Evidence for an $$\eta _c(1S) \pi ^-$$ η c ( 1 S ) π - resonance in $$B^0 \rightarrow \eta _c(1S) K^+\pi ^-$$ B 0 → η c ( 1 S ) K + π - 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.