A large number of experimental discoveries especially in the heavy quarkonium sector that did not at all fit to the expectations of the until then very successful quark model led to a renaissance of hadron spectroscopy. Among various explanations of the internal structure of these excitations, hadronic molecules, being analogues of light nuclei, play a unique role since for those predictions can be made with controlled uncertainty. We review experimental evidences of various candidates of hadronic molecules, and methods of identifying such structures. Nonrelativistic effective field theories are the suitable framework for studying hadronic molecules, and are discussed in both the continuum and finite volumes. Also pertinent lattice QCD results are presented. Further, we discuss the production mechanisms and decays of hadronic molecules, and comment on the reliability of certain assertions often made in the literature.

We study the scattering of light pseudoscalar mesons (π, K) off charmed mesons (D,

Among the newly observed structures in the heavy-quarkonium mass region, some have been proposed to be hadronic molecules. We investigate the consequences of heavy-quark flavor symmetry on these heavy meson hadronic molecules. The symmetry allows us to predict new hadronic molecules on one hand, and test the hadronic molecular assumption of the observed structures on the other hand. We explore the consequences of the flavor symmetry assuming the X(3872) and Zb(10 610) as an isoscalar DD̅ * and isovector BB̅ * hadronic molecule, respectively. A series of hadronic molecules composed of heavy mesons are predicted. In particular, there is an isoscalar 1++ BB̅ * bound state with a mass about 10 580 MeV which may be searched for in the Υ(1S,2S)π+π-π0 mass distribution; the isovector charmonium partners of the Zb(10 610) and the Zb(10 650) are also predicted, which probably corresponds to the very recently observed Zc(3900) and Zc(4025) resonances by the BESIII Collaboration

The LHCb Collaboration announced two pentaquark-like structures in the J/ψ p invariant mass distribution. We show that the current information on the narrow structure at 4.45 GeV is compatible with kinematical effects of the rescattering from χc1 p to J/ψ p: First, it is located exactly at the χc1 p threshold. Second, the mass of the four-star well-established Λ(1890) is such that a leading Landau singularity from a triangle diagram can coincidentally appear at the χc1 p threshold, and third, there is a narrow structure at the χc1 p threshold but not at the χc0 p and χc2 p thresholds. In order to check whether that structure corresponds to a real exotic resonance, one can measure the process Λ 0 b → K − χc1 p. If the Pc(4450) structure exists in the χc1 p invariant mass distribution as well, then the structure cannot be just a kinematical effect but is a real resonance, otherwise, one cannot conclude the Pc(4450) to be another exotic hadron. In addition, it is also worthwhile to measure the decay Υ(1S) → J/ψ pp: a narrow structure at 4.45 GeV but not at the χc0 p and χc2 p thresholds would exclude the possibility of a pure kinematical effect. *

Abstract. The assumption that the newly observed charged bottomonia states Z b (10610) and Z b (10650) are of molecular nature is confronted with the measured invariant mass distributions for the transitions of the Υ (5S) to the final statesIt is shown that the assumption that the Z b (10610) and Z b (10650) are BB * + c.c. and B * B * bound states, respectively, with very small binding energies is consistent with the data. The calculation is based on a power counting for bottom meson loops, which is explicitly given up to two-loop in the framework of a nonrelativistic effective field theory. We also show that if the Z b states are of molecular nature, then the data should not be analyzed by using a Breit-Wigner parametrization.PACS. 14.40.Rt Exotic mesons -13.25.Gv Decays of J/ψ, Υ , and other quarkonia

