of several resonances of interest.

In this work, the complete one-loop calculation of meson-meson scattering amplitudes within U(3)$\bigotimes${}U(3) chiral perturbation theory with explicit resonance states is carried out for the first time. Partial waves are unitarized from the perturbative calculation employing a nonperturbative approach based on the N/D method. Once experimental data are reproduced in a satisfactory way we then study the resonance properties, such as the pole positions, corresponding residues and their NC behaviors. The resulting NC dependence is the first one in the literature that takes into account the fact that the $\eta${}1 becomes the ninth Goldstone boson in the chiral limit for large NC. Within this scheme the vector resonances studied, $\rho${}(770), K*(892) and $\phi${}(1020), follow an NC trajectory in agreement with their standard q\ifmmode\bar\else\textasciimacron\fi{}q interpretation. The scalars f0(1370), a0(1450) and K*(1430) also have for large NC a q\ifmmode\bar\else\textasciimacron\fi{}q pole position trajectory and all of them tend to a bare octet of scalar resonances around 1.4~GeV. The f0(980) tends asymptotically to the bare pole position of a singlet scalar resonance around 1~GeV. The $\sigma${}, $\kappa${} and a0(980) scalar resonances have a very different NC behavior. The case of the $\sigma${} resonance is analyzed with special detail.

Through the scattering of light-pseudoscalar mesons $(\pi ,K,\eta ,{\eta }^{\prime })$ off charmed mesons $(D,D_s)$, we study the $D_{s0}^{*}(2317)$ state and other relevant charm scalar mesons in a unitarized chiral effective field theory approach. We investigate the charm scalar meson poles with different strangeness ($S$) and isospin ($I$) quantum numbers as well as their corresponding residues, which provide the coupling strengths of the charm scalar mesons. Both the light-quark mass and $N_C$ dependences of the pole positions of the $D_{s0}^{*}(2317)$ and the poles with $(S,I)=(0,1/2)$ are analyzed in detail in this work. Interestingly we observe quite similar pion mass trajectories for the resonance pole at around 2.1 GeV with $(S,I)=(0,1/2)$ to those of the $f_0(500)$ given in the literature. When increasing the values of $N_C$ we find that a bound state and a virtual state in the $(S,I)=(1,0)$ channel asymmetrically approach the $DK$ threshold for $N_C<6$, and they meet at this threshold at $N_C=6$. When $N_C>6$, the bound and virtual states move into the complex plane on the second Riemann sheet and become a symmetric pair of resonance poles. For large enough values of $N_C$, neither the $D_{s0}^{*}(2317)$ pole nor the poles with $(S,I)=(0,1/2)$ tend to fall down to the real axis, indicating that they do not behave like a standard quark-antiquark meson at large $N_C$.

We study the scattering of the light-flavor pseudoscalar mesons ([equation]) off the ground-state charmed mesons ([equation]) within chiral effective field theory. The recent lattice simulation...