The new LHCb beauty-strange state as the bottom partner of the $D_{s0}^{\ast }(2317)$: a LQCD constrained coupled-channel chiral analysis

The LHCb Collaboration has observed a narrow state in the ${\bar{B}}_s^0{\pi }^0$ spectrum, about 74 MeV below the $B^{-}{K}^{+}$ threshold, which is the natural candidate for the bottom partner of the $D_{s0}^{\ast }(2317)$. Such a state had been predicted as a $\bar{B}K$ bound state by unitarized heavy-meson chiral perturbation theory with lattice-QCD input. We analyze the LHCb spectrum with these coupled-channel amplitudes, including isospin breaking through the physical meson masses, ${\pi }^0$-$\eta$ mixing, and $m_u\mathrm{\ne }{m}_d$ in the chiral amplitudes, thereby allowing the ${\bar{B}}_{s0}^{\ast }$ to decay through its only open strong channel, ${\bar{B}}_s^0{\pi }^0$. With the parameters fixed by lattice QCD and no experimental input from the LHCb measurement, the predicted state lies within 1.1-1.7 standard deviations of the observed peak. Adjusting a single parameter, the spectrum is described as well as with the zero-width limit of the LHCb analysis. The new state thus emerges naturally from these amplitudes, with a large $\bar{B}K$ component and an isospin-violating width of tens of keV, and its $J^P=1^{+}$ heavy-quark spin partner is predicted with a precision comparable to that of the measured mass.