The light quarkonium and charmonium mass shifts in an unquenched quark model

Abstract: The unquenched quark model for the light quarkonium and the charmonium states is explored in the present work. The quark-pair creation operator in the $^3P_0$ model, which mix the two-quark and four-quark components is modified by considering the effects of the created quark pair's energy, as well as the separation between the created quark pair and the valence quark pair. All the wave functions needed including the mesons and the relative motion between two mesons are all obtained by solving the corresponding Schr\"{o}dinger equation with the help of the Gaussian expansion method. Our aim of the present work is to find a new set of parameters which can give a good description of the mass spectrum of the low-lying light quarkonium and charmonium states. Moreover, some exotic states, for example $X(3872)$ can be described well in the unquenched quark model.