Probing the nature of the χc1(3872) state using radiative decays

Abstract

The radiative decays χc1(3872) → ψ(2S) γ and χc1(3872) → J/ψγ are used to probe the nature of the χc1(3872) state using proton-proton collision data collected with the LHCb detector, corresponding to an integrated luminosity of 9 fb−1. Using the B+ → χc1(3872)K+ decay, the χc1(3872) → ψ(2S) γ process is observed for the first time and the ratio of its partial width to that of the χc1(3872) → J/ψγ decay is measured to be Γ χ c 1 3872 → ψ 2 S γ Γ χ c 1 3872 → J / ψ γ = 1.67 ± 0.21 ± 0.12 ± 0.04 , where the first uncertainty is statistical, the second systematic and the third is due to the uncertainties on the branching fractions of the ψ(2S) and J/ψ mesons. The measured ratio makes the interpretation of the χc1(3872) state as a pure D0 D ¯ ∗ 0 + D ¯ 0 D*0 molecule questionable and strongly indicates a sizeable compact charmonium or tetraquark component within the χc1(3872) state.