Evidence for $f_0(980)f_0(980)$ production in ${\chi }_{c0}$ decays

Using a sample of $14\times {10}^6$ $\psi (2S)$ events accumulated with the BES II detector, evidence for $f_0(980)f_0(980)$ production in ${\chi }_{c0}$ decays is obtained for the first time; the branching ratio is determined to be $\mathcal{B}[{\chi }_{c0}\to f_0(980)f_0(980)\to {\pi }^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}]=[7.6\pm 1.9(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 1.6(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})]\times {10}^{-4}$. The significance of the $f_0(980)$ signal is about 4.6$\sigma$.

Figure 1

The ${\pi }^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}$ invariant mass spectrum. The shaded histogram shows the spectrum for Monte Carlo simulated background events.

Figure 2

Scatter plots of ${\pi }^{+}{\pi }^{-}$ versus ${\pi }^{+}{\pi }^{-}$ invariant mass for selected $\gamma {\pi }^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}$ events with ${\pi }^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}$ mass in (a) the ${\chi }_{c0}$ and (b) the ${\chi }_{c2}$ mass regions.

Figure 3

Projections of ${\pi }^{+}{\pi }^{-}$ invariant mass under the (a) ${\chi }_{c0}$ and (b) ${\chi }_{c2}$ peaks (two entries per event).

Figure 4

Plot of ${\pi }^{+}{\pi }^{-}$ mass recoiling against the $f_0(980)(0.88\mathrm{G}\mathrm{e}\mathrm{V}<m_{{\pi }^{+}{\pi }^{-}}<1.04\mathrm{G}\mathrm{e}\mathrm{V})$ for events in the ${\chi }_{c0}$ mass region (up to two entries per event), where the dashed line histogram indicates a rough estimation of background determined from sidebands.

Figure 5

Scatter plot of ${\pi }^{+}{\pi }^{-}$ versus ${\pi }^{+}{\pi }^{-}$ invariant mass in the $f_0(980)$ region for ${\chi }_{c0}$ candidate events, showing the definition of signal and background regions.

Figure 6

A fit to the ${\pi }^{+}{\pi }^{-}$ mass recoiling against the $f_0(980)(0.88\mathrm{G}\mathrm{e}\mathrm{V}<m_{{\pi }^{+}{\pi }^{-}}<1.04\mathrm{G}\mathrm{e}\mathrm{V})$ for events in the ${\chi }_{c0}$ mass region (up to two entries per event).