The doubly heavies: Q̄Qq̄q and QQq̄q̄ 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 → ∞, M _Q → ∞ the Q̄Qq̄q tetraquarks (TQ-s) are expected to be narrow and slightly below or above the Q̄ and Qq̄ two-meson threshold. The isoscalar TQ-s manifest themselves by decay to Q̄Q ππ, and the ∼30 MeV heavier charged isotriplet TQ-s by decays into (Q̄Q) π. 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 also consider "baryon-like" states QQqq, 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.