Strangeness Xenicity

There are also some conservation principles that are observed to hold for strong interactions but not for weak interactions. This matter is discussed in the following section. 

In Section 20-4 it was pointed out that the close similarity in properties of the neutron and the proton, except for electric charge, suggests that these two particles represent two aspects of the same particle, the nucleon. The nucleon may be said to have intrinsic electric charge -Hi and electric-charge vector i, which can have the component-Hi or—i in ordinary space, leading to the resultant electric charge +1 for the proton and 0 for the neutron. The proton and neutron can then be described as a charge doublet. 

The idea of strangeness was introduced by Gell-Mann and Nishijima to explain in a rough way the rates of decay reactions. Some of the unstable particles are expected to decay by virtue of the strong interactions (Section 20-3), and this decomposition should be very rapid, with half-lives of the order of 10 s. An example is the decay of the 17 particle, to form three pions its half-life is about 10 s. 

Many other particles, however, are observed to have much longer half-lives, of the order of lO s. These particles accordingly live 10 times as long as predicted for them on the basis of the theory of strong interactions. 

A diagram representing the masses and xenicities of some of the elementary particles. 

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Name Electric Charge Mass Xenicity (strangeness) Spin... 

Electric Charge Intrinsic Charge Charge Spin Xenicity (strangeness) ... 

The three quarks are the positive quark, p, the negative quark, n, and the strange quark, k. (Note that we use p and n for the quarks, p and n for the nucleons.) All three are fermions, with spin h p has electric charge 4-f, and n and k have charge —i. The antiquarks p, h, and X have charges —f, and +5, respectively. Each quark has baryon number +5 (each antiquark—i) k has xenicity 1 and k has xenicity —1. 

Let us consider the diquarks with baryon number 0 —that is, the compounds of a quark and an antiquark. The most stable diquarks are expected to be those in which both particles are in a Is orbital, as they move about their common center of mass. The quarks and antiquarks are different particles hence the Pauli exclusion principle does not forbid-parallel spins for a quark and its antiquark, and a I5- diquark can have resultant spin 0 or resultant spin 1. The mesons tt, and tt are ph, pp, (or nn), and pn, respectively, and various other mesons are similarly represented. The proton is represented by p-n and the neutron by pn-. The strange quark k (and its antiparticle k) are found in the mesons and baryons with xenicity different from zero. 

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