Observation of Λbar⁢𝑝 →𝐾+⁢𝜋+⁢𝜋−⁢𝜋0 and Λ⁢bar𝑝→𝐾+⁢𝜋+⁢𝜋−⁢2⁢𝜋0

Using $(10087 \pm 44)\times10^{6}$ $J/\psi$ events collected with the BESIII detector at a center-of-mass energy of $\sqrt{s}=3.097~\mathrm{GeV}$, the antihyperon-nucleon annihilation processes $\bar{\Lambda}p \to K^+\pi^+\pi^- + k\pi^0$ $(k=1,2,3)$ are studied at an incident $\bar{\Lambda}$ momentum of approximately $1.074~\mathrm{GeV}/c$. The reactions $\bar{\Lambda}p \to K^+\pi^+\pi^-\pi^0$ and $\bar{\Lambda}p \to K^+\pi^+\pi^-2\pi^0$ are observed for the first time, with corresponding cross sections $$\sigma(\bar{\Lambda}p \to K^+\pi^+\pi^-\pi^0) = \left(8.5^{+1.2}_{-1.1}\,(\mathrm{stat}) \pm 0.4\,(\mathrm{syst})\right)\,\mathrm{mb}$$ and $$\sigma(\bar{\Lambda}p \to K^+\pi^+\pi^-2\pi^0) = \left(7.9^{+1.9}_{-1.7}\,(\mathrm{stat}) \pm 0.4\,(\mathrm{syst})\right)\,\mathrm{mb}.$$ No significant signal is found for $\bar{\Lambda}p \to K^+\pi^+\pi^-3\pi^0$, and an upper limit of $7.2~\mathrm{mb}$ is set at the $90\%$ confidence level. Evidence for the $K(892)^+$ resonance is observed in the $K^+\pi^0$ invariant mass spectrum for $k=1$, and the corresponding cross section is measured to be $$\sigma(\bar{\Lambda}p \to K(892)^+\pi^+\pi^-) = \left(12.5^{+3.8}_{-3.4}\,(\mathrm{stat}) \pm 1.2\,(\mathrm{syst})\right)\,\mathrm{mb}.$$ Owing to the limited statistics, possible interference effects are not considered. These findings provide important input for improving our understanding of antihyperon-nucleon interactions.