Philosophers have contemplated the basic constituents
of matter for thousands of years. At first, it was believed by the ancient
Greeks that all matter was created with a magical substance that no one
could comprehend. Later, when the scientists were more advanced, the building
blocks were theorized to be fire, earth, water, and air. Over a thousand
years later, there is evidence for experimentally verified fundamental
constituents: quarks and leptons.
All matter consists of molecules. All molecules
consist of elements/ atoms. All atoms consist of a nucleus and orbiting
electrons. The nucleus consists of protons and neutrons. And finally, protons
and neutrons consist of quarks. There are four forces that bind these particles
to form matter: the electromagnetic force, the strong force, the weak force,
and the force of gravity (even though this force is not included in the
Standard Model). These factors are the basic parts of what is known as
the Standard Model.
The Standard Model postulates that quarks and leptons
are the fundamental particles of the universe. In other words, these particles
are thought to be structure less.
There are three families or flavors of quarks. The first consists of
the up (+2/3 charge) and down (-1/3 charge) quarks. The next and heavier
family consists of the charm (+2/3 charge) and the strange (-1/3 charge)
quarks. The last and heaviest family consists of top (+2/3 charge) and
bottom (-1/3 charge) quarks.
Like the quarks, there are three families of leptons. The first family
is the electron (-1 charge) and the electron neutrino ( neutral). The second
and heavier family consists of the muon (-1 charge) and the muon neutrino
(neutral charge). The third and heaviest family is the tau (-1 charge)
and the tau neutrino (neutral).
A combination of the first flavors of quarks and
leptons make up all of the matter that we have encountered naturally in
the universe. Two up quarks and a down quark make up a proton since the
net charge of this particle is +1. Two down quarks and an up quark are
the constituents of a neutron (giving a net charge of zero). The combination
of protons and neutrons make up a nucleus, and the lepton particle, an
electron, orbits the small dense nucleus. This shows that an atom is completely
configured with the fundamental particles mentioned in the fourth and fifth
paragraphs (excluding the force mediators that will be explained later).
Now that the fundamental particles of all matter
have been briefly discussed, it is necessary to describe the forces that
govern these particles. The most overwhelming force at the subatomic level
is known as the strong force. In a proton there are two up quarks and