Measurements of branching fractions of ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}\eta$ and ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\eta }^{\mathrm{\prime }}$ and asymmetry parameters of ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\pi }^0$, ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}\eta$, and ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\eta }^{\mathrm{\prime }}$

We report a study of ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\pi }^0$, ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}\eta$, and ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\eta }^{\mathrm{\prime }}$ using the data sample corresponding to an integrated luminosity of 980 $\mathrm{f}{\mathrm{b}}^{-1}$ collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The branching fractions relative to ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\pi }^0$ are measured as: ${\mathcal{B}}_{{\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}\eta }\mathrm{/}{\mathcal{B}}_{{\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\pi }^0}=0.25\pm 0.03\pm 0.01$ and ${\mathcal{B}}_{{\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\eta }^{\mathrm{\prime }}}\mathrm{/}{\mathcal{B}}_{{\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\pi }^0}=0.33\pm 0.06\pm 0.02$. Using ${\mathcal{B}}_{{\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\pi }^0}=(1.25\pm 0.10)\mathrm{\%}$, we obtain ${\mathcal{B}}_{{\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}\eta }=(3.14\pm 0.35\pm 0.11\pm 0.25)\times {10}^{-3}$ and ${\mathcal{B}}_{{\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\eta }^{\mathrm{\prime }}}=(4.16\pm 0.75\pm 0.21\pm 0.33)\times {10}^{-3}$. Here the uncertainties are statistical, systematic, and from ${\mathcal{B}}_{{\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\pi }^0}$, respectively. The ratio of the branching fraction of ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\eta }^{\mathrm{\prime }}$ with respect to that of ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}\eta$ is measured to be ${\mathcal{B}}_{{\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\eta }^{\mathrm{\prime }}}\mathrm{/}{\mathcal{B}}_{{\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}\eta }=1.34\pm 0.28\pm 0.06$. We update the asymmetry parameter of ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\pi }^0$, ${\alpha }_{{\mathrm{\Sigma }}^{+}{\pi }^0}=-0.48\pm 0.02\pm 0.02$, with a considerably improved precision. The asymmetry parameters of ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}\eta$ and ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\eta }^{\mathrm{\prime }}$ are measured to be ${\alpha }_{{\mathrm{\Sigma }}^{+}\eta }=-0.99\pm 0.03\pm 0.05$ and ${\alpha }_{{\mathrm{\Sigma }}^{+}{\eta }^{\mathrm{\prime }}}=-0.46\pm 0.06\pm 0.03$ for the first time.