"...., van Beveren discussed the effect of coupling the discrete, stable hadron basis states of the naive quark model to the two-hadron continuum. This program, often referred to as unquenching the quark model, is an extremely important topic in hadron physics. It is clear that coupling naive quark model states (or quenched lattice QCD states) to the continuum of real and virtual decay channels will induce many important effects, such as mass shifts, nonzero decay widths, and modified electromagnetic couplings due to continuum components in the state vector."

"In their contribution [2] to HADRON05, van Beveren et al. describe a T-matrix formalism for iterating the effect of these meson loops, and apply it to the calculation of 2-->2 scattering amplitudes. Their results appear to be successful [3] in describing many puzzling aspects of meson spectroscopy, such as the low-mass sigma and kappa scalar enhancements and the masses of the narrow D_sJ states. This type of model shows considerable promise in attempts to understand the importance of hadron loops in the physics of hadrons, and in answering such long-standing questions as why the valence quark model works as well as it does, and under what circumstances it should fail."