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

TL;DR

This study uses radiative decay measurements from proton-proton collisions to investigate the internal structure of the $ ext{chi}_{c1}(3872)$ state, providing evidence against it being a pure molecular state.

Contribution

First observation of the $ ext{chi}_{c1}(3872) ightarrow ext{psi}(2S) ext{gamma}$ decay and measurement of its ratio to $ ext{J}/ extpsi ext{gamma}$ decay, informing its possible internal composition.

Findings

The decay $ ext{chi}_{c1}(3872) ightarrow ext{psi}(2S) ext{gamma}$ is observed for the first time.

The ratio of partial widths is measured to be approximately 1.67.

Results suggest the $ ext{chi}_{c1}(3872)$ has a significant charmonium or tetraquark component.

Abstract

The radiative decays $\chi_{c1}(3872)\rightarrow\psi(2S)\gamma$ and $\chi_{c1}(3872)\rightarrow J/\psi\gamma$ 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~$B^+\rightarrow \chi_{c1}(3872)K^+$decay, the $\chi_{c1}(3872)\rightarrow \psi(2S)\gamma$ process is observed for the first time and the ratio of its partial width to that of the $\chi_{c1}(3872)\rightarrow J/\psi\gamma$ decay is measured to be $$ \frac{\Gamma_{\chi_{c1}(3872)\rightarrow \psi(2S)\gamma}} {\Gamma_{\chi_{c1}(3872)\rightarrow J/\psi\gamma}} = 1.67 \pm 0.21 \pm 0.12 \pm0.04 , $$ 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 $J/\psi$ mesons. The measured ratio makes the interpretation of the $\chi_{c1}(3872)$ state as a~pure $D^0\bar{D}^{*0}+\bar{D}^0D^{*0}$ molecule questionable and strongly indicates a sizeable compact charmonium or tetraquark component within the $\chi_{c1}(3872)$ state.