Observation of a Resonancelike Structure in the ${\pi }^{+-}{\psi }^{\prime }$ Mass Distribution in Exclusive $B\to K{\pi }^{+-}{\psi }^{\prime }$ Decays

A distinct peak is observed in the ${\pi }^{\pm }{\psi }^{\prime }$ invariant mass distribution near 4.43 GeV in $B\to K{\pi }^{\pm }{\psi }^{\prime }$ decays. A fit using a Breit-Wigner resonance shape yields a peak mass and width of $M=4433\pm 4(\mathrm{stat})\pm 2(\mathrm{syst})\mathrm{MeV}$ and $\Gamma ={45}_{-13}^{+18}(\mathrm{stat}{)}_{-13}^{+30}(\mathrm{syst})\mathrm{MeV}$. The product branching fraction is determined to be $\mathcal{B}\mathbf{(}{B}^0\to K^{\mp }{Z}^{\pm }(4430)\mathbf{)}\times \mathcal{B}\mathbf{(}{Z}^{\pm }(4430)\to {\pi }^{\pm }{\psi }^{\prime }\mathbf{)}=(4.1\pm 1.0(\mathrm{stat})\pm 1.4(\mathrm{syst}))\times {10}^{-5}$, where $Z^{\pm }(4430)$ is used to denote the observed structure. The statistical significance of the observed peak is $6.5\sigma$. These results are obtained from a $605{\mathrm{fb}}^{-1}$ data sample that contains $657\times {10}^6$ $B\overline{B}$ pairs collected near the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric energy $e^{+}{e}^{-}$ collider.

Figure 1

The $M^2(K\pi )$ (horizontal) vs $M^2(\pi {\psi }^{\prime })$ (vertical) Dalitz-plot distribution for $B^0\to K^{-}{\pi }^{+}{\psi }^{\prime }$ candidate events.

Figure 2

The $M({\pi }^{+}{\psi }^{\prime })$ distribution for events in the $M_{\mathrm{bc}}-\Delta E$ signal region and with the $K^{*}$ veto applied. The shaded histogram show the scaled results from the $\Delta E$ sideband. The solid curves show the results of the fit described in the text.

Figure 3

The $M(K\pi )$ distribution for events within $\pm 0.03\mathrm{GeV}$ of the 4.43 GeV peak. Here the $K^{*}$ veto, which excludes the regions indicated by double-sided arrows, has been removed. The shaded histogram indicates the $\Delta E$-sideband-determined non-$B$ background.