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

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~9fb⁻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_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 χc1(3872) state as a~pure D⁰D̄*0+D̄⁰D*0 molecule questionable and strongly indicates a sizeable compact charmonium or tetraquark component within the χc1(3872) state.