Study of the reaction $e^{+}{e}^{-}\to J/\psi {\pi }^{+}{\pi }^{-}$ via initial-state radiation at BABAR

We study the process $e^{+}{e}^{-}\to J/\psi {\pi }^{+}{\pi }^{-}$ with initial-state-radiation events produced at the PEP-II asymmetric-energy collider. The data were recorded with the BABAR detector at center-of-mass energies 10.58 and 10.54 GeV, and correspond to an integrated luminosity of $454{\mathrm{fb}}^{-1}$. We investigate the $J/\psi {\pi }^{+}{\pi }^{-}$ mass distribution in the region from 3.5 to $5.5\mathrm{GeV}/c^2$. Below $3.7\mathrm{GeV}/c^2$ the $\psi (2S)$ signal dominates, and above $4\mathrm{GeV}/c^2$ there is a significant peak due to the $Y(4260)$. A fit to the data in the range $3.74–5.50\mathrm{GeV}/c^2$ yields a mass value $4245\pm 5(\mathrm{stat})\pm 4(\mathrm{syst})\mathrm{MeV}/c^2$ and a width value ${114}_{-15}^{+16}(\mathrm{stat})\pm 7(\mathrm{syst})\mathrm{MeV}$ for this state. We do not confirm the report from the Belle Collaboration of a broad structure at $4.01\mathrm{GeV}/c^2$. In addition, we investigate the ${\pi }^{+}{\pi }^{-}$ system which results from $Y(4260)$ decay.

Figure 1

(a) The background-subtracted data, and MC simulation modified as described in the text, for the $\psi (2S)$ peak region. (b) The corresponding distribution for the mass region below $4.0\mathrm{GeV}/c^2$.

Figure 2

(a) The $J/\psi {\pi }^{+}{\pi }^{-}$ mass spectrum from $3.74\mathrm{GeV}/c^2$ to $5.5\mathrm{GeV}/c^2$; the points represent the data and the shaded histogram is the background from the $J/\psi$ sidebands; the solid curve represents the fit result, and the dashed curve results from the simultaneous fit to the background; (b) the measured $e^{+}{e}^{-}\to J/\psi {\pi }^{+}{\pi }^{-}$ cross section as a function of c.m. energy; the solid curve results from the fit shown in (a).

Figure 3

(a) The background-subtracted ${\pi }^{+}{\pi }^{-}$ mass distribution for the $Y(4260)$ signal region; the dashed vertical line is at the nominal $f_0(980)$ mass value [18]; (b) the corresponding $\cos {\theta }_{\pi }$ distribution; the fitted line is for an $S$-wave description; (c) the result of the fit using the model of Eq. (3).