Radiative decay of $χ_{c1}$ states in effective Lagrangian approach

Abstract: The $\chi_{c1}(3872)$ state, first observed by the Belle collaboration with its quantum numbers identified as $J^{PC} = 1<sup>{++}$, has been the subject of extensive research due to its intriguing properties. Several theoretical interpretations have been proposed to explain its unique characteristics, including the $\chi_{c1}(2P)$ assignment, a molecular $\bar{D}<sup>{<em>}D/\bar{D}D^{</em>}$ configuration, a coupled-channel framework incorporating $c\bar{c}$ and di-meson degrees of freedom, and the compact tetraquark hypothesis. However, challenges remain in reconciling its mass coincidence with the threshold and the observed isospin violation within both the pure $c\bar{c}$ and compact tetraquark models. In this study, we examine the radiative decays of the $\chi_{c1}(1P)$ and $\chi_{c1}(3872)$ states in an effective field theory framework, incorporating triangle loops of $D$ and $D<sup>{*}$ mesons. The model parameters are calibrated based on the observed branching fraction of the radiative decay mode $\chi_{c1}(1P) \to J/\psi \gamma$. Utilizing these fixed parameters, we predict the branching fractions $$R_{\chi_{c1}(3872) \to J/\psi \gamma} \sim 10<sup>{-1}$$ and $$R_{\chi_{c1}(3872) \to \psi(2S) \gamma} \sim 10<sup>{-2}$$, and the relative fraction $\mathcal{R}<em>{\Psi\gamma} \approx 0.109$. The work supports the argument that the $\chi</em>{c1}(3872)$ is unlikely a $c\bar{c}$ state.