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The analyses of the LHCb data on X(2900) in the D^- K^+ spectrum are performed. Both dynamically generated and explicitly introduced X_1(2900) are considered. The results show that both these two approaches support the interpretation of X_1(2900) as a bar{D}_1 K molecular state, with J^{P}=1^- and an iso-singlet interpretation is much more favorable. The effect of triangle singularity on the production of X_1(2900) is also discussed, and it is found that it cannot be interpreted as a pure triangle cusp.
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The LHCb collaboration reported the observation of a narrow peak in the $D^- K^+$ invariant mass distributions from the $B^+to D^+ D^- K^+$ decay. The peak is parameterized in terms of two resonances $X_0(2900)$ and $X_1(2900)$ with the quark contents $bar{c}bar{s}ud$, and their spin-parity quantum numbers are $0^+$ and $1^-$, respectively. We investigate the rescattering processes which may contribute to the $B^+to D^+ D^- K^+$ decays. It is shown that the $D^{*-}K^{*+}$ rescattering via the $chi_{c1}K^{*+}D^{*-}$ loop or the $bar{D}_{1}^{0}K^{0}$ rescattering via the $D_{sJ}^{+}bar{D}_{1}^{0}K^{0}$ loop simulate the $X_0(2900)$ and $X_1(2900)$ structures. Such phenomena are due to the analytical property of the scattering amplitudes with the triangle singularities located to the vicinity of the physical boundary.
We interpret the $B_{s1}(5778)$ as an $S$-wave $B^astbar{K}$ molecular state in the Bethe-Salpeter equation approach. In the ladder and instantaneous approximations, and with the kernel containing one-particle-exchange diagrams and introducing three different form factors (monopole, dipole, and exponential form factors) in the vertex, we find the bound state exists. We also study the decay widths of the decay $B_{s1}(5778)rightarrow B_s^astpi$ and the radiative decays $B_{s1}(5778)rightarrow B_sgamma$ and $B_{s1}(5778)rightarrow B_s^{ast}gamma$, which will be instructive for the forthcoming experiments.
Recently, the LHCb Collaboration reported on the evidence for a hidden charm pentaquark state with strangeness, i.e., $P_{cs}(4459)$, in the $J/psiLambda$ invariant mass distribution of the $Xi_b^-to J/psi Lambda K^-$ decay. In this work, assuming that $P_{cs}(4459)$ is a $bar{D}^*Xi_c$ molecular state, we study this decay via triangle diagrams $Xi_brightarrow bar{D}_s^{(*)}Xi_cto (bar{D}^{(*)}bar{K})Xi_cto P_{cs} bar{K}to (J/psiLambda) bar{K}$. Our study shows that the production yield of a spin 3/2 $bar{D}^*Xi_c$ state is approximately one order of magnitude larger than that of a spin $1/2$ state due to the interference of $bar{D}_sXi_c$ and $bar{D}_s^*Xi_c$ intermediate states. We obtain a model independent constraint on the product of couplings $g_{P_{cs}bar{D}^*Xi_c}$ and $g_{P_{cs}J/psiLambda}$. With the predictions of two particular molecular models as inputs, we calculate the branching ratio of $Xi_b^-to (P_{cs}to)J/psiLambda K^- $ and compare it with the experimental measurement. We further predict the lineshape of this decay which could be useful to future experimental studies.
We revisit, improve and complete some recent estimates of the $0^{+}$ and $1^-$ open charm $(bar c bar d)(us)$ tetraquarks and the corresponding molecules masses and decay constants from QCD spectral sum rules (QSSR) by using QCD Laplace sum rule (LSR) within stability criteria where the factorised perturbative NLO corrections and the contributions of quark and gluon condensates up to dimension-6 in the OPE are included. We confront our results with the $D^-K^+$ invariant mass recently reported by LHCb from $B^+to D^+(D^-K^+)$ decays. We expect that the bump near the $D^-K^+$ threshold can be originated from the $0^{++}(D^-K^+)$ molecule and/or $D^-K^+$ scattering. The prominent $X_{0}$(2900) scalar peak and the bump $X_J(3150)$ (if $J=0$) can emerge from a {it minimal mixing model}, with a tiny mixing angle $theta_0simeq (5.2pm 1.9)^0$, between a scalar {it Tetramole} (${cal T_M}_0$) (superposition of nearly degenerated hypothetical molecules and compact tetraquarks states with the same quantum numbers) having a mass $M_{{cal T_M}_0}$=2743(18) MeV and the first radial excitation of the $D^-K^+$ molecule with mass $M_{(DK)_1}=3678(310)$ MeV. In an analogous way, the $X_1$(2900) and the $X_J(3350)$ (if $J=1$) could be a mixture between the vector {it Tetramole} $({cal T_M}_1)$ with a mass $M_{{cal T_M}_1}=2656(20)$ MeV and its first radial excitation having a mass $M_{({cal T_M}_1)_1}=4592(141)$ MeV with an angle $theta_1simeq (9.1pm 0.6)^0$. A (non)-confirmation of the previous {it minimal mixing models} requires an experimental identification of the quantum numbers of the bumps at 3150 and 3350 MeV.
We investigate the decay and the production mechanism of the resonance X(1812) recently observed in the $jptogamma X(1812), X(1812)toomegaphi$ at BESII. The decay widths of $X(1812)toetaeta$,$ etaeta$,$ omegaphi$,$ K^+K^-$,$ rho^+rho^-$, $ omegaomega$,$ K^{*+}K^{*-}$ and $pi^+pi^-$ are evaluated based on the scenario of the X(1812) as a candidate of $(ksks)$ molecule. It turns out that the quark exchange mechanism plays an important role in the understanding of the large decay width for the $X(1812)toomegaphi$. It is also found that the decay widths for $X(1812)toetaeta$ and $etaeta$ are enhanced by the quark exchange mechanism. These channels are suggested to be the tools to test the molecular scenario in experiment. The branching fraction of $Br(Xtoomegaphi)$ is evaluated to be about 4.60%. Searches for additional evidence about the X(1812) in $jp$ radiative decays are reviewed. In the molecular scenario, the X(1812) production rate is also evaluated to be $Gamma(jptogamma X)/Gamma(jptogamma K^{*+}K^{*-})=2.13^{+7.41}_{-1.85}$, which is close to the measured value $2.83pm0.92$.
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