Pole of the doubly charmed $T_{cc}+$ from lattice QCD
In plain words
The $T_{cc}+$, found in 2021, contains two charm quarks and two light antiquarks and lies just below the energy of a $D*+$ and D0 meson pair. Lattice QCD (QCD computed numerically on a space-time grid) calculations near the real quark masses disagree on whether it is a true bound state or a virtual state (a near-bound state that does not quite bind).
Precise statement
Compute $D D*$ scattering in the $I = 0, J^{P} = 1+$ channel in lattice QCD at physical pion mass, including the left-hand cut from one-pion exchange, with continuum extrapolation, and determine whether T_cc+ is a bound state, virtual state or resonance. Experiment: about 0.36 MeV below the $D*+ D0$ threshold, width about 48 keV. Answer: pole position with uncertainty below 0.2 MeV.
What would settle it
A lattice QCD calculation at the physical point with a finite-volume formalism that handles the one-pion-exchange left-hand cut, extrapolated to the continuum.
Status in the literature
Unverified note
Padmanath and Prelovsek (2022, $m_{\pi}$ about $280\,\mathrm{MeV}$) found a virtual state; HAL QCD (Lyu et al., PRL 2023, $m_{\pi} = 146\,\mathrm{MeV}$, potential method) found a near-threshold virtual state at $E = -59 (+53/-99)\,\mathrm{keV}$; a finite-volume treatment with the one-pion-exchange left-hand cut at the physical point was still lacking as of 2026.