Heavy quark hadrons and exotics a challenge for QCD

Exotic hadrons with heavy quarks can confirm in the same multiquark system that the qq̄ interaction is much stronger than the qq interaction and produces color structures totally different from those of normal hadrons. Unusual heavy tetraquark structure may explain unusual properties of newly discovered mesons like the X(3872). Both charged and neutral B decays produce this narrow neutral resonant state that decays to both J/ψρ and J/ψω, while no charged resonances in the same multiplet are found. This suggests that the X is an isoscalar resonance whose production conserves isospin, while isospin is violated only in the decay by an electromagnetic interaction allowing the isospin-forbidden J/ψρ decay. New data for X production in B decays provide a separation between production and decay, sharpen several experimental puzzles and impose serious constraints on all models. A proposed isoscalar tetraquark model agrees with all present data, conserves isospin in its production and breaks isospin only in an electromagnetic X(3872)→J/ψρ^o decay. The narrow X decay width results from the tiny phase space available for the J/ψω decay and enables competition with the electromagnetic isospin-forbidden J/ψρ decay which has much larger phase space. Experimental tests are proposed for this isospin production invariance.