In this paper, we present a review of recent works on weak decay of heavy mesons and baryons with two mesons, or a meson and a baryon, interacting strongly in the final state. The aim is to learn about the interaction of hadrons and how some particular resonances are produced in the reactions. It is shown that these reactions have peculiar features and act as filters for some quantum numbers which allow to identify easily some resonances and learn about their nature. The combination of basic elements of the weak interaction with the framework of the chiral unitary approach allow for an interpretation of results of many reactions and add a novel information to different aspects of the hadron interaction and the properties of dynamically generated resonances.

The so far only known charmed non-strange scalar meson is dubbed as $D_0^*(2400)$ in the Review of Particle Physics. We show, within the framework of unitarized chiral perturbation theory, that there are in fact two $(I=1/2, J^P=0^+)$ poles in the region of the $D_0^*(2400)$ in the coupled-channel $D\pi$, $D\eta$ and $D_s\bar K$ scattering amplitudes. With all the parameters previously fixed, we predict the energy levels for the coupled-channel system in a finite volume, and find that they agree remarkably well with recent lattice QCD calculations. This successful description of the lattice data is regarded as a strong evidence for the two-pole structure of the $D_0^*(2400)$. With the physical quark masses, the poles are located at $\left(2105^{+6}_{-8}-i\,102^{+10}_{-12}\right)$~MeV and $\left(2451^{+36}_{-26}-i\,134^{+7}_{-8}\right)$~MeV, with the largest couplings to the $D\pi$ and $D_s\bar K$ channels, respectively. Since the higher pole is close to the $D_s\bar K$ threshold, we expect it to show up as a threshold enhancement in the $D_s\bar K$ invariant mass distribution. This could be checked by high-statistic data in future experiments. We also show that the lower pole belongs to the same SU(3) multiplet as the $D_{s0}^*(2317)$ state. Predictions for partners in the bottom sector are also given.Comment: 6 pages, 5 figure

We perform a coupled channel study of the meson-meson $S$ waves with isospin ($I$) 0 and $1/2$ up to 2 GeV. A new approach is derived that allows one to include the many channels $\pi \pi$, $K\overline{K}$, $\eta \eta$, $\sigma \sigma$, $\eta {\eta }^{\prime }$, ${\eta }^{\prime }{\eta }^{\prime }$, $\rho \rho$, $\omega \omega$, $\omega \varphi$, $\varphi \varphi$, $a_1\pi$, and ${\pi }^{*}\pi$ with still few free parameters. It follows that coupled channel dynamics is strong and cannot be neglected in order to study resonance properties in the region 1.4–1.6 GeV. All the resonances with masses below 2 GeV and $I=0$ and $1/2$ are generated. We identify the $f_0(1710)$ and an important contribution to the $f_0(1500)$ as an unmixed glueball. This is based on an accurate agreement of our results with predictions of lattice QCD and the chiral suppression of the coupling of a scalar glueball to $\overline{q}q$. Another pole, mainly corresponding to the $f_0(1370)$, is a pure octet state.

Mapping states with explicit gluonic degrees of freedom in the light sector is a challenge, and has led to controversies in the past. In particular, the experiments have reported two different hybrid candidates with spin-exotic signature, ${\pi }_1(1400)$ and ${\pi }_1(1600)$, which couple separately to $\eta \pi$ and ${\eta }^{\prime }\pi$. This picture is not compatible with recent Lattice QCD estimates for hybrid states, nor with most phenomenological models. We consider the recent partial wave analysis of the ${\eta }^{(\prime )}\pi$ system by the COMPASS Collaboration. We fit the extracted intensities and phases with a coupled-channel amplitude that enforces the unitarity and analyticity of the $S$ matrix. We provide a robust extraction of a single exotic ${\pi }_1$ resonant pole, with mass and width $1564\pm 24\pm 86$ and $492\pm 54\pm 102\mathrm{MeV}$, which couples to both ${\eta }^{(\prime )}\pi$ channels. We find no evidence for a second exotic state. We also provide the resonance parameters of the $a_2(1320)$ and $a_2^{\prime }(1700)$.

The Zc±(3900)/Zc±(3885) resonant structure has been experimentally observed in the Y(4260)→J/ψππ and Y(4260)→D¯⁎Dπ decays. This structure is intriguing since it is a prominent candidate of an exotic hadron. Yet, its nature is unclear so far. In this work, we simultaneously describe the D¯⁎D and J/ψπ invariant mass distributions in which the Zc peak is seen using amplitudes with exact unitarity. Two different scenarios are statistically acceptable, where the origin of the Zc state is different. They correspond to using energy dependent or independent D¯⁎D S-wave interaction. In the first one, the Zc peak is due to a resonance with a mass around the DD¯⁎ threshold. In the second one, the Zc peak is produced by a virtual state which must have a hadronic molecular nature. In both cases the two observations, Zc±(3900) and Zc±(3885), are shown to have the same common origin, and a D¯⁎D bound state solution is not allowed. Precise measurements of the line shapes around the DD¯⁎ threshold are called for in order to understand the nature of this state