Probing the nature of the χ_c1(3872) state using radiative decays

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

Γ_{χc1(3872)→ψ(2S)γ/}Γ_{χc1(3872)→J/ψγ}= 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 χ_c1(3872) state as a pure D⁰

D̅^∗0 + D̅ ⁰ D^*0 molecule questionable and strongly indicates a sizeable compact charmonium or tetraquark component within the χ_c1(3872) state.