Observation of $\bar{\mathrm{\Lambda }}p\to K^{+}{\pi }^{+}{\pi }^{-}{\pi }^0$ and $\bar{\mathrm{\Lambda }}p\to K^{+}{\pi }^{+}{\pi }^{-}2{\pi }^0$

Using $(10087\pm 44)\times {10}^6$ $J\mathrm{/}\psi$ events collected with the BESIII detector at a center-of-mass energy of $\sqrt{s}=3.097$ GeV, the antihyperon-nucleon annihilation processes $\bar{\mathrm{\Lambda }}p\to K^{+}{\pi }^{+}{\pi }^{-}+k{\pi }^0$ ($k=1,2,3$) are studied at an incident $\bar{\mathrm{\Lambda }}$ momentum of approximately 1.074 GeV/$c$. The reactions $\bar{\mathrm{\Lambda }}p\to K^{+}{\pi }^{+}{\pi }^{-}{\pi }^0$ and $\bar{\mathrm{\Lambda }}p\to K^{+}{\pi }^{+}{\pi }^{-}2{\pi }^0$ are observed for the first time, with corresponding cross sections ${\sigma }_{\bar{\mathrm{\Lambda }}p\to K^{+}{\pi }^{+}{\pi }^{-}{\pi }^0}=8.5_{-1.1}^{+1.2}(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 0.4(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})$ mb and ${\sigma }_{\bar{\mathrm{\Lambda }}p\to K^{+}{\pi }^{+}{\pi }^{-}2{\pi }^0}=7.9_{-1.7}^{+1.9}\pm 0.4$ mb. No significant signal is found for $\bar{\mathrm{\Lambda }}p\to K^{+}{\pi }^{+}{\pi }^{-}3{\pi }^0$, and an upper limit of 7.2 mb is set at a 90% confidence level. An evidence for the $K^{\ast }(892{)}^{+}$ resonance is seen in the $K^{+}{\pi }^0$ invariant mass spectrum $M_{K^{+}{\pi }^0}$ for $k=1$, and the corresponding cross section for $\bar{\mathrm{\Lambda }}p\to K^{\ast }(892{)}^{+}{\pi }^{+}{\pi }^{-}$ is measured to be ${\sigma }_{\bar{\mathrm{\Lambda }}p\to K^{\ast }(892{)}^{+}{\pi }^{+}{\pi }^{-}}=12.5_{-3.4}^{+3.8}\pm 1.2$ mb. Owing to the limited statistics, possible interference effects are not considered. These findings offer crucial input to deepen our understanding of the antihyperon-nucleon interactions.