Coupled Channel Effects of the $Sigma_c^{(*)}bar{D}^{(*)}$-$Lambda_c(2595)bar{D}$ System and Molecular Nature of the $P_c$ Pentaquark States from One-Boson Exchange Model

The effects of the $Sigma_cbar{D}^*$-$Lambda_{c}(2595)bar{D}$ coupled-channel dynamics and various one-boson-exchange (OBE) forces for the LHCb pentaquark states, $P_c(4440)$ and $P_c(4457)$, are reinvestigated. Both the pion and $rho$-meson exchanges are considered for the $Sigma_cbar{D}^*$-$Lambda_{c}(2595)bar{D}$ coupled-channel dynamics. It is found that the role of the $Lambda_{c}(2595)bar{D}$ channel in the descriptions of the $P_c(4440)$ and $P_c(4457)$ states is not significant with the OBE parameters constrained by other experimental sources. The naive OBE models with the short-distance $delta(vec{r})$ term of the one-pion exchange (OPE) kept fail to reproduce the $P_c(4440)$ and $P_c(4457)$ states simultaneously. The OPE potential with the full $delta(vec{r})$ term results in a too large mass splitting for the $J^P=1/2^-$ and $3/2^-$ $Sigma_cbar{D}^*$ bound states with total isospin $I=1/2$. The OBE model with only the OPE $delta(vec{r})$ term dropped may fit the splitting much better, but somewhat underestimates the splitting. Since the $delta(vec r)$ potential is from short-distance physics, which also contains contributions from the exchange of mesons heavier than those considered explicitly, we vary the strength of the $delta(vec r)$ potential and find that the masses of the $P_c(4312)$, $P_c(4440)$, and $P_c(4457)$ can be reproduced simultaneously with the $delta(vec r)$ term in the OBE model reduced by about 80%. While two different spin assignments are possible to produce their masses, in the preferred description the spin-parities of the $P_c(4440)$ and $P_c(4457)$ are $3/2^-$ and $1/2^-$, respectively.

A survey of heavy-heavy hadronic molecules

The spectrum of hadronic molecules composed of heavy-antiheavy charmed hadrons has been obtained in our previous work. The potentials are constants at the leading order, which are estimated from resonance saturation. The experimental candidates of hadronic molecules, say $X(3872)$, $Y(4260)$, three $P_c$ states and $P_{cs}(4459)$, fit the spectrum well. The success in describing the pattern of heavy-antiheavy hadronic molecules stimulates us to give more predictions for the heavy-heavy cases, which are less discussed in literature than the heavy-antiheavy ones. Given that the heavy-antiheavy hadronic molecules, several of which have strong experimental evidence, emerge from the dominant constant interaction from resonance saturation, we find that the existence of many heavy-heavy hadronic molecules is natural. Among these predicted heavy-heavy states we highlight the $DD^*$ molecule and the $D^{(*)}Sigma_c^{(*)}$ molecules, which are the partners of famous $X(3872)$ and $P_c$ states. Quite recently, LHCb collaboration reported a doubly charmed tetraquark state, $T_{cc}$, which is in line with our results for the $DD^*$ molecule. With the first experimental signal of this new kind of exotic states, the upcoming update of the LHCb experiment as well as other experiments will provide more chances of observing the heavy-heavy hadronic molecules.

Is the existence of a $J/psi J/psi$ bound state plausible?

In a recent measurement LHCb reported pronounced structures in the $J/psi J/psi$ spectrum. One of the various possible explanations of those is that they emerge from non-perturbative interactions of vector charmonia. It is thus important to understand whether it is possible to form a bound state of two charmonia interacting through the exchange of gluons, which hadronise into two pions at the longest distance. In this paper, we demonstrate that, given our current understanding of hadron-hadron interactions, the exchange of correlated light mesons (pions and kaons) is able to provide sizeable attraction to the di-$J/psi$ system, and it is possible for two $J/psi$ mesons to form a bound state. As a side result we find from an analysis of the data for the $psi(2S)to J/psi pipi$ transition including both $pipi$ and $Kbar K$ final state interactions an improved value for the $psi(2S)to J/psi$ transition chromo-electric polarisability: $|alpha_{psi(2S)J/psi}|= (1.8pm 0.1)~mbox{GeV}^{-3}$, where the uncertainty also includes the one induced by the final state interactions.

Confirmation of the existence of an exotic state in the $pi D$ system

In recent years many candidates for states beyond the most simple realization of the quark model were found in various experiments around the world. However, so far no consensus exists on their structure, although there is strong evidence that at least some of those are dynamically generated from meson-meson interactions. In this Letter we provide an important missing piece from the theoretical side to prove that the lightest open charm strange and non-strange scalars $D_{s0}^*(2317)$ and $D_0^*$ as well as their axial-vector partner states can all be understood as emerging from the interactions between Goldstone bosons stemming from the spontaneous breaking of chiral symmetry and the ground state charmed mesons. For that purpose we exploit the flavor multiplet structure of the lightest open-charm positive-parity scalar states in an SU(3) symmetric lattice QCD simulation at large pion masses to establish that there exists a bound state in the flavor-sextet representation, which cannot emerge for quark-antiquark states but appears naturally for four-quark configurations. Moreover, we find repulsion in the $[15]$ representation and thus no single-particle state in this representation exists, falsifying the expectation for tetraquark models. The findings establish the pattern predicted for the interactions of Goldstone bosons with $D$ mesons from chiral symmetry and the paradigm of the lowest-lying positive-parity charmed mesons as dynamically generated states.

A survey of heavy-antiheavy hadronic molecules

Many efforts have been made to reveal the nature of the overabundant resonant structures observed by the worldwide experiments in the last two decades. Hadronic molecules attract special attention because many of these seemingly unconventional resonances are located close to the threshold of a pair of hadrons. To give an overall feature of the spectrum of hadronic molecules composed of a pair of heavy-antiheavy hadrons, namely, which pairs are possible to form molecular states, we take charmed hadrons for example to investigate the interaction between them and search for poles by solving the Bethe-Salpeter equation. We consider all possible combinations of hadron pairs of the $S$-wave singly-charmed mesons and baryons as well as the narrow $P$-wave charmed mesons. The interactions, which are assumed to be meson-exchange saturated, are described by constant contact terms which are resummed to generate poles. It turns out that if a system is attractive near threshold by the light meson exchange, there is a pole close to threshold corresponding to a bound state or a virtual state, depending on the strength of interaction and the cutoff. In total, 229 molecular states are predicted. The observed near-threshold structures with hidden-charm, like the famous $X(3872)$ and $P_c$ states, fit into the spectrum we obtain. We also highlight a $Lambda_cbar Lambda_c$ bound state that has a pole consistent with the cross section of the $e^+e^-toLambda_cbar Lambda_c$ precisely measured by the BESIII Collaboration.

$D^{pm}D^{*mp}$ Hadronic Atom as a Key to Revealing the $X(3872)$ Mystery

The $X(3872)$, whose mass coincides with the $D^0bar D^{*0}$ threshold, is the most extended hadron object. Since its discovery in 2003, debates have never stopped regarding its internal structure. We propose a new object, the $X$ atom, which is the $D^pm D^{*mp}$ composite system with positive charge parity and a mass of $(3879.89pm0.07)$ MeV, formed mainly due to the Coulomb force. We show that a null signal of the $X$ atom can be used to put a lower limit on the binding energy of the $X(3872)$. From the current knowledge of the $X(3872)$ properties, the production rate for the $X$ atom relative to the $X(3872)$ in $B$ decays and at hadron colliders should be at least $1times10^{-3}$. New insights into the $X(3872)$ will be obtained through studying the $X$ atom.

Explaining the Many Threshold Structures in the Heavy-Quark Hadron Spectrum

Tremendous progress has been made experimentally in the hadron spectrum containing heavy quarks in the last two decades. It is surprising that many resonant structures are around thresholds of a pair of heavy hadrons. There should be a threshold cusp at any $S$-wave threshold. By constructing a nonrelativistic effective field theory with open channels, we discuss the generalities of threshold behavior, and offer an explanation of the abundance of near-threshold peaks in the heavy quarkonium regime. We show that the threshold cusp can show up as a peak only for channels with attractive interaction, and the width of the cusp is inversely proportional to the reduced mass relevant for the threshold. We argue that there should be threshold structures at any threshold of a pair of heavy-quark and heavy-antiquark hadrons, which have attractive interaction at threshold, in the invariant mass distribution of a heavy quarkonium and light hadrons that couple to that open-flavor hadron pair. The structure becomes more pronounced if there is a near-threshold pole. Predictions of the possible pairs are also given for the ground state heavy hadrons. Precisely measuring the threshold structures will play an important role in revealing the heavy-hadron interactions, and thus understanding the puzzling hidden-charm and hidden-bottom structures.

Two-Meson Form Factors in Unitarized Chiral Perturbation Theory

We present a comprehensive analysis of form factors for two light pseudoscalar mesons induced by scalar, vector, and tensor quark operators. The theoretical framework is based on a combination of unitarized chiral perturbation theory and dispersion relations. The low-energy constants in chiral perturbation theory are fixed by a global fit to the available data of the two-meson scattering phase shifts. Each form factor derived from unitarized chiral perturbation theory is improved by iteratively applying a dispersion relation. This study updates the existing results in the literature and explores those that have not been systematically studied previously, in particular the two-meson tensor form factors within unitarized chiral perturbation theory. We also discuss the applications of these form factors as mandatory inputs for low-energy phenomena, such as the semi-leptonic decays $B_sto pi^+pi^-ell^+ell^-$ and the $tau$ lepton decay $taurightarrowpi^{-}pi^{0} u_{tau}$, in searches for physics beyond the Standard Model.

Possible precise measurements of the $X(3872)$ mass with the $e^+e^-topi^0gamma X(3872)$ and $pbar ptogamma X(3872)$ reactions

It was recently proposed that the $X(3872)$ binding energy, the difference between the $D^0bar D^{*0}$ threshold and the $X(3872)$ mass, can be precisely determined by measuring the $gamma X(3872)$ line shape from a short-distance $D^{*0}bar D^{*0}$ source produced at high-energy experiments. Here, we investigate the feasibility of such a proposal by estimating the cross sections for the $e^+e^-topi^0gamma X(3872)$ and $pbar ptogamma X(3872)$ processes considering the $D^{*0}bar D^{*0}D^0/bar D^{*0}D^{*0}bar D^0$ triangle loops. These loops can produce a triangle singularity slightly above the $D^{*0}bar D^{*0}$ threshold. It is found that the peak structures originating from the $D^{*0}bar D^{*0}$ threshold cusp and the triangle singularity are not altered much by the energy dependence introduced by the $e^+e^-topi^0D^{*0}bar D^{*0}$ and $pbar ptobar D^{*0}D^{*0}$ production parts or by considering a finite width for the $X(3872)$. We find that $sigma(e^+e^-topi^0gamma X(3872)) times {rm Br}(X(3872)topi^+pi^-J/psi)$ is $mathcal{O}(0.1~{rm fb})$ with the $gamma X(3872)$ invariant mass integrated from 4.01 to 4.02 GeV and the c.m. energy of the $e^+e^-$ pair fixed at 4.23 GeV. The cross section $sigma(pbar ptogamma X(3872))times {rm Br}(X(3872)topi^+pi^-J/psi)$ is estimated to be of $mathcal{O}(10~{rm pb})$. Our results suggest that a precise measurement of the $X(3872)$ binding energy can be done at PANDA.

Exploring possible triangle singularities in the $Xi^-_{b} to K^- J/psi Lambda$ decay

We analyze possible singularities in the $J/psi Lambda$ invariant mass distribution of the $Xi^-_{b}~to~K^- J/psi Lambda$ process via triangle loop diagrams. Triangle singularities in the physical region are found in 18 different triangle loop diagrams. Among those with $Xi^*$-charmonium-$Lambda$ intermediate states, the one from the $chi_{c1} Xi(2120) Lambda$ loop, which is located around 4628 MeV, is found the most likely to cause observable effects. One needs $S$- and $P$-waves in $chi_{c1} Lambda$ and $J/psi Lambda$ systems, respectively, when the quantum numbers of these systems are $1/2^+$ or $3/2^+$. When the quantum numbers of the $Xi(2120)$ are $J^P=1/2^+$, $1/2^-$ or $3/2^+$, the peak structure should be sharper than the other $J^P$ choices. This suggests that although the whole strength is unknown, we should pay attention to the contributions from the $Xi^*$-charmonium-$Lambda$ triangle diagram if structures are observed in the $J/psi Lambda$ invariant mass spectrum experimentally. In addition, a few triangle diagrams with the $D_{s1}^*(2700)$ as one of the intermediate particles can also produce singularities in the $J/psiLambda$ distribution, but at higher energies above 4.9 GeV.