Sigma Particle
In 1947 physicists observed new particles associated with cosmic ray events. The observed particle-streaming from these highly energetic cosmic events came in two types--a neutral particle that decayed into positive and negative particles and another, more massive positively charged particle that decayed into a positively charged heavy electron and a photon. What was surprising to scientists was the decay time associated with these particles. The decay times due to strong interactions are measured in femto seconds (10-15 s). The decay time of the observed K mesons was, however, about 10-10 s, and was, therefore, more typical of a weak decay. Since the initial discovery both mesonic and baryonic type particles (e.g., pions), similar particles have been discovered. Because of their decay times these particles are described as strange particles.
The sigma particle was one of the first three strange particles discovered. A particle is considered "strange" if it is produced by the strong force, but decays by the weak or the electromagnetic force. Experiments showed that strange particles were always produced in pairs. Their behavior was initially explained by Murray Gell-Mann in the United States and, independently, by Kazuhiko Nishijima (1929-) in Japan. If one of the particles was given strangeness (s) s+1 and the other -1 (similar to electric charge and the way electrons and positrons are produced), then when an S=+1 and an S=-1 particle were produced by the strong reaction, the total strangeness was zero, and strangeness was conserved. Strangeness is not preserved in the weak or electromagnetic decay reactions. The sigma particle was designated S=-1.
In 1961, Gell-Mann and independently, the Israeli physicist Yuval Ne'eman (1925-) began using some of the conserved characteristics in hadrons and mesons to group them into multiplets of eight. Gell-Mann called his classification "the eightfold way" borrowing the term from Buddhism, which stated the way to nirvana was the eightfold way.
Gell-Mann and others developed a classification scheme similar to Mendeleev's (1834-1907) periodic table of the elements. From these schemes Gell-Mann was able to predict nearly exactly the omega minus particle which was found in 1964 at the Brookhaven accelerator laboratory. Gell-Mann believed there was an internal structure to the hadrons. In 1963, he published his paper on quarks, the building blocks of hadrons. The sigma particle was known to be a family of three, one with a positive charge, one with a negative charge, and one neutral. They were all strange particles and were baryons, about 10% more massive than a proton. They each had ½ unit of spin.
The sigma-plus is made up of two up (u) quarks and one strange (s) quark, the sigma-minus of two down (d) quarks and one strange (s) quark, and the sigma zero of one up (u), one down (d), and one strange (s) quark. The sigma particle played a key role in filling out the classification scheme of Gell-Mann and others, and helped lead the way to an understanding of one of the fundamental elements of matter, the quark.
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