# Observation of the decay $X(3872) \to \pi^0 \chi_{c1}(1P)$

## TL;DR

This paper reports the first observation of the decay mode $X(3872) o ^0 hi_{c1}$, expanding understanding of the $X(3872)$ particle's decay channels using data from the BESIII detector.

## Contribution

It presents the first observation of a new decay mode of $X(3872)$, specifically $X(3872) o ^0 hi_{c1}$, with quantitative measurements of its branching ratio relative to known decays.

## Key findings

- First observation of $X(3872) o ^0 hi_{c1}$ decay with >5$$ significance.
- Measured ratio of branching fractions: $0.88^{+0.33}_{-0.27}\u00b1 0.10$.
- Upper limits set for decays to $^0 hi_{c0}$ and $^0 hi_{c2}$.

## Abstract

Using a total of $9.0~\mathrm{fb}^{-1}$ of $e^+e^-$ collision data with center-of-mass energies between 4.15 and 4.30 GeV collected by the BESIII detector, we search for the processes $e^+e^-\to \gamma X(3872)$ with $X(3872)\to\pi^0\chi_{cJ}$ for $J=0,1,2$. We report the first observation of $X(3872)\to\pi^{0}\chi_{c1}$, a new decay mode of the $X(3872)$, with a statistical significance of more than 5$\sigma$. Normalizing to the previously established process $e^+e^-\to \gamma X(3872)$ with $X(3872)\to\pi^+\pi^-J/\psi$, we find ${\cal B}(X(3872) \to \pi^0 \chi_{c1})/{\cal B}(X(3872) \to \pi^+\pi^- J/\psi) = 0.88^{+0.33}_{-0.27}\pm0.10$, where the first error is statistical and the second is systematic. We set 90% confidence level upper limits on the corresponding ratios for the decays to $\pi^0\chi_{c0}$ and $\pi^0\chi_{c2}$ of 19 and 1.1, respectively.