No abstract

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.

Since 2003 many new hadrons, including the lowest-lying positive-parity charm-strange mesons D * s0 (2317) and Ds1 (2460), were observed that do not conform with quark model expectations. It was recently demonstrated that various puzzles in the charm meson spectrum find a natural resolution, if the SU(3) multiplets for the lightest scalar and axial-vector states, amongst them the D * s0 (2317) and the Ds1 (2460), owe their existence to the nonperturbative dynamics of Goldstone-Boson scattering off D (s) and D * (s) mesons. Most importantly the ordering of the lightest strange and nonstrange scalars becomes natural. In this work we demonstrate for the first time that this mechanism is strongly supported by the recent high quality data on the B − → D + π − π − provided by the LHCb experiment. This implies that the lowest quark-model positive-parity charm mesons, together with their bottom counterparts, if realized in nature, do not form the ground-state multiplet. This is similar to the pattern that has been established for the scalar mesons made from light up, down and strange quarks, where the lowest multiplet is considered to be made of states not described by the quark model. In a broader view, the hadron spectrum must be viewed as more than a collection of quark model states.One of the currently most challenging problems in fundamental physics is to understand the nonperturbative regime of the Quantum Chromodynamics (QCD), the fundamental theory for the interaction of quarks and gluons. However, since the quark and gluon fields are confined inside color-neutral hadrons, what needs to be achieved is a quantitative understanding of the hadron spectrum. First principle lattice QCD (LQCD) calculations are indispensable in this regard. In many cases, one can efficiently bridge the computationally intensive LQCD framework with complicated experimental observables using chiral perturbation theory (ChPT)-the effective field theory for QCD at low energies-and its unitarization to fulfill probability conservation. In this work we demonstrate how such a combination leads to the resolution of a number of longstanding puzzles in charmmeson spectroscopy. It also paves the way towards a new paradigm in the spectroscopy for heavy-light mesons.Until the beginning of the millennium heavy-hadron spectroscopy was assumed to be well understood by means of the quark model [1,2], which describes the positive-parity ground state charm mesons as bound systems of a heavy quark and a light antiquark in a Pwave. This belief was put into question in 2003, when the charm-strange scalar (J P = 0 + ) and axial-vector (1 + ) mesons D * s0 (2317) [3] and D s1 (2460) [4] were discov- * fkguo@itp.ac.cn ered (for recent reviews on new hadrons, see Refs.[5-11]), since the states showed properties at odds with the quark model. Moreover, attempts to adjust the quark model raised more questions [12]. Various alternative proposals were put forward about the nature of these new states including D ( * ) K hadronic molecules (loosely bound states of two ...