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...