Doubly charmed tetraquark T₂₂^+ in (2+1) -flavor QCD near physical point

We study the doubly charmed tetraquark state T₂₂^+ by the HAL QCD method applied to the D^*D system in (2+1) flavor lattice QCD at nearly physical pion mass, m_¥pi= 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¥ₑ₌ ₏₎₋₄ = -59 (^+53-₉₉) (^+2-₆₇) keV and an inverse scattering length 1/a₀=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_¥pi =135 MeV. The potential at the physical pion mass is shown to give a semi-quantitative description of the D⁰ D⁰¥pi^+ mass spectrum at the LHCb.