With the discovery of a doubly charmed $\Xi_{ cc}$ baryon a somewhat forgotten issue of tetraquarks containing two heavy and two light (anti) quarks, ${ \cal T}_{ QQ}$, triggered theorist's interest. We discuss quark model estimates of ${ \cal T}_{ QQ}$ masses and such objects including a model where the light sector is treated as a soliton. We show that this model has different large $N_c$ limit than other approaches.
Categories: hep-ph