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

Abstract

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 9 fb−1 . Using the B + → χc1(3872)K+ decay, the χc1(3872) → ψ(2S)γ process is observed for the frst time and the ratio of its partial width to that of the χc1(3872) → J/ψγ decay is measured to be Γχc1(3872)→ψ(2S)γ Γχc1(3872)→J/ψγ = 1.67 ± 0.21 ± 0.12 ± 0.04, where the frst 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 D0D¯ ∗0 + D¯ 0D∗0 molecule questionable and strongly indicates a sizeable compact charmonium or tetraquark component within the χc1(3872) state.