Leptons and HadronsElementary particles fall into two classes, leptonsand hadrons,depending on whether
they respond to the strong interaction (hadrons) or do not (leptons).
The simplest particles are the leptons (Greek: “light,” “swift”), which seem to be
truly elementary with no hint of internal structures or even of extension in space.
Leptons are affected only by the electromagnetic (if charged), weak, and gravitational
interactions. Of the particles to which we have already been introduced, the electron
and the neutrino are leptons; there are four other types.
Hadrons (Greek: “heavy,” “strong”) are subject to the strong interaction as well as
to the others. They also differ from leptons in that they occupy space, rather than be-
ing infinitesimal in size: hadrons seem to be a little over 1 fm (10^15 m) across. Hadrons
are composed of either two or three quarks,which, like leptons, are structureless and
as close to being point particles as present measurements can establish. Hadrons that
consist of three quarks, such as the proton and neutron, are called baryons; mesons,
such as the pion, consist of two quarks. Like nothing else in nature, quarks have charges
of^13 eor^23 e, and their combination in hadrons is always such that the hadron charges
are either 0 ore. Quarks have never been observed outside of hadrons, but, as we
shall see, there is convincing evidence that they do exist. The strong force that acts
between hadrons is the external manifestation of the more basic interactions among
the quarks they contain and is mediated by the exchange of mesons, as described in
Sec.11.7.13.2 LEPTONS
Three pairs of truly elementary particlesTable 13.2 lists the six known leptons and their antiparticles. Because the neutrinos
involved in beta decays, which were discussed in Chap. 12, are associated with elec-
trons, their proper symbol is e.
The electron was the first elementary particle for which a satisfactory theory was
developed. This theory was proposed in 1928 by Paul A. M. Dirac, who obtained a
relativistically correct wave equation for a charged particle in an electromagnetic field.
When the observed mass and charge of the electron are inserted in the solutions of
this equation, the intrinsic angular momentum of the electron is found to be ^12 (thatElementary Particles 477
Table 13.2 Leptons. All are unaffected by the strong interaction and are
fermions. The neutrinos are uncharged; their masses are unknown but
unlikely to exceed a few eVc^2.Lepton Symbol Antiparticle Mass, MeV/c^2 Mean Life, s SpinElectron e e 0.511 Stable ^12
e-neutrino e e Very small Stable ^12
Muon 106 2.2 10 ^6 ^12
-neutrino Very small Stable ^12
Ta u 1777 2.9 10 ^23 ^12
-neutrino Very small Stable ^12 bei48482_ch13.qxd 1/23/02 8:06 PM Page 477