Observation of a new charged charmoniumlike state in ${\overline{B}}^0\to J/\psi K^{-}{\pi }^{+}$ decays

We present the results of an amplitude analysis of ${\overline{B}}^0\to J/\psi K^{-}{\pi }^{+}$ decays. A new charged charmoniumlike state $Z_c(4200{)}^{+}$ decaying to $J/\psi {\pi }^{+}$ is observed with a significance of $6.2\sigma$. The mass and width of the $Z_c(4200{)}^{+}$ are $4196_{-29-13}^{+31+17}\mathrm{MeV}/c^2$ and $370_{-70-132}^{+70+70}\mathrm{MeV}$, respectively; the preferred assignment of the quantum numbers is $J^P=1^{+}$. In addition, we find evidence for $Z_c(4430{)}^{+}\to J/\psi {\pi }^{+}$. The analysis is based on a $711{\mathrm{fb}}^{-1}$ data sample collected by the Belle detector at the asymmetric-energy $e^{+}{e}^{-}$ collider KEKB.

Figure 1

The $\mathrm{\Delta }E$ distribution; the signal and sideband regions are hatched.

Figure 2

Dalitz plots of the signal region (a), sidebands (b) and signal efficiency (c).

Figure 3

Efficiency as a function of the angular variables.

Figure 4

Dalitz plot slices used to present results. Vertical divisions are at $1.2{\mathrm{GeV}}^2/c^4$, $(1.432\mathrm{GeV}/c^2{)}^2\approx 2.05{\mathrm{GeV}}^2/c^4$ and $3.2{\mathrm{GeV}}^2/c^4$ [the second division is chosen to be at the $K_2^{*}(1430)$ mass since the interference of the $K^{*}$ resonances and the $Z_c(4200{)}^{+}$ changes at this mass]. Horizontal divisions are at $16{\mathrm{GeV}}^2/c^4$ and $19{\mathrm{GeV}}^2/c^4$.

Figure 5

Fit to the background events. The solid line is the fit result; the dashed line is the $K^{*}(892)$ component; the dotted line is the $K_S^0\to {\pi }^{+}{\pi }^{-}$ component. The slices are defined in Fig. 4.

Figure 6

The fit results with (solid line) and without (dashed line) the $Z_c(4430{)}^{+}$ [the $Z_c(4200{)}^{+}$ is not included in the model] for the second and third vertical slices that are defined in Fig. 4.

Figure 7

The fit results with (solid line) and without (dashed line) the $Z_c(4200{)}^{+}$ ($J^P=1^{+}$) in the default model. The points with error bars are data; the hatched histograms are the $J/\psi$ sidebands. The slices are defined in Fig. 4.

Figure 8

The fit results with the $Z_c(4200{)}^{+}$ ($J^P=1^{+}$) in the default model. The points with error bars are data; the solid histograms are fit results, the dashed histograms are the $Z_c(4430{)}^{+}$ contributions, the dotted histograms are the $Z_c(4200{)}^{+}$ contributions and the dash-dotted histograms are contributions of all $K^{*}$ resonances. The slices are defined in Fig. 4.

Figure 9

Projections of the fit results with (solid line) and without (dashed line) the $Z_c(4200{)}^{+}$ ($J^P=1^{+}$) onto the angular variables in the default model for the region defined by $M_{K\pi }^2>1.2{\mathrm{GeV}}^2/c^4$, $16{\mathrm{GeV}}^2/c^4<M_{J/\psi \pi }^2<19{\mathrm{GeV}}^2/c^4$. Points with error bars are data.

Figure 10

The fit results with (solid line) and without (dashed line) the $Z_c(4430{)}^{+}$ [the $Z_c(4200{)}^{+}$ is included in the model] for the second and third vertical slices that are defined in Fig. 4.

Figure 11

Argand plots for the $Z_c(4200{)}^{+}$ helicity amplitudes. The bin central mass values (in $\mathrm{GeV}/c^2$) are shown near the points.

Figure 12

Comparison of the $2^{-}$ and $1^{+}$ hypotheses in the default model. The histograms are distributions of $\mathrm{\Delta }(-2\ln \mathit{L})$ in MC pseudoexperiments generated in accordance with the fit results with $2^{-}$ (open histogram) and $1^{+}$ (hatched histogram) $Z_c^{+}$ signals. The $\mathrm{\Delta }(-2\ln \mathit{L})$ value observed in data is indicated with an arrow.

Figure 13

Results of a fit to a $\mathrm{\Delta }(-2\ln \mathit{L})$ distribution done as part of the $Z_c(4200{)}^{+}$ global significance calculation for the $J^P=1^{+}$ hypothesis.