The doubly charmed tetraquark $T^+_{ cc}$ recently discovered by the LHCb Collaboration is studied on the basis of $(2+1)$-flavor lattice QCD simulations of the $D^*D$ system with nearly physical pion mass $m_\pi=146$ MeV. The interaction of $D^*D$ in the isoscalar and $S$-wave channel, derived from the hadronic spacetime correlation by the HAL QCD method, is attractive for all distances and leads to a near-threshold virtual state with a pole position $E_\text{ pole}=-59\left(^{ +53}_{ -99}\right)\left(^{ +2}_{ -67}\right)$ keV and a large scattering length $1/a_0=0.05(5)\left(^{ +2}_{ -2}\right)~\text{ fm}^{ -1}$. The virtual state is shown to evolve into a loosely bound state as $m_\pi$ decreases to its physical value by using a potential modified to $m_\pi=135$ MeV based on the pion-exchange interaction. Such a potential is found to give a semiquantitative description of the LHCb data on the $D^0D^0\pi^+$ mass spectrum. Future study is necessary to perform physical-point simulations with the isospin-breaking and open three-body-channel effects taken into account.
Categories: hep-lat hep-ex hep-ph nucl-th