He is primarily associated with two concepts that directly contributed to major breakthroughs in his field. The first was the idea of "associated production" to explain the puzzling properties of strange particles. His ideas and those of Murray Gell-Mann resulted in the idea of a quantum number called strangeness. The second concept was Pais's and Gell-Mann's theory regarding the composition of the neutral kaons, proposing that the observed states were admixtures of particles and antiparticles, having different lifetimes; this was experimentally confirmed in the following year by Lederman and collaborators.