In Cheung et al [JHEP 1612 (2016) 089, arXiv:1610.01073] we computed spectra of highly excited charmonia and charmed mesons with light (up and down) quark masses, corresponding to m_π ≈ 240 MeV, closer to their physical masses than in our earlier study where m_π ≈ 390 MeV. A highlight of the results was the presence of hybrid mesons (where the gluonic field is excited), some of which had exotic quantum numbers (cannot be solely a quark-antiquark pair). The resulting phenomenology of hybrid mesons gave insight into the effective low-energy gluonic degrees of freedom. One main conclusion was that the spectra showed only mild dependence on the light-quark mass, and the pattern of states, in particular the hybrid mesons, did not change significantly as the light-quark mass was reduced. This is an important systematic check and suggests that the interesting phenomenology of excited and hybrid mesons we found is not an artefact of working with unphysically-heavy light-quark masses.

Figure 1 shows the spectrum of excited D mesons (containing a charm quark and a light antiquark) labelled by J^P (J is spin, P is parity): green and red boxes are the computed masses and one-sigma statistical uncertainties with red highlighting states identified as hybrid mesons; black lines are experimental values.

In Moir et al [JHEP 1610 (2016) 011, arXiv:1607.07093], we presented the first ab-initio study of coupled-channel scattering involving charm mesons (and the first lattice QCD study of three coupled scattering channels). Working with light-quark masses corresponding to m_π ≈ 390 MeV, from computations of finite-volume spectra we determined infinite-volume D_π,D_η,D_s K isospin-1/2 scattering amplitudes. Figure 2 shows the various S-wave (orbital angular momentum = 0) cross sections: upper (lower) panel shows quantities proportional to the diagonal (off-diagonal) cross sections. The singularity structure of the amplitudes showed a near-threshold bound state in J^P=0⁺ corresponding to the physical D₀^*(2400) resonance, a deeply bound state with J^P=1^– corresponding to the D^*, and evidence for a narrow J^P=2⁺ resonance. This study represents a significant step toward addressing some puzzles in the spectroscopy of charmed mesons.