`` $\Lambda - \Sigma$ conversion in $^4_{\Lambda}$He and $^4_{\Lambda}$H based on four-body calculation

Precise four-body calculations for $^4_{\Lambda}$He and $^4_{\Lambda}$H have been performed in the framework of the variational method with Jacobian-coordinate Gaussian-basis functions. All the rearrangement channels of both $NNN \Lambda$ and $NNN \Sigma$ are explicitly taken into account for the first time with the use of realistic $NN$ and $YN$ interactions. The role of $\Lambda$-$\Sigma$ conversion and the amount of the virtual $\Sigma$-component in $^4_{\Lambda}$He and $^4_{\Lambda}$H are discussed.