The LHCb collaboration recently reported the observation of a narrow peak in the $$D^- K^+$$ D - K + invariant mass distributions from the $$B^+\rightarrow D^+ D^- K^+$$ B + → D + D - K + decay. The peak is parameterized in terms of two resonances $$X_0(2900)$$ X 0 ( 2900 ) and $$X_1(2900)$$ X 1 ( 2900 ) with the quark contents $${\bar{c}}{\bar{s}}ud$$ c ¯ s ¯ u d , and their spin-parity quantum numbers are $$0^+$$ 0 + and $$1^-$$ 1 - , respectively. We investigate the rescattering processes which may contribute to the $$B^+\rightarrow D^+ D^- K^+$$ B + → D + D - K + decays. It is shown that the $$D^{*-}K^{*+}$$ D ∗ - K ∗ + rescattering via the $$\chi _{c1}K^{*+}D^{*-}$$ χ c 1 K ∗ + D ∗ - loop and the $${\bar{D}}_{1}^{0}K^{0}$$ D ¯ 1 0 K 0 rescattering via the $$D_{sJ}^{+}{\bar{D}}_{1}^{0}K^{0}$$ D sJ + D ¯ 1 0 K 0 loop can simulate the $$X_0(2900)$$ X 0 ( 2900 ) and $$X_1(2900)$$ X 1 ( 2900 ) with consistent quantum numbers. Such phenomena are due to the analytical property of the scattering amplitudes with the triangle singularities located to the vicinity of the physical boundary.

The discovery of Ξ ++ cc by the LHCb Collaboration triggers predictions of more doubly charmed baryons. By taking into account both the P -wave excitations between the two charm quarks and the scattering of light pseudoscalar mesons off the ground state doubly charmed baryons, a set of negative-parity spin-1/2 doubly charmed baryons are predicted already from a unitarized version of leading order chiral perturbation theory. Moreover, employing heavy antiquark-diquark symmetry the relevant low-energy constants in the next-to-leading order are connected with those describing light pseudoscalar mesons scattering off charmed mesons, which have been well determined from lattice calculations and experimental data. Our calculations result in a spectrum richer than that of heavy mesons. We find two very narrow J P = 1/2 − Ω P cc , which very likely decay into Ωccπ 0 breaking isospin symmetry. In the isospin-1/2 Ξ P cc sector, three states are predicted to exist below 4.2 GeV with the lowest one being narrow and the other two rather broad. We suggest to search for the Ξ P cc states in the Ξ ++ cc π − mode. Searching for them and their analogues are helpful to establish the hadron spectrum.

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