The pion was followed two years later by the K+ (1949), the po (1950), and the Lo and Ko (1951). The development of accelerators in the early 1950's opened the door to the so-called "population explosion," a bewildering proliferation of particles. Among the particles discovered in the 1950's were the baryons D, S±, X–, So, and Xo, plus the antiproton and the antineutron.
	The K mesons were recognized in 1955-56 by Gell-Mann and by Nishijima as having a new characteristic called "strangeness." The early 1960's saw the discovery of many more mesons such as the r, w, h, K*, f, f, a2, and h' along with many more baryons. Gell-Mann sought an understanding of this morass of particles by developing (in 1961) the "eightfold way," a precursor to the quark model, incorporating strangeness and isospin in an SU(3) group theoretic framework.
	The quark model itself was proposed by Gell-Mann and by Zweig in 1964. With it came a great simplification and understanding of all the known mesons and baryons: They were all composites of only three types of quarks, the u, d, and s quarks. The s quark carried strangeness and was a constituent of "strange" particles such as the K mesons. A critical prediction of the eightfold way had been the existence of a new particle, the W–, which was understood later in the quark model as a baryon made of three strange quarks (an sss state). The W– was indeed discovered in 1964.