Probing the nature of the χ _c1(3872) state using radiative decays

Abstract: The radiative decays χ _c1(3872) → ψ(2S) γ and χ _c1(3872) → J/ψγ are used to probe the nature of the χ _c1(3872) state using proton-proton collision data collected with the LHCb detector, corresponding to an integrated luminosity of 9 fb ⁻¹. Using the B ⁺ → χ _c1(3872)K ⁺ decay, the χ _c1(3872) → ψ(2S) γ process is observed for the first time and the ratio of its partial width to that of the χ _c1(3872) → J/ψγ decay is measured to be (Formula presented.) 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 χ _c1(3872) state as a pure D ⁰D¯ ^∗0 + D¯ ⁰ D ^*0 molecule questionable and strongly indicates a sizeable compact charmonium or tetraquark component within the χ _c1(3872) state.