From strings to QCD: The study of doubly heavy hadrons

We will discuss the string theory constructions for the $QQq$ and $QQ\bar q\bar q $systems. Following the hadro-quarkonium picture the potential is a function of separation between the heavy quarks. The results show the universality of the string tension and factorization at small separations expected from heavy quark-diquark symmetry. The potentials look very similar at small quark separations but at larger separations they differ. The reason for this is that the flattening of the potentials happens at two well-separated scales as follows from the two different mechanisms: string breaking by light quarks for QQq and string junction annihilation for $QQ\bar q\bar q$.