Unquenching the Quark Model

We present the formalism for a new generation of unquenched quark models in which quark-antiquark pair effects are taken into account in an explicit form via a microscopic QCD-inspired quark-antiquark creation mechanism. No truncation in the sum over all the big tower of states is necessary since these states are automatically generated by means of powerful group-theoretical techniques. An important check on the formalism and the numerical results is provided by the closure limit. As an application, the effect of quark-antiquark pairs on the strange content of the proton spin, $\Delta s$, is discussed. The contributions of the up and down quarks, $\Delta u$ and $\Delta d$, are also calculated. This has become possible after solving the difficult problem of permutational symmetry related to quark rearrangements. Finally, we present some preliminary results in the closure limit as well as an outlook for future applications.Comment: Invited talk at XI Conference on Problems in Theoretical Nuclear Physics (Cortona 2006), Cortona, Italy, 11-14 Oct, 200

Threshold corrections of χc(2P)\chi_{\rm c}(2P) and χb(3P)\chi_{\rm b}(3P) states and J/ψρJ/\psi \rho and J/ψωJ/\psi \omega transitions of the X(3872)X(3872) in a coupled-channel model

We calculate the masses of $\chi_{\rm c}(2P)$ and $\chi_{\rm b}(3P)$ states with threshold corrections in a coupled-channel model. Here, the meson quarkonium core is augmented by higher Fock components due to pair-creation effects. According to our results, we interpret the resonances characterized by very small threshold corrections, like $\chi_{\rm b}(3P)$'s, as almost pure quarkonia, and those states characterized by non-negligible threshold corrections, like the $X(3872)$, as quarkonium cores plus meson-meson components. We also study the $J/\psi \rho$ and $J/\psi \omega$ hidden-flavor strong decays of the $X(3872)$. The decays are calculated as the dissociation of one of these components ($D^0 \bar D^{0*}$) into a $c \bar c$ state ($J/\psi$) plus a light meson ($\rho$ or $\omega$) in a potential model. In particular, our result for the ratio between the $X(3872) \rightarrow J/\psi \omega$ and $J/\psi \rho$ widths (0.6) is compatible with the present experimental data ($0.8\pm0.3$) within the experimental error

The unquenched quark model

In this contribution, we briefly discuss the results for charmonium and bottomonium spectra with self-energy corrections in the unquenched quark model, due to the coupling to the meson-meson continuum. The UQM formalism can be extended to include also the effects of hybrid mesons, i.e. hybrid loops. Finally, we discuss the results of a calculation of hybrid mesons spectrum in Coulomb Gauge QCD.Comment: Talk given at the conference "Dark Matter, Hadron Physics and Fusion Physics", Messina (Italy), September 24-26, 201