The world we live in is made from a handful
of elementary particles. The current understanding
has twelve particles and their corresponding
antiparticles out of which normal matter, like
atoms, is composed. The twelve particles can
be separated into two groups, leptons and quarks, of six based on how
they are affected by the strong nuclear force. Both leptons and quarks
have spin 1/2 and obey the Pauli exclusion principle.
Leptons are unaffected by
the strong force under any circumstances. The most
familiar lepton is the electron. Other members of this
group include neutrinos and the muon and tau particles.
The second group consists of quarks. Quarks do feel
the strong nuclear force and it dominates their existence
so much that we can not directly observe single quarks,
but only combinations of quarks bound together by the strong
force. Protons and neutrons are examples of baryons, particles made
from three bound quarks. A proton consists of two up quarks and a down
quark bound together while an antiproton consists of the
corresponding three antiquarks. Bound states of quark-antiquark pairs
are called mesons.
There are also particles which are associated with each of the forces
and in some sense carry the force between particles. The photon
is the carrier of the electromagnetic force, the gluon carries the strong
nuclear force, and the W and Z carry the weak nuclear force. These are
all Bosons meaning they have spin 1 and do not obey the Pauli exclusion
principle.
This Table shows the relationship
between the particles:
The six quarks and six leptons are each grouped
in three pairs by the weak nuclear force which causes
transitions between the two members of a pair. Some
forms of radioactive decay are due to this. The three
pairs of quarks and three pairs of leptons are ordered
by mass. For leptons, the electron and its
neutrino constitute the lowest mass pair, which we
designate as the first generation of leptons.
Similarly, the muon and its neutrino are the second
generation, and the tau and its neutrino the third
generation. The three quark generations are the up and
down quark (from which protons and neutrons are made),
the strange and charm quarks, and the top and bottom quarks.
As the second and third generation particles are heavier,
the weak nuclear force causes these particles to
decay to the lightest leptons and quarks. Thus normal
atoms are made from only first generation particles with
the heavier generations existing only briefly either as the
results of high energy interactions, or in the very hot
early universe.
For futher information contact Prof. David Hedin, Northern
Illinois University, email:
hedin@niu.edu
Last modified: July 2010