Elementary Particles, Quarks and Quantum Chromodynamics 249
An explanation of why the strange particles are produced strongly yet
decay weakly was first made by A. Pais who suggested that these
particles could only be produced strongly in pairs and since they decay
singly into non-strange particles, they can not decay strongly but only
weakly. Gell-Mann and Nishijima each separately worked out an
identical scheme in which they assigned a strangeness quantum number
to each elementary particle. The pion and nucleon which do not behave
in a strange way were assigned 0 strangeness. The nucleon has two
charge states +e and 0 and hence has a center of charge at e/2. The Σ
particle has three charge states, +e, 0 and –e and hence has a center of
charge at 0, as does the Λ particle which has only one charge state, 0.
The center of charge of the ≡ is at –e/2 since it has two charge states, ≡-
and ≡o. The Ω– has only the charge state –e and hence the center of
charge at –e. these particles were assigned positive strangeness according
to how many units of e/2 their charge center differed from that of the
nucleon.
In a similar manner the strangeness of the kaon was determined by
the fact that its center of charge +e/2 for the K+ and Ko is one unit from
the 0 center of charge of the π+, πo and π– and hence has strangeness –1.
The strangeness of the K– and antiKo on the other hand is +1 since
their center of charge is at –e/2. According to the Gell Mann–Nishijima
scheme the strong interaction conserves strangeness and baryon number
and the weak interaction violates strangeness conservation but still
maintains baryon conservation. All the decays of the strange particles are
weak because there are no energetically possible decay modes, which do
not violate strangeness. The Λ does not have enough energy to decay
into a K– and a proton. All of the decay modes of the strange particles
involve products with one less unit of strangeness. Nishijima and Gell
Mann therefore predicted that the strong production of strange particles
would always involve a combination of particles such that the total
strangeness before and after the collision was the same. Hence the
Λ or Σ would be produced with a K+ or a Ko such as π– + p → Ko + Λ or
π– + p → K+ + Σ-. But a reaction such as π– + p → K– + Σ+ could never
take place, or for that matter p + p → p + n + K+. Their predictions
were confirmed almost immediately by the results from the 3 GeV
Cosmotron built at Brookhaven Labs.
The first baryons that were discovered, namely, p, n, Λ, Σ+, Σ^0 , Σ–,
Ξ–, and Ξ^0 all were spin ½ particles with baryon number 1. The
first mesons that were discovered, namely, π+, π^0 , π–, η, Κ+, Κ^0 , K–,