Doubly charmed tetraquark T_cc^+ in (2+1)-flavor QCD near physical point

We study the doubly charmed tetraquark state T_cc^+ by the HAL QCD method applied to the D^*D system in (2+1) flavor lattice QCD at nearly physical pion mass, m_π= 146 MeV. We obtain the attractive potential at all distances in the S-wave of the isoscalar D^* D system, whose long distance behavior is well described by the two-pion exchange (TPE), and it generates a virtual pole near D^* D threshold with a pole position E_ pole = -59 (^+53_-99) (^+2_-67) keV and an inverse scattering length 1/a_0=0.05(5)(^+2_-2) fm^-1. The virtual pole turns into a loosely bound state pole if the pion mass in the TPE potential is extrapolated to the physical value, m_π =135 MeV. The potential at the physical pion mass is shown to give a semi-quantitative description of the D^0 D^0π^+ mass spectrum at the LHCb.