Evidence for an $\eta_c(1S) \pi^-$ resonance in $B^0 \to \eta_c(1S) K^+\pi^-$ decays

A Dalitz plot analysis of ${{B} ^0} \!\rightarrow \eta _c(1S) {{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}\mathrm {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 $\eta _c(1S) \pi ^-$ resonant state. The significance of this exotic resonance is more than three standard deviations, while its mass and width are $4096 \pm 20~^{+18-22} \,\mathrm {Me}\mathrm {V} $ and $152 \pm 58~^{+60-35} \,\mathrm {Me}\mathrm {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} \!\rightarrow \eta _c(1S) {{K} ^+} {{\pi } ^-} $ branching fraction is performed and gives $\begin{aligned} \displaystyle \mathcal {B}({{B} ^0} \!\rightarrow \eta _c(1S) {{K} ^+} {{\pi } ^-} ) = (5.73 \pm 0.24 \pm 0.13 \pm 0.66) \times 10^{-4}, \end{aligned}$ where the first uncertainty is statistical, the second systematic, and the third is due to limited knowledge of external branching fractions.