Unquenched quark-model calculation of X (3872) electromagnetic decays

Abstract

A recent quark-model description of X (3872) as an unquenched 2 3P1 (Formula Presented) state is generalised by now including all relevant meson–meson configurations, in order to calculate the widths of the experimentally observed electromagnetic decays X (3872) → γ J/ ψ and X(3872) → γ ψ (2S). Interestingly, the inclusion of additional two-meson channels, most importantly D ± D ⋆ ∓, leads to a sizeable increase of the (Formula Presented) probability in the total wave function, although the (Formula Presented) component remains the dominant one. As for the electromagnetic decays, unquenching strongly reduces the γ ψ (2S) decay rate; yet it even more sharply enhances the γ J/ψ rate, resulting in a decay ratio compatible with one experimental observation but in slight disagreement with two others. Nevertheless, the results show a dramatic improvement as compared to a quenched calculation with the same confinement force and parameters. Concretely, we obtain Γ(X(3872) →γ ψ(2S) = 28.9 keV and ΓX(3872)→ γJ/ψ = 24.7 keV, with branching ratio R γ ψ = 1.17.