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

We report the observation of a narrow state decaying into $J/\psi {\pi }^{+}{\pi }^{-}$ and produced in $220{\mathrm{p}\mathrm{b}}^{-1}$ of $\bar{p}p$ collisions at $\sqrt{s}=1.96\mathrm{T}\mathrm{e}\mathrm{V}$ in the CDF II experiment. We observe $730\pm 90$ decays. The mass is measured to be $3871.3\pm 0.7\left(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\right)\pm 0.4\left(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\right)\mathrm{M}\mathrm{e}\mathrm{V}/c^2$, with an observed width consistent with the detector resolution. This is in agreement with the recent observation by the Belle Collaboration of the $X\left(3872\right)$ meson.

Figure 1

The mass distributions of $J/\psi {\pi }^{+}{\pi }^{-}$ and $J/\psi {\pi }^{\pm }{\pi }^{\pm }$ candidates passing the selection described in the text. A large peak for the $\psi \left(2S\right)$ is seen in the $J/\psi {\pi }^{+}{\pi }^{-}$ distribution as well as a small signal near a mass of $3872\mathrm{M}\mathrm{e}\mathrm{V}/c^2$. The curve is a fit using two Gaussians and a quadratic background to describe the data. The inset shows an enlargement of the $J/\psi {\pi }^{+}{\pi }^{-}$ data and fit around $3872\mathrm{M}\mathrm{e}\mathrm{V}/c^2$.

Figure 2

The mass distributions of $J/\psi {\pi }^{+}{\pi }^{-}$ candidates with $m\left({\pi }^{+}{\pi }^{-}\right)>0.5\mathrm{G}\mathrm{e}\mathrm{V}/c^2$ (points) and $m\left({\pi }^{+}{\pi }^{-}\right)<0.5\mathrm{G}\mathrm{e}\mathrm{V}/c^2$ (open circles). The curve is a fit with two Gaussians and a quadratic background. The inset shows an enlargement of the high dipion-mass data and fit.