We have analyzed the singularities of a triangle loop integral in detail and derived a formula for an easy evaluation of the triangle singularity on the physical boundary. It is applied to the Λ b → J/ψK − p process via Λ * -charmonium-proton intermediate states. Although the evaluation of absolute rates is not possible, we identify the χ c1 and the ψ(2S) as the relatively most relevant states among all possible charmonia up to the ψ(2S). The Λ(1890) χ c1 p loop is very special as its normal threshold and triangle singularities merge at about 4.45 GeV, generating a narrow and prominent peak in the amplitude in the case that the χ c1 p is in an S-wave. We also see that loops with the same charmonium and other Λ * hyperons produce less dramatic peaks from the threshold singularity alone. For the case of χ c1 p → J/ψ p and quantum numbers 3/2 − or 5/2 + one needs Pand D-waves, respectively, in the χ c1 p, which drastically reduce the strength of the contribution and smooth the threshold peak. In this case we conclude that the singularities cannot account for the observed narrow peak. In the case of 1/2 + , 3/2 + quantum numbers, where χ c1 p → J/ψ p can proceed in an S-wave, the Λ(1890) χ c1 p triangle diagram could play an important role, though can neither assert their strength without further input from experiments and lattice QCD calculations. 1 I. INTRODUCTIONTriangle singularities in physical processes were introduced by Landau [1] and stem from Feynman diagrams involving three intermediate particles when the three particles can be placed simultaneously on shell and the momenta of these particles are collinear (parallel or antiparallel) in the frame of an external decaying particle at rest. In one of the cases (we call it parallel), two of the particles in the loop will go in the same direction and might fuse into other external outgoing particle(s) [2], so that the rescattering process can even happen as a classical process. In this case, the decay amplitude has a singularity close to the physical region 1 and, thus, can produce an enhancement. One of the classical cases would be given when the two on shell particles move in the same direction and with similar velocities. In the center-of-mass frame of the rescattering particles, these two particles would also be at rest and the triangle singularity is then located around the threshold.One very successful example of effects of the triangle singularity was shown in the decay of η(1405) → πa 0 (980) and η(1405) → πf 0 (980) in Refs. [3,4]. The second reaction breaks isospin symmetry. However, the process η(1405) → K * K followed by K * → Kπ and the fusion of KK → f 0 (980) enhances drastically the rate of η(1405) → πf 0 (980) relative to other isospin violating processes. Experimentally the ratio of rates for η(1405), a huge number for an isospin breaking magnitude. The work of [3,4] was continued in [6] where the precise rates, as well as the shapes of the two reactions, are well described.Another striking example of triangle singularities is the one ...

We calculate the S-wave scattering lengths for charmed mesons scattering off Goldstone bosons and explore their quark mass dependence using chiral perturbation theory up to next-to-leading order as well as a unitarized version of it. The quark mass dependence of all scattering lengths determined in a recent lattice calculation can be reproduced by the unitarized version. We also discuss signals of possible bound states in these observables. PACS. 12.39.Fe Chiral Lagrangians -13.75.Lb Meson-meson interactions -14.40.Lb Charmed mesons

The spectrum of hadrons is the manifestation of color confinement of quantum chromodynamics. Hadronic resonances correspond to poles of the S-matrix. Since 2003, lots of new hadron resonant structures were discovered in the mass regions from light mesons to hadrons containing a pair of a heavy quark and an antiquark. Many of them are candidates of exotic hadrons, and they are usually observed as peaks in invariant mass distributions. However, the S-matrix also has kinematical singularities due to the on-shellness of intermediate particles for a process, such as two-body thresholds and triangle singularities (TSs), and they can produce peaks as well. On the one hand, such singularities may be misidentified as resonances; on the other hand, they can be used as tools for precision measurements. In this paper, we review the threshold cusps and various triangle singularities in hadronic reactions, paying attention to their manifestations in phenomena related to exotic hadron candidates. * fkguo@itp.ac.cn † xiaohai.liu@tju.edu.cn ‡ shsakai@itp.ac.cn 1 The X(3872) is named χ c1 (3872) according to its quantum numbers I G (J P C ) = 0 + (1 ++ ) by the PDG [10]. Similarly, the vector charmonium-like states Y (4260) and Y (4660) mentioned below are called ψ(4260) and ψ(4660), respectively. This naming scheme does not mean that the PDG assumes them to be normal cc charmonium states. Here we follow the XY Z naming scheme that is still used in most of the relevant publications.2 A conventional explanation of the observed peaks was given in Refs. [26,27].3 region Z c (4430) [30], Z c (3900) [31,32] and Z c (4020) [33], the charged bottomonium-like structures Z b (10610) and Z b (10650) [34], and the pentaquark candidates with hidden charm P c (4312), P c (4440) and P c (4457) [35,36]. Most of these new structures were observed in the heavy-flavor sector. In particular, the heavy quarkonium-like ones are often called XY Z states in the literature due to the undetermined internal structure. On the one hand, these discoveries enlarged the known QCD spectrum to a large extent; on the other hand, they became a nice showcase of the intricate nonperturbative nature of QCD at low energies 3 : most of them fall off the expectations from quark model, which despite being just a model had provided useful guidance in classifying a large amount of hadrons into various multiplets. Therefore, they are regarded as prominent candidates of exotic hadrons. However, how the spectrum of exotic hadrons should be organized and even what types of exotic hadrons can be well defined are still unclear. Partly because of this, the observation of each of these new structures leads to different models such as compact tetraquarks (or pentaquarks), hadronic molecules, hybrid states, hadro-charmonia, and kinematic effects, etc. Nevertheless, a deeper understanding of how the hadron spectrum, in particular that of the excited hadrons above (or at least close to) strong decay thresholds, is organized can shed light on the color confinement problem of QCD. For th...