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

TL;DR

This paper reports the observation of an exotic $ ext{η}_c(1S) ext{π}^-$ resonance in $B^0$ decays, with a significance over three standard deviations, and measures the decay's branching fraction using LHCb data.

Contribution

It provides the first evidence for an exotic resonance in this decay mode and measures its properties and the decay's branching fraction.

Findings

Exotic resonance with mass ~4096 MeV and width ~152 MeV observed

Resonance significance exceeds three standard deviations

Branching fraction of $B^0 o ext{η}_c(1S) K^+ ext{π}^-$ measured as approximately 5.73 x 10^{-4}

Abstract

A Dalitz plot analysis of $B^0 \to \eta_c(1S) K^+\pi^-$ decays is performed using data samples of $pp$ collisions collected with the LHCb detector at centre-of-mass energies of $\sqrt{s}=7,~8$ and $13$ TeV, 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 $4096 \pm 20~^{+18}_{-22}$ MeV and $152 \pm 58~^{+60}_{-35}$ MeV, 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 $B^0 \to \eta_c(1S) K^+\pi^-$ branching fraction is performed and gives $\displaystyle \mathcal{B}(B^0 \to \eta_c(1S) K^+\pi^-) = (5.73 \pm 0.24 \pm 0.13 \pm 0.66) \times 10^{-4}$, where the first uncertainty is statistical, the second systematic, and the third is due to limited knowledge of external branching fractions.