Observation of two resonancelike structures in the ${\pi }^{+}{\chi }_{c1}$ mass distribution in exclusive ${\overline{B}}^0\to K^{-}{\pi }^{+}{\chi }_{c1}$ decays

We report the first observation of two resonancelike structures in the ${\pi }^{+}{\chi }_{c1}$ invariant mass distribution near $4.1\mathrm{GeV}/c^2$ in exclusive ${\overline{B}}^0\to K^{-}{\pi }^{+}{\chi }_{c1}$ decays. From a Dalitz plot analysis in which the ${\pi }^{+}{\chi }_{c1}$ mass structures are represented by Breit-Wigner resonance amplitudes, we determine masses and widths of: $M_1=(4051\pm {14}_{-41}^{+20})\mathrm{MeV}/c^2$, ${\Gamma }_1=({82}_{-17-22}^{+21+47})\mathrm{MeV}$, $M_2=({4248}_{-29-35}^{+44+180})\mathrm{MeV}/c^2$, and ${\Gamma }_2=({177}_{-39-61}^{+54+316})\mathrm{MeV}$; and product branching fractions of $\mathcal{B}({\overline{B}}^0\to K^{-}{Z}_{1,2}^{+})\times \mathcal{B}(Z_{1,2}^{+}\to {\pi }^{+}{\chi }_{c1})=({3.0}_{-0.8-1.6}^{+1.5+3.7})\times {10}^{-5}$ and $({4.0}_{-0.9-0.5}^{+2.3+19.7})\times {10}^{-5}$, respectively. Here the first uncertainty is statistical, the second is systematic. The significance of each of the ${\pi }^{+}{\chi }_{c1}$ structures exceeds $5\sigma$, including the systematic uncertainty from various fit models. This analysis is based on $657\times {10}^6$ $B\overline{B}$ events collected at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider.