Evidence of a Broad Structure at an Invariant Mass of $4.32\mathrm{GeV}/c^2$ in the Reaction $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}\psi (2S)$ Measured at BABAR

We present a measurement of the cross section of the process $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}\psi (2S)$ from threshold up to 8 GeV center-of-mass energy using events containing initial-state radiation, produced at the SLAC PEP-II $e^{+}{e}^{-}$ storage rings. The study is based on $298{\mathrm{fb}}^{-1}$ of data recorded with the BABAR detector. A structure is observed in the cross section not far above threshold, near 4.32 GeV. We also investigate the compatibility of this structure with the $Y(4260)$ previously reported by this experiment.

Figure 1

The invariant-mass distribution for all ${\pi }^{+}{\pi }^{-}J/\psi$ candidates where more than one entry per event is allowed. The solid curve is a fit to the distribution in which the $\psi (2S)$ signal is described by a Cauchy function and the background by a quadratic function (represented by the dashed curve). The arrows indicate the $\psi (2S)$ mass window.

Figure 2

The distributions of (a) $\Delta p^{*}$ and (b) $\cos {\theta }^{*}$ of the $2({\pi }^{+}{\pi }^{-})J/\psi$ combination in the $e^{+}{e}^{-}$ c.m. frame are shown for data (solid dots) and Monte Carlo simulation of the signal (histogram) normalized to the total number of the observed data events.

Figure 3

The $2({\pi }^{+}{\pi }^{-})J/\psi$ invariant-mass spectrum up to $5.7\mathrm{GeV}/c^2$ for the final sample. The shaded histogram represents the fixed background and the curves represent the fits to the data (see text).

Figure 4

The primary ${\pi }^{+}{\pi }^{-}$ invariant-mass spectrum within region $m(2({\pi }^{+}{\pi }^{-})J/\psi )<5.7\mathrm{GeV}/c^2$ for the final sample. Only one entry per event is included in the plot, as described in the text. The histogram shows the distribution for Monte Carlo events (see text).

Figure 5

The measured c.m. energy dependence of the cross section (points with error bars) for $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}\psi (2S)$ after background subtraction. The solid histogram shows the energy-dependent selection efficiency.