Kaonic nuclei -- dense and cold nuclear systems

We have predicted strongly bound kaonic states in few-body nuclei, the bindings of which are on the order of 100 MeV: the separation energies of a $K^{-}$ in $^{3}$He and $^{4}$He are calculated to be 108 and 86 MeV with widths of 20 and 34 MeV, respectively. Substantial contraction of the system is induced due to the strong \textit{KN} attraction, thus forming an unusually-dense nuclear object. Since these kaonic nuclei have large densities more than 3-times the normal density, they provide a unique playground for studying possible QCD structure in dense and cold nuclear medium. We discuss the implication of recently discovered strange tri-baryons in $^{4}$He(stopped-$K^{-}$, $p)$S$^{0 }$(3115) and $^{4}$He(stopped-$K^{-}$, $n)$ S$^{+}$ (3140) within the framework of deeply bound Kaonic states formed on shrunk nuclear cores. The S$^{+}$ (3140) corresponds to $T$=0 \textit{ppnK}$^{-}$, whereas the S$^{0 }$(3115) to $T$=1 \textit{pnnK}$^{-}$, which is an isobaric analog state of \textit{pppK}$^{-}$. The observed binding energies can be accounted for by including the relativistic effect and by invoking a medium-enhanced \textit{KN} interaction by 15{\%}. A new paradigm is discussed, which would be closely related to important issues of ``chiral symmetry restoration,'' ``kaon condensation'' and ``strange matter.''