Gell-Mann's work in the 1950s involved recently discovered cosmic ray particles that came to be called kaons and hyperons. Classifying these particles led him to propose that a quantum number called strangeness would be conserved by the strong and the electromagnetic interactions, but not by the weak interactions. (Kazuhiko Nishijima arrived at this idea independently, calling the quantity formula_1-charge after the eta meson.) Another of Gell-Mann's ideas is the Gell-Mann–Okubo formula, which was, initially, a formula based on empirical results, but was later explained by his quark model. Gell-Mann and Abraham Pais were involved in explaining the puzzling aspect of the neutral kaon mixing.