Observation of the Decay $X(3872)\to {\pi }^0{\chi }_{c1}(1P)$

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 $5\sigma$ for all systematic fit variations. Normalizing to the previously established process $e^{+}{e}^{-}\to \gamma X(3872)$ with $X(3872)\to {\pi }^{+}{\pi }^{-}J/\psi$, we find $\mathcal{B}\mathbf{(}X(3872)\to {\pi }^0{\chi }_{c1}\mathbf{)}/\mathcal{B}\mathbf{(}X(3872)\to {\pi }^{+}{\pi }^{-}J/\psi \mathbf{)}=0.88_{-0.27}^{+0.33}\pm 0.10$, 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.

Figure 1

Distribution of ${\pi }^{+}{\pi }^{-}J/\psi$ mass, $M({\pi }^{+}{\pi }^{-}J/\psi )$, from the normalization process $e^{+}{e}^{-}\to \gamma {\pi }^{+}{\pi }^{-}J/\psi$ for (a) $4.15<E_{\mathrm{c}.\mathrm{m}.}<4.30\mathrm{GeV}$ and (b) $4.00<E_{\mathrm{c}.\mathrm{m}.}<4.15$ or $4.30<E_{\mathrm{c}.\mathrm{m}.}<4.60\mathrm{GeV}$. Points are data; lines are fits (solid is the total and dotted is the polynomial background). The darker histogram is a MC estimate of peaking $J/\psi$ backgrounds; the lighter stacked histogram is an estimate of nonpeaking backgrounds using $J/\psi$ sidebands from data.

Figure 2

Distribution of ${\pi }^0{\chi }_{cJ}$ mass, $M({\pi }^0{\chi }_{cJ})$, from the process $e^{+}{e}^{-}\to \gamma {\pi }^0{\chi }_{cJ}$ for (a) $4.15<E_{\mathrm{c}.\mathrm{m}.}<4.30\mathrm{GeV}$ and (b) $4.00<E_{\mathrm{c}.\mathrm{m}.}<4.15$ or $4.30<E_{\mathrm{c}.\mathrm{m}.}<4.60\mathrm{GeV}$. The ${\chi }_{cJ}$ are selected using a broad region of $\gamma J/\psi$ mass. Points, lines, and histograms follow the same convention as Fig. 1. The dashed line is the total background contribution to the fit, including signal events with ${\gamma }_1$ and ${\gamma }_2$ interchanged.

Figure 3

Distribution of $M({\gamma }_{1,2}J/\psi )$ after selecting the $X(3872)$ signal region from Fig. 2. Points and shaded histograms follow the same convention as Fig. 1. The solid line is the signal MC and is scaled using subsequent fits; the dashed line is the component of the signal MC where ${\gamma }_1$ and ${\gamma }_2$ are interchanged. Vertical lines show the ${\chi }_{cJ}$ selection regions.

Figure 4

Distributions of ${\pi }^0{\chi }_{cJ}$ mass, $M({\pi }^0{\chi }_{cJ})$, from the process $e^{+}{e}^{-}\to \gamma {\pi }^0{\chi }_{cJ}$ for (a) $J=0$, (b) $J=1$, and (c) $J=2$. Points, lines, and histograms follow the same convention as Fig. 1. The dashed line is the total background in the fit and includes contributions from events with interchanged ${\gamma }_1$ and ${\gamma }_2$ and cross feed among the search channels.