Narrow exotic hadrons in the heavy quark limit of QCD

Understand

This paper focuses on tetraquarks containing two heavy quarks in the formal limit where the heavy quark masses are taken to be arbitrarily large.

• There are well-established model-independent arguments for the existence of deeply bound exotic $\bar{q}\bar{q}'QQ$ tetraquark in the formal limit of arbitrarily large heavy quark masses.
• However, these previous arguments did not address the question whether tetraquark states exist close to the threshold for breaking up into two heavy mesons.
• Such states are not stable under strong interactions---they can emit pions and decay to lower-lying tetraquark states.