Elementary Particles, Quarks and Quantum Chromodynamics 253
The electrons and the muons and their associated neutrinos form a
family of particles known as leptons. The muon is basically a heavy
electron with a rest mass energy of 105.7 MeV compared to the 0.5 MeV
rest mass energy of the electron. There is one more member of
this family that has so far been discovered namely the tauon and
its associated tau-neutrino. The rest mass of the tauon is 1,777 MeV and
it has a mean lifetime of 2.9 × 10-13 seconds and decays via the weak
interaction into leptons and sometimes into hadrons.
The weak interaction is mediated by the exchange of three kinds of
bosons the W–, the W+, the antiparticle of the W- and the Z, which is its
own antiparticle because it has no charge. All three bosons are very
short-lived with a mean lifetime of about 3 × 10−25 seconds. They are
extremely heavy particles with masses almost 100 times that of the
proton (940 MeV), namely with a rest mass energy of 80,400 MeV for
the W and 91,200 MeV for the Z.
Summing up the Interactions of Elementary Particles
There are four basic interactions in the universe, gravity, weak,
electromagnetic and strong. The W+, W−, and Z bosons play the same
role for the weak interaction as the photon does for the electromagnetic
force and together they form the four gauge bosons of the electroweak
interaction and hence unite the electromagnetic force and the weak
interaction as one force. The electroweak interaction and gravity are
forces that impact all the particles of the universe as opposed to the
strong interaction, which only operates on hadrons consisting of baryons
and mesons.
Given that hadrons are composites of quarks and antiquarks the
number of known elementary particles can be reduced to the following
sets of quarks, leptons and bosons, namely
- The following quarks and their associated antiquarks: up,
down, strange, charm, top and bottom. - The following leptons and their associated antiparticles: e, μ,
τ, νe, νμ, and ντ. - The four gauge bosons, W–, W+, Z and γ.
Some elementary particle physicists believe that there is another
boson out there called the Higgs particle, whose existence would explain