According to our present knowledge, the world is made of two
elementary particle types, quarks and leptons. “Elementary” here
means that they are not made of any other, smaller particles
themselves. Hence, they are the fundamental building blocks in Nature.
They can interact via the exchange of bosons, resulting in attractive or
repulsive forces and consequently in bound states. The four
fundamental forces are mediated via different bosons: The
electromagnetic force (sun light, rainbow, batteries, red or blue poles of
a lego toy train etc.) is mediated via the exchange of photons. The
exchange of gluons (engl. glue = sticky material) yields the strong force
which acts in large atomic nuclei and holds the protons and neutrons at
very small distances together, although the protons carry same-sign
positive electric charge and should therefore repell each other with
immense force. The exchange of W+, W- or Z0 bosons mediates the
weak force, which is responsible for radioative decay or the burning
process of the sun. Gravity is not yet understood on a quantum
mechanics level, but presumably mediated vie the exchange of
Quarks participate in all three known fundamental interactions, while
leptons only participate in the electromagnetic and the weak interaction.
Quarks make up protons and neutrons, which in turn form atomic
nuclei. The electron was discovered in 1897 and is by now the best
known lepton. One or more electrons form the electron shell of atoms.
Moving electrons represent an electrical current, for example in
lightning in a thunderstorm. When rubbing balloons on a woolen
jumper, we pull out electrons from its atoms, leaving an electrostatically
charged balloon or jumper. Electrons and electrical currents are
omnipresent these days and dominate our modern life. The partner of
the electrically charged electron is the electrically neutral neutrino with
only a tiny mass.
Quarks and leptons come in different varieties, which appear to differ
only in their respective mass. Otherwise they have identical properties.
The varieties are called families or generations. The electron is a
member of the lightest, first generation. The muon (μ) is the charged
lepton of the second generation and has its own partner lepton, the
muon neutrino. The third generation lepton is the tau with its
corresponding tau neutrino. The muon is approximately 200-times
heavier than the electron. Consequently, it decays in about 2.2
microseconds into an electron and two neutrinos and is therefore
difficult to study. Very fast, highly-relativistic muons, however, can be
studies easily, as their lifetime observed from a laboratory rest system
appears to be stretched by the factor βɣ with β=v/c and ɣ=1/sqrt(1-β2)
(time dilatation). Via this relation of the special theory of relativity, Albert
Einstein managed to explain the muon paradoxon, i.e. the question why
muons, which are created at an altitude of about 13 km, even at the
speed of light should only travel about 660 m and should hence not
reach the ground. Some muon properties are summarised in the
following. Further information can be retrieved from a review article by
the Particle Data Group.
m = 105.7 MeV, i.e. ~ 200-times the electron mass
τ = 2.2 10−6 s
1936 discovered by Carl D. Andersen and Seth Neddermayer in studes
of cosmic radiation using a cloud chamber. Muons can be produced for
example in the decay of pions, which are produced in vast quantities in
proton collisions with matter, for example the atmosphere.