Observation and Properties of the $X(3872)$ Decaying to $J/\psi {\pi }^{+}{\pi }^{-}$ in $p\overline{p}$ Collisions at $\sqrt{s}=1.96\mathrm{T}\mathrm{e}\mathrm{V}$

We report the observation of the $X(3872)$ in the $J/\psi {\pi }^{+}{\pi }^{-}$ channel, with $J/\psi$ decaying to ${\mu }^{+}{\mu }^{-}$, in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{T}\mathrm{e}\mathrm{V}$. Using approximately $230{\mathrm{p}\mathrm{b}}^{-1}$ of data collected with the Run II D0 detector, we observe $522\pm 100$ $X(3872)$ candidates. The mass difference between the $X(3872)$ state and the $J/\psi$ is measured to be $774.9\pm 3.1(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 3.0(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\mathrm{M}\mathrm{e}\mathrm{V}/c^2$. We have investigated the production and decay characteristics of the $X(3872)$ and find them to be similar to those of the $\psi (2S)$ state.

Figure 1

$\Delta M=M({\mu }^{+}{\mu }^{-}{\pi }^{+}{\pi }^{-})-M({\mu }^{+}{\mu }^{-})$ for all candidates satisfying the selection requirements. The solid curve is a fit to the data, and the dashed curve represents the background under each peak. The inset shows the mass distribution of the $J/\psi$ candidates used in the analysis.

Figure 2

$\Delta M$ distribution for two ranges of rapidity of the ${\mu }^{+}{\mu }^{-}{\pi }^{+}{\pi }^{-}$ system.

Figure 3

Comparison of event-yield fractions for $X(3872)$ and $\psi (2S)$ in the following regions: (a) $p_T(J/\psi {\pi }^{+}{\pi }^{-})>15\mathrm{G}\mathrm{e}\mathrm{V}/c$; (b) $|y|$ of $J/\psi {\pi }^{+}{\pi }^{-}<1$; (c) $\cos ({\theta }_{\pi })<0.4$; (d) effective proper decay length, $dl<0.01\mathrm{c}\mathrm{m}$; (e) $\mathrm{\text{isolation}}=1$; (f) $\cos ({\theta }_{\mu })<0.4$.