When the particles with which we are already familiar (pions, protons, neutrons, etc.) go into high-energy collisions a whole flock of particles are produced, and together with the a mesons and nucleons there appear new particles, specifically "superheavy" particles, the hyperons, whose mass exceeds that of the nucleons (the protons and neutrons) and new types of K mesons ("kaons"), which are heavier than the pions. It is significant that such processes once again produce a variety of "antiparticles", or charge-conjugate particles, which are "mirror images" of the ordinary particles.
Let us write down a couple of typical reactions of the production of new particles: where is a lambda hyperon and is a sigma hyperon.
The K mesons (the kaons) also turned out to be quite interesting particles, though strictly speaking there is no such thing as a dull particle, which does not display any distinctive characteristics. Furthermore, it appears that the only way to understand the properties of each particle is to consider them in their ensemble, similarly to a symphony orchestra, in which it is im possible to eliminate any individual instrument without impairing the total effect. The K mesons decay in diverse ways—into two or three a mesons, and alternatively into two or three particles together with leptons. Given below are some typical instances of K-meson decay: A single kaon track was discovered in 1941 by Leprince-Ringuet. After that investigators began to find many new particles in the range of 900 1200 rn, and it was thought for a long time that there was a T particle which decays into three pions, and a 0 particle which transforms into two pions.
It finally turned out that these are charged and neutral K mesons of virtually identical mass (966 m). To be precise, the neutral kaon is slightly heavier, with a mass of 974 m.
As we climb up the scale of particle masses, we encounter for the first time the "strange" particles, in the guise of the K mesons. To the class of the strange particles belong all the hyperons. The first unusual property of the strange particles is the fact that they are produced in pairs. For instance, when high-energy IC mesons collide with protons, among the particles produced there are always either two K mesons, or two hyperons, or a hyperon and a K meson. This is suggestive of the fact that the strange particles might possess some new property which is conserved, in a way similar to the electric or lepton charge. Thus, for instance, electrons are
produced in pairs with positrons or neutrinos. The new conserved property of the K mesons and the hyperons was half-jokingly called strangeness. If the K+meson is assigned the strangeness S = +1, then its antiparticle, the K - meson, will have the strangeness S = —1, K° will have S = +1, and K° will have S = —1 .
We will not dwell in detail on all the phases of development in the discovery of the hyperons, but only mention that the first superheavy particle, the neutral A hyperon, was discovered in the analysis of cosmic ray tracks by Butler and Rochester in 1947, which was a very eventful year for physics. The mass of the A hyperon is equal to 2183 m, and its strangeness S=—I. Shortly afterward the heavier E hyperons, both charged and neutral, were discovered, and, finally, the heaviest of all the known particles—the cascade E hyperons. All the baryons, i. e. , the nucleons and the hyperons, have a spin which means that they are fermions. The concept of strangeness was first introduced by Gell-Mann and Nishijima and stands as a landmark in the physics of elementary particles, as it may be possible to analyze and interpret the complicated reactions in the pro duction and decay of the strange particles, i. e. , the hyperons and kaons, and of some nonstrange particles as well—the pions and nucleons.
The 1c particles, K particles, and the baryons form together a group known as the strongly interacting particles. They are distinguished from the leptons, or the light particles, i. e. , the electron, the two types of neutrino, the muon, and the corresponding antiparticles, which are characterized by interactions of much lower intensity, the so-called weak interactions. In addition, of course, all the charged particles, whether light (the leptons), medium (the mesons), or heavy (the baryons), interact with each other by means of the electromagnetic field.
When strange particles are produced by strong interaction the strangeness is conserved (two particles of opposite strangeness are produced), but in the decay of strange particles, which is due to weak interactions, the strangeness is not conserved. For instance, The hyperons thus live relatively long , compared to nuclear lifetimes of R their lifetime is about sec, and not or sec, like the "resonons", for instance.