Probing the nature of the χc1(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 $$ \frac{\Gamma_{\chi_{c1}(3872)\to \uppsi \left(2\textrm{S}\right)\gamma }}{\Gamma_{\chi_{c1}(3872)\to \textrm{J}/\uppsi \gamma }}=1.67\pm 0.21\pm 0.12\pm 0.04, $$ 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 χ_c1(3872) state as a pure D⁰ $$ {\overline{\textrm{D}}}^{\ast 0} $$ D ¯ ∗ 0 + $$ {\overline{\textrm{D}}}^0 $$ D ¯ 0 D^*0 molecule questionable and strongly indicates a sizeable compact charmonium or tetraquark component within the χ_c1(3872) state.