Constraints on Sigma Nucleus Dynamics from Dirac Phenomenology of Sigma^- Atoms

Strong interaction level shifts and widths in $\Sigma^-$ atoms are analyzed by using a $\Sigma$ nucleus optical potential constructed within the relativistic mean field approach. The analysis leads to potentials with a repulsive real part in the nuclear interior. The data are sufficient to establish the size of the isovector meson--hyperon coupling. Implications to $\Sigma$ hypernuclei are discussed.