Speaker
Description
Chiral effective field theory (EFT) provides a natural framework to explore the properties of near-threshold exotics with reliable error estimates at both physical and unphysical pion masses, and to connect these regimes via chiral extrapolations once the low-energy constants are fixed. We present a chiral EFT approach for extracting two-body scattering information from finite-volume energy levels obtained in lattice QCD. This framework allows for an explicit incorporation of long-range physics governed by one-pion exchange (OPE) and a model-independent treatment of coupled channels, serving as an alternative to Lüscher's method. We apply the chiral EFT approach to coupled-channel $B^{(*)}\bar D^{(*)}$ scattering to analyse the recent lattice QCD results by Alexandrou et al. [Phys. Rev. Lett. 132, 151902 (2024)] and obtain shallow bound states in both channels, in agreement with the lattice findings [arXiv:2602.02176]. The finite-volume spectra and extracted pole positions show a near-degeneracy in $J=0$ and $J=1$ channels, consistent with heavy-quark spin symmetry (HQSS). Using HQSS, we predict additional shallow bound states near the $B \bar D^*$ and $B^* \bar D^*$ thresholds, which are accessible to future lattice simulations. The corresponding effective range parameters support a molecular interpretation of the $T_{bc}$ state. In contrast to our findings in similar studies of the $T_{cc}^+$, the effect of OPE on the finite volume-spectra is found to be small for $T_{bc}$, with only moderate impact on HQSS partners.