Brueckner rearrangement effects in ${}_{\Lambda }^5\mathrm{He}$ and ${}_{\Lambda \Lambda }^6\mathrm{He}$

Rearrangement effects in light hypernuclei are investigated in the framework of the Brueckner theory. We can estimate without detailed numerical calculations that the energy of the $\alpha$ core is reduced by more than 2.5 MeV when the $\Lambda$ adheres to ${}^4\mathrm{He}$ to form ${}_{\Lambda }^5\mathrm{He}.$ Similar assessment of rearrangement contributions is essential to deduce the strength of $\Lambda \Lambda$ interaction from experimentally observed $\Delta B_{\Lambda \Lambda }.$ The recently observed experimental value of $\sim 1\mathrm{MeV}$ for the $\Delta B_{\Lambda \Lambda }$ of ${}_{\Lambda \Lambda }^6\mathrm{He}$ suggests that the matrix element of $〈\Lambda \Lambda |v|\Lambda \Lambda 〉$ in ${}_{\Lambda \Lambda }^6\mathrm{He}$ is around $-2\mathrm{MeV}.$