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I discuss the properties of some representative XYZ mesons in the context of the most commonly proposed models for their underlying nature.

With large J /ψ and ψ(2 S ) data samples, the BESIII Collaboration has performed many studies toward conventional charmonium states below energy 3.686 GeV, which provide information on the internal structure and the interactions of the mesons. In this proceeding, just a few of them, related to (1) OZI suppressed decay search or measurement, (2) “12% rule” test, and (3) theoretical different predictions test, is focused on.

Two-ordinary-meson scattering in large-Nc QCD implies consistency criteria for intermediate-tetraquark contributions. Their fulfilment at Nc-leading order constrains the nature of the spectrum of genuinely exotic tetraquark states.

We discuss the necessity and the emergence of quantum reference frames when at tempting to define a rest frame for mixed particles. We analyze the corresponding concept of frame dependent entanglement and how it could affect measurements on mixed mesons and neutrinos [1].

We show that the one-loop chiral corrections for heavy light mesons in the potential model can explain the small mass of Ds(2317) as well as the small mass gap between Ds(2317) and D(2308).

We review results from the Jefferson Laboratory (JLab) program with the 12 GeV Upgrade of the CEBAF accelerator, with a particular emphasis on processes involving mesons. We include associated efforts in theory and phenomenology. We also discuss expected results from future analyses and data taking, including new instrumentation, in particular SoLID and the planned 22 GeV Upgrade of CEBAF.

We study the \\[J/\\psi \\rightarrow K^+ K^- f_0(980)(a_0(980))\\] reaction and find that the mechanism to produce this decay develops a triangle singularity around \\[M_{\\mathrm{inv}}(K^- f_0/K^- a_0) \\approx 1515\\] MeV. The differential width \\[d\\varGamma / dM_{\\mathrm{inv}}(K^- f_0/K^- a_0)\\] shows a rapid growth around the invariant mass being 1515 MeV as a consequence of the triangle singularity of this mechanism, which is directly tied to the nature of the \\[f_0(980)\\] and \\[a_0(980)\\] as dynamically generated resonances from the interaction of pseudoscalar mesons. The branching ratios obtained for the \\[J/\\psi \\rightarrow K^+ K^- f_0(980)(a_0(980))\\] decays are of the order of \\[10^{-5}\\], accessible in present facilities, and we argue that their observation should provide relevant information concerning the nature of the low-lying scalar mesons.

We search for ψ(3770)→π+π−η, K+K−η, \\(p\\bar{p}\\eta\\), ρ0π+π−η, K+K−π+π−η, \\(p\\bar{p}\\pi^{+}\\pi^{-}\\eta\\), \\(p\\bar{p}K^{+}K^{-}\\eta\\) and \\(p\\bar{p}K^{+}K^{-}\\pi^{0}\\) using data samples of 17.3 and 6.5 pb−1 integrated luminosities recorded at the center-of-mass energies of 3.773 and 3.65 GeV, respectively, by the BES-II detector operating at the BEPC collider. We obtain cross section measurements at both energies and upper limits on ψ(3770) decay branching fractions to the final states studied.

A bstract We provide results for the full set of form factors describing semileptonic B -meson transitions to pseudoscalar mesons π , K , D ¯ and vector mesons ρ , K ∗ , D ¯ ∗ . Our results are obtained within the framework of QCD Light-Cone Sum Rules with B -meson distribution amplitudes. We recalculate and confirm the results for the leading-twist two-particle contributions in the literature. Furthermore, we calculate and provide new expressions for the two-particle contributions up to twist four. Following new developments for the three-particle distribution amplitudes, we calculate and provide new results for the complete set of three-particle contributions up to twist four. The form factors are computed numerically at several phase space points using up-to-date input parameters, including correlations across phase space points and form factors. We use a model ansatz for all contributing B -meson distribution amplitudes that is self-consistent up to twist-four accuracy. We find that the higher-twist two-particle contributions have a substantial impact on the results, and dominate over the three-particle contributions. Our numerical results, including correlations, are provided as machine-readable files in the supplementary material. We discuss the qualitative phenomenological impact of our results on the present b anomalies.

In this work, we interpret the newly observed η[sub.1](1855) resonance with exotic J[sup.PC]=1[sup.−+] quantum numbers in the I=0 sector, reported by the BESIII Collaboration, as a dynamically generated state from the interaction between the lightest pseudoscalar mesons and axial-vector mesons. The interaction is derived from the lowest order chiral Lagrangian from which the Weinberg–Tomozawa term is obtained, describing the transition amplitudes among the relevant channels, which are then unitarized using the Bethe–Salpeter equation, according to the chiral unitary approach. We evaluate the η[sub.1](1855) decays into the ηη[sup.′] and KK¯[sup.*]π channels and find that the latter has a larger branching fraction. We also investigate its SU(3) partners, and according to our findings, the π[sub.1](1400) and π[sub.1](1600) structures may correspond to dynamically generated states, with the former one coupled mostly to the b[sub.1]π component and the latter one coupled to the K[sub.1](1270)K¯ channel. In particular, our result for the ratio Γ(π[sub.1](1600)→f[sub.1](1285)π)/Γ(π[sub.1](1600)→η[sup.′]π) is consistent with the measured value, which supports our interpretation for the higher π[sub.1] state. We also report two poles with a mass about 1.7 GeV in the I=1/2 sector, which may be responsible for the K[sup.*](1680). We suggest searching for two additional η[sub.1] exotic mesons with masses around 1.4 and 1.7 GeV. In particular, the predicted η[sub.1](1700) is expected to have a width around 0.1 GeV and can decay easily into KK¯ππ.