248 The Poetry of Physics and The Physics of Poetry
the charge on the π+ and π– like a handle, leading them around with
magnets.
The search for the intermediate meson predicted by Yukawa led to the
discovery of a number of other particles, which like the pion and the
nucleon also exert nuclear forces. These strongly interacting particles
came to be known as strange particles for reasons, which will be clear
shortly. The same type of cosmic ray studies which lead to the discovery
of the muon and the pion lead to the discovery of four types of strange
particles: the K meson or kaon, and two baryons, the Λ and Σ particles.
The ≡, or cascade particle and the Ω particle, were discovered using
accelerators.
The K mesons or kaons all have a mass of approximately 500 MeV,
which is about 3.5 times, the mass of the pion. There is a charged kaon,
the K+, and a neutral kaon, the Ko. In addition these two particles have
antiparticles, the K– and the antiKo. The Ko and the antiKo are two
separate particles, which behave differently. The K mesons, like the
pions, have baryon number zero. The K mesons are different than the π
mesons, however, in that the π- is the antiparticle of the π+ but the πo is its
own antiparticle. There are, therefore, only three pions but four kaons,
the K+ and the K^0 and their antiparticles. The Λ, with mass 1115 MeV;
the Σ with mass 1190 MeV; the ≡ with mass 1315 and the Ω with mass
1670 all have masses greater than the proton and the neutron whose
masses are approximately 940 MeV. These particles are referred to as
hyperons because they have masses greater than the nucleons. Because
they all decay eventually into one nucleon, they are baryons each with
baryon number equal to 1.
The reason that the strange particles received their nickname is
because of the fact that they must be produced in pairs and that they
carry a strange charge called strangeness. They are produced quite
readily by the strong interaction as long as strangeness is conserved,
however, they decay very slowly via the weak interaction into pions or
muons or both if they are kaons and into nucleons and pions if they are
hyperons. We shall return to the story of their decay when we discuss the
weak interaction. For the purposes of the present discussion it is
important to note that the decay of all the strange particles is of the order
of 10-10 or 10-8 sec that is characteristic of the weak interaction and that
these decays violate strangeness conservation that was characteristic of
their production via the strong interaction.