Measurement of the $e^{+}{e}^{-}\to J/\psi c\overline{c}$ cross section at $\sqrt{s}\approx 10.6\mathrm{GeV}$

We present a new measurement of the $e^{+}{e}^{-}\to J/\psi c\overline{c}$ cross section where the $c\overline{c}$ pair can fragment either into charmed hadrons or a charmonium state. In the former case the $J/\psi$ and a charmed hadron are reconstructed, while the latter process is measured using the recoil mass technique, which allows the identification of two-body final states without reconstruction of one of the charmonia. The measured $e^{+}{e}^{-}\to J/\psi c\overline{c}$ cross section is $(0.74\pm 0.08\genfrac{}{}{0ex}{}{+0.09}{-0.08})\mathrm{pb}$, and the $e^{+}{e}^{-}\to J/\psi X_{\mathrm{non}\mathrm{\text{−}}c\overline{c}}$ cross section is $(0.43\pm 0.09\pm 0.09)\mathrm{pb}$. We note that the measured cross sections are obtained from a data sample with the multiplicity of charged tracks in the event larger than 4; corrections for the effect of this requirement are not performed as this cannot be done in a model-independent way. The analysis is based on a data sample with an integrated luminosity of $673{\mathrm{fb}}^{-1}$ recorded near the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ asymmetric-energy collider.

Figure 1

The $M_{\mathrm{rec}}((c\overline{c}{)}_{\mathrm{tag}})$ spectra: $(c\overline{c}{)}_{\mathrm{tag}}=$ (a) $J/\psi$, (b) ${\psi }^{\prime }$, (c) ${\chi }_{c1}$, and (d) ${\chi }_{c2}$. The curves are described in the text.

Figure 2

$M_{{\ell }^{+}{\ell }^{-}}$ spectra for $H_c$ signal (points with errors) and scaled $H_c$ sideband windows (histograms), where $H_c=$ (a) $D^0$, (b) $D^{+}$, (c) $D_s^{+}$, and (d) ${\Lambda }_c^{+}$. The curves represent the result of the fit; solid curves correspond to the $H_c$ signal windows, and dashed curves to the $H_c$ sidebands.

Figure 3

$p_{J/\psi }^{*}$ spectra: (a) inclusive (open circles), from $e^{+}{e}^{-}\to J/\psi H_{c}X$ (filled squares) and from double charmonium production (filled circles); (b) the sum of all $e^{+}{e}^{-}\to J/\psi c\overline{c}$ processes (open squares), from the $e^{+}{e}^{-}\to J/\psi X_{\mathrm{non}\mathrm{\text{−}}c\overline{c}}$ processes (filled triangles). The fit results are shown in (a) for the inclusive spectrum, and in (b) for the processes $e^{+}{e}^{-}\to J/\psi c\overline{c}$ (solid curve) and $e^{+}{e}^{-}\to J/\psi X_{\mathrm{non}\mathrm{\text{−}}c\overline{c}}$ (dashed curve).

Figure 4

Angular distributions [$|\cos {\theta }_{\mathrm{hel}}|$ in (a), $|\cos {\theta }_{\mathrm{prod}}|$ in (b)] for inclusive $e^{+}{e}^{-}\to J/\psi X$ (open circles), $e^{+}{e}^{-}\to J/\psi c\overline{c}$ (open squares), and $e^{+}{e}^{-}\to J/\psi X_{\mathrm{non}\mathrm{\text{−}}c\overline{c}}$ processes (filled triangles). The results of the fits described in the text are shown with the dash-dotted, solid, and dashed curves, respectively.