Probing the nature of the ${\chi }_{c1}(3872)$ state using radiative decays

The radiative decays ${\chi }_{c1}(3872)\to \psi (2S)\gamma$ and ${\chi }_{c1}(3872)\to J\mathrm{/}\psi \gamma$ are used to probe the~nature of the~${\chi }_{c1}(3872)$ state using proton-proton collision data collected with the LHCb detector, corresponding to an~integrated luminosity of~9fb${}^{-1}$. Using the~$B^{+}\to {\chi }_{c1}(3872)K^{+}$decay, the ${\chi }_{c1}(3872)\to \psi (2S)\gamma$ process is observed for the first time and the ratio of its partial width to that of the ${\chi }_{c1}(3872)\to J\mathrm{/}\psi \gamma$ decay is measured to be $$\frac{{\mathrm{\Gamma }}_{{\chi }_{c1}(3872)\to \psi (2S)\gamma }}{{\mathrm{\Gamma }}_{{\chi }_{c1}(3872)\to J\mathrm{/}\psi \gamma }}=1.67\pm 0.21\pm 0.12\pm 0.04,$$ where the first uncertainty is statistical, the second systematic and the third is due to the uncertainties on the branching fractions of the $\psi (2S)$ and $J\mathrm{/}\psi$ mesons. The measured ratio makes the interpretation of the ${\chi }_{c1}(3872)$ state as a~pure $D^0{\bar{D}}^{\ast 0}+{\bar{D}}^0{D}^{\ast 0}$ molecule questionable and strongly indicates a sizeable compact charmonium or tetraquark component within the ${\chi }_{c1}(3872)$ state.