Probing the nature of the ₂₁ (3872) state using radiative decays

The radiative decays ₂₁ (3872) (2S) and ₂₁ (3872) J/ are used to probe the~nature of the~₂₁ (3872) state using proton-proton collision data collected with the LHCb detector, corresponding to an~integrated luminosity of~9fb^-1. Using the~B^+ ₂₁ (3872) K^+decay, the ₂₁ (3872) (2S) process is observed for the first time and the ratio of its partial width to that of the ₂₁ (3872) J/ decay is measured to be _₂₁ (₃₈₇₂) (₂ₒ) _{₂₁ (3872) J/} = 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 (2S) and J/ mesons. The measured ratio makes the interpretation of the ₂₁ (3872) state as a~pure D⁰D^*0+D⁰D^*0 molecule questionable and strongly indicates a sizeable compact charmonium or tetraquark component within the ₂₁ (3872) state.