Quasi-bound states in the continuum: a dynamical coupled-channel calculation of axial-vector charmed mesons

Understand

Masses and widths of the charmed axial-vector mesons $D_1(2420)$, $D_1(2430)$, $D_{s1}(2536)$, and $D_{s1}(2460)$ are calculated nonperturbatively in the Resonance-Spectrum-Expansion model, by coupling various open and closed meson-meson channels to the bare $J^P=1^+$ $c\bar{q}$ ($q=u,d$) and $c\bar{s}$ states.

• The coupling to two-meson channels dynamically mixes and lifts the mass degeneracy of the spectroscopic $^3P_1$ and $^1P_1$ states, as an alternative to the usual spin-orbit splitting.
• Of the two resulting $S$-matrix poles in either case, one stays very close to the energy of the bare state, as a quasi-bound state in the continuum, whereas the other shifts considerably.
• This is in agreement with the experimental observation that the $D_1(2420)$ and $D_{s1}(2536)$ have much smaller widths than one would naively expect.