Thursday, 9 July 2009

Elementary Particles

Elementary Particles :
One of the primary goals in modern physics is to answer the question "What is the Universe made of?"
Often that question reduces to "What is matter and what holds it together?" This continues the line of
investigation started by Democritus, Dalton and Rutherford.
Modern physics speaks of fundamental building blocks of Nature, where fundamental takes on a
reductionist meaning of simple and structureless. Many of the particles we have discussed so far appear
simple in their properties. All electrons have the exact same characteristics (mass, charge, etc.), so we call
an electron fundamental because they are all non-unique.
The search for the origin of matter means the understanding of elementary particles. And with the advent
of holism, the understanding of elementary particles requires an understanding of not only their
characteristics, but how they interact and relate to other particles and forces of Nature, the field of physics
called particle physics.

The study of particles is also a story of advanced technology begins with the search for the primary
constituent. More than 200 subatomic particles have been discovered so far, all detected in sophisicated
particle accerlators. However, most are not fundamental, most are composed of other, simplier particles.
For example, Rutherford showed that the atom was composed of a nucleus and orbiting electrons. Later
physicists showed that the nucleus was composed of neutrons and protons. More recent work has shown
that protons and neutrons are composed of quarks.
Quarks and Leptons:
The two most fundamental types of particles are quarks and leptons. The quarks and leptons are divided
into 6 flavors corresponding to three generations of matter. Quarks (and antiquarks) have electric charges
in units of 1/3 or 2/3's. Leptons have charges in units of 1 or 0.

Normal, everyday matter is of the first generation, so we can concentrate our investigation to up and
down quarks, the electron neutrino (often just called the neutrino) and electrons.

Note that for every quark or lepton there is a corresponding antiparticle. For example, there is an up
antiquark, an anti-electron (called a positron) and an anti-neutrino. Bosons do not have antiparticles since
they are force carriers (see fundamental forces).

Baryons and Mesons:
Quarks combine to form the basic building blocks of matter, baryons and mesons. Baryons are made of
three quarks to form the protons and neutrons of atomic nuclei (and also anti-protons and anti-neutrons).
Mesons, made of quark pairs, are usually found in cosmic rays. Notice that the quarks all combine to
make charges of -1, 0, or +1.

Thus, our current understanding of the structure of the atom is shown below, the atom contains a nucleus
surrounded by a cloud of negatively charged electrons. The nucleus is composed of neutral neutrons and
positively charged protons. The opposite charge of the electron and proton binds the atom together with
electromagnetic forces.

The protons and neutrons are composed of up and down quarks whose fractional charges (2/3 and -1/3)
combine to produce the 0 or +1 charge of the proton and neutron. The nucleus is bound together by the
nuclear strong force (that overcomes the electronmagnetic repulsion of like-charged protons)
Color Charge:
Quarks in baryons and mesons are bound together by the strong force in the form of the exchange of
gluons. Much like how the electromagnetic force strength is determined by the amount of electric charge,
the strong force strength is determined by a new quantity called color charge.
Quarks come in three colors, red, blue and green (they are not actually colored, we just describe their
color charge in these terms). So, unlike electromagnetic charges which come in two flavors (positive and
negative or north and south poles), color charge in quarks comes in three types. And, just to be more
confusing, color charge also has its anti-particle nature. So there is anti-red, anti-blue and anti-green.
Gluons serve the function of carrying color when they interact with quarks. Baryons and mesons must
have a mix of colors such that the result is white. For example, red, blue and green make white. Also red
and anti-red make white.

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