Lattice perspectives on doubly heavy tetraquarks

Doubly heavy tetraquarks have emerged as new probes to study the heavy hadron spectrum. With the experimental observation of the J P = 1 + T c c + , they pose a unique opportunity to bring together efforts in experiment, phenomenology, and lattice QCD . In lattice calculations they are accessible as ground states , unlike hidden flavor tetraquarks, and this enables accurate determinations of the scattering parameters alongside the binding energies of these tetraquarks. Today, lattice calculations firmly predict J P = 1 + T b b u d and T b b u s as QCD bound states, while recent studies approaching the J P = 1 + T c c u d find it to be a virtual bound state at slightly non-physical input quark masses. Studies of the J P = 1 + T b c u d are ongoing and a new focus area. In light of these developments the evolution of this field until this point is reviewed. Emphasis is put on the methods in lattice spectroscopy that enable a robust evaluation of the lattice studies gathered. They are further reviewed towards their limitations and achievements. Current challenges and opportunities are discussed, including possibilities to approach the left-hand cut in the scattering analysis of the charm candidates and towards understanding the structure of those including two bottom quarks.