Still Another Conservation Law
Particles have intrinsic properties in addition to the ones we have listed so far: mass, “charge, spin, lepton number, and baryon number. The find of these additional properties was discovered when researchers observed that certain new particles, such as the bone (K) and the sigma (~), always seemed to be produced in pairs. It seemed impossible to produce only one of them at a time. Thus, if a beam of energetic pions interacts with the protons in a bubble chamber, the reaction
which violates no conservation law moon in the early days of particle physics, never occurs. It was eventually proposed (by Murray Gell-Marm in the United States and independently by K. Nishijima in Japan) that certain particles possess a new property, called strangeMll, with its own quantum number S and its own conservation law.
(Be careful not to confuse the symbol S here with spin.) Tile name strangeness arises from the fact that, before” the identities of these particles were pinned down, they were knoWn IS “strange particles,” and the label stuck, Tile proton, neutron, and pion have S = 0; that is, they are not “strange.” It was proposed, however, that the KC particle has strangeness S = +1 and that ~ + has S = -I. In the reaction of Eq. 43-16, the net strangeness is initially zero and finally zero; thus, the reaction conserves strangeness. However, in the reaction shown in Eq. 45-17, the final net strangeness is – 1; thus, thaI reaction does not conserve strangeness and cannot occur. Apparently”, then, we must add one more conservation law to our list-the conservation of strangeness:
Strangeness is c:conserved interactions involving the It1″Oil force
It may seem heavy-handed 10 invent a new property of particles just 10 account for a little puzzle like that posed by Eqs. 45-16 and 45-17. However, strangeness and its quantum number soon revealed themselves in many other areas of particle physics, and strangeness is now fully accepted as a legitimate particle attribute, on a par with charge and spin. Do not be misled by the whimsical character of the name. Strangeness is no more mysterious a property of particles than is charge. Both are properties that particles may (or may not) have; each is described by an appropriate quantum number. Each obeys a conservation law. Still other properties of particles have been discovered and given even more whimsical names, such as charm and bottomless, but all are perfectly legitimate properties.Let us see, as an example, how the new property of strangeness “earns its keep” by leading us to uncover important regularities in the properties of the particles. Strangeness is c:observed interactions involving the It1″OIlI force