Bucknell Physics 212 Supplement

Section 9.3 Classification of Particles

Historically, particles were classified by their masses. Two particles with exactly the same mass are almost surely identical, unless some other obvious difference (like different charges) turns up. Even then, physicists give the particles the same symbol (like \(e^-\) for electrons, \(e^+\) for positrons) and look for some underlying symmetry that causes them to have the same mass.

The first rough classification assigned the labels leptons, mesons, and baryons to particles, names derived from the Greek words for light, medium, and heavy. Thus low mass particles like electrons were called leptons, middle mass particles like pions were called mesons, and heavy particles (protons and neutrons) were called baryons.

As more particles entered the scene, a scheme based just on mass became inadequate. For instance, a muon has a mass close to that of a pion, but behaves much more like an electron. The present classification of particles is now based on two factors: the type of force the particle “feels,”¹ and the spin of the particle. Apparently all particles feel the weak interaction, but only some can feel the strong interaction. The constituents of matter that feel the strong interaction are called hadrons; those that do not are leptons. (Messenger particles,² like photons and \(W\)'s, are neither; they are not constituents of matter but force carriers.) As for a classification based on spin, those particles with integral spin are called bosons, those with half-integral spin are called fermions, as we have seen. Let's now look at the present definitions of leptons, mesons, and baryons.

Leptons. Leptons are those particles that do not participate in the strong interaction. All leptons have spin-\(\textstyle{\frac{1}{2}}\) and are thus fermions. Their masses range from 0 to \(1.78\Xunits{GeV/ c^2 }\text{.}\) Leptons appear to have no internal structure (down to \(10^{-18}\Xunits{m}\) at least) and thus appear to be true elementary particles. There are only a handful of different kinds and they come in just three groups, or families, as seen in Table 9.1. Some examples are electrons, neutrinos, and muons.

Mesons. Mesons are those particles that participate in the strong interaction and have integer spin. That is, those hadrons that are also bosons are called mesons. Their masses range from \(0.135\Xunits{GeV/ c^2 }\) up to \(11\Xunits{GeV/ c^2 }\) or more, and more are still being discovered. Only a few have lifetimes longer than about \(10^{-20}\Xunits{s}\text{.}\) Unlike leptons or baryons the number of mesons is not conserved — mesons can be created or destroyed in reactions. Pions and kaons are two examples of meson types. Table 9.2 lists several others.

Baryons. Baryons are those particles that participate in the strong interaction and have half-integer spin. That is, hadrons that are also fermions are called baryons. As a class, they are the heaviest and most numerous type of particle with hundreds of members and masses ranging from \(0.938\Xunits{GeV/ c^2 }\) up to \(2.6\Xunits{GeV/ c^2 }\text{.}\) Both mesons and baryons seem to have a size and internal structure on the scale of \(10^{-15}\)–\(10^{-16}\Xunits{m}\text{.}\) Baryons are “proton-like,” in that all baryons eventually decay into protons, the lightest baryon. Some other baryons are the neutron, the lambda (\(\Lambda\)) and the omega minus (\(\Omega^-\)). A few baryons are listed in Table 9.3.