The doubly heavies: QQ(q)over-barq and QQ(q)over-bar(q)over-bar tetraquarks and QQq baryons

Recent discoveries by Belle and BESIII of charged exotic quarkonium-like resonances provide fresh impetus for study of heavy exotic hadrons. In the limit N-c -> infinity, M-Q -> infinity the (Q) over barQ (q) over barq' tetraquarks (TQ-s) are expected to be narrow and slightly below or above the (Q) over barq' and Q (q) over bar two-meson threshold. The isoscalar TQ-s manifest themselves by decay to (QQ) pi pi, and the similar to 30 MeV heavier charged isotriplet TQ-s by decays into ((Q) over barQ)pi. The new data strongly suggest that the real world with N-c = 3, Q = c, b and q,q' = u, d is qualitatively described by the above limit. We discuss the relevant theoretical estimates and suggest new signatures for TQ-s in light of the recent discoveries. We alsoconsider "baryon-like" states QQ'(q) over bar(q) over bar', which if found will be direct evidence not just for near-threshold binding of two heavy mesons, but for genuine tetraquarks with novel color networks. We stress the importance of experimental search for doubly-heavy baryons in this context.