Evidence for an η c ( 1 S ) π - resonance in B 0 → η c ( 1 S ) K + π - decays

Abstract

A Dalitz plot analysis of $B^0\to {\eta }_c\left(1S\right)K^{+}{\pi }^{-}$ decays is performed using data samples of pp collisions collected with the $\text{LHCb}$ detector at centre-of-mass energies of $\sqrt{s}=7,8$ and $13\mathrm{Te}V$ , corresponding to a total integrated luminosity of $4.7\text{fb}{}^{-1}$ . A satisfactory description of the data is obtained when including a contribution representing an exotic ${\eta }_c\left(1S\right){\pi }^{-}$ resonant state. The significance of this exotic resonance is more than three standard deviations, while its mass and width are $4096\pm 20{}_{-22}^{+18}\mathrm{Me}V$ and $152\pm 58{}_{-35}^{+60}\mathrm{Me}V$ , respectively. The spin-parity assignments $J^P=0^{+}$ and $J^P=1^{-}$ are both consistent with the data. In addition, the first measurement of the $B^0\to {\eta }_c\left(1S\right)K^{+}{\pi }^{-}$ branching fraction is performed and gives $\begin{array}{c}B(B^0\to {\eta }_c\left(1S\right)K^{+}{\pi }^{-})=(5.73\pm 0.24\pm 0.13\pm 0.66)\times {10}^{-4},\end{array}$ where the first uncertainty is statistical, the second systematic, and the third is due to limited knowledge of external branching fractions.