The newly observed hidden-charm tetraquark state $Z_{cs}(3985)$, together with $Z_c(3900)$ and $X(4020)$, are studied in the combined theoretical framework of the effective range expansion, compositeness relation and the decay width saturation. The e
lastic effective-range-expansion approach leads to sensible results for the scattering lengths, effective ranges and the compositeness coefficients, $i.e.$, the probabilities to find the two-charm-meson molecule components in the tetraquark states. The coupled-channel formalism by including the $J/psipi$ and $Dbar{D}^*/bar{D}D^*$ to fulfill the constraints of the compositeness relation and the decay width, confirms the elastic effective-range-expansion results for the $Z_c(3900)$, by using the experimental inputs for the ratios of the decay widths between $Dbar{D}^*/bar{D}D^*$ and $J/psipi$. With the results from the elastic effective-range-expansion study as input for the compositeness, we generalize the discussions to the $Z_{cs}(3985)$ by including the $J/psi K^{-}$ and $D_s^{-}D^{*0}/D_s^{*-}D^{0}$, and predict the partial decay widths of the $J/psi K^{-}$. Similar calculations are also carried out for the $X(4020)$ by including the $h_cpi$ and $D^*bar{D}^*$, and the partial decay widths of the $h_cpi$ is predicted. Our results can provide useful guidelines for future experimental measurements.
The recently discovered fully charmed tetraquark candidate $X(6900)$ is analyzed within the frameworks of effective-range expansion, compositeness relation and width saturation, and a coupled multichannel dynamical study. By taking into account const
raints from heavy-quark spin symmetry, the coupled-channel amplitude including the $J/psi J/psi,~ chi_{c0}chi_{c0}$ and $chi_{c1}chi_{c1}$ is constructed to fit the experimental di-$J/psi$ event distributions around the energy region near $6.9$ GeV. Another dynamical two-coupled-channel amplitude with the $J/psi J/psi$ and $psi(3770) J/psi$ is also considered to describe the same datasets. The three different theoretical approaches lead to similar conclusions that the two-meson components do not play dominant roles in the $X(6900)$. Our determinations of the resonance poles in the complex energy plane from the refined coupled-channel study are found to be consistent with the experimental analyses. The coupled-channel amplitudes also have another pole corresponding to a narrow resonance $X(6825)$ that we predict sitting below the $chi_{c0}chi_{c0}$ threshold and of molecular origin. We give predictions to the line shapes of the $chi_{c0}chi_{c0}$ and $chi_{c1}chi_{c1}$ channels, which could provide a useful guide for future experimental measurements.
We study the newly reported hidden-charm pentaquark candidates $P_c(4312)$, $P_c(4440)$ and $P_c(4457)$ from the LHCb Collaboration, in the framework of the effective-range expansion and resonance compositeness relations. The scattering lengths and e
ffective ranges from the $S$-wave $Sigma_cbar{D}$ and $Sigma_cbar{D}^*$ scattering are calculated by using the experimental results of the masses and widths of the $P_c(4312)$, $P_c(4440)$ and $P_c(4457)$. Then we calculate the couplings between the $J/psi p,,Sigma_cbar{D}$ channels and the pentaquark candidate $P_c(4312)$, with which we further estimate the probabilities of finding the $J/psi p$ and $Sigma_cbar{D}$ components inside $P_c(4312)$. The partial decay widths and compositeness coefficients are calculated for the $P_c(4440)$ and $P_c(4457)$ states by including the $J/psi p$ and $Sigma_cbar{D}^*$ channels. Similar studies are also carried out for the three $P_c$ states by including the $Lambda_cbar{D}^{*}$ and $Sigma_cbar{D}^{(*)}$ channels.
We use the chiral effective field theory to study the lattice finite-volume energy levels from the meson-meson scattering. The hadron resonance properties and the scattering amplitudes at physical masses are determined from the lattice energy levels
calculated at unphysically large pion masses. The results from the $pieta, Kbar{K}$ and $pieta$ coupled-channel scattering and the $a_0(980)$ resonance are explicitly given as a concrete example.
Within the framework of the effective Lagrangian approach, we perform a thorough analysis of the $J/psi to Pgamma(gamma^*)$, $J/psi to VP$, $Vto Pgamma(gamma^*)$, $Pto Vgamma(gamma^*)$ and $Ptogammagamma(gamma^*)$ processes, where $V$ stand for light
vector resonances, $P$ stand for light pseudoscalar mesons, and $gamma^*$ subsequently decays into lepton pairs. The processes with light pseudoscalar mesons $eta$ and $eta$ are paid special attention to and the two-mixing-angle scheme is employed to describe their mixing. The four mixing parameters both in singlet-octet and quark-flavor bases are updated in this work. We confirm that the $J/psi to eta(eta^{prime})gamma^{(*)}$ processes are predominantly dominated by the $J/psito eta_c gamma^{*} to eta(eta^{prime})gamma^{(*)}$ mechanism. Predictions for the $J/psi to P mu^+mu^-$ are presented. A detailed discussion on the interplay between electromagnetic and strong transitions in the $J/psi to VP$ decays is given.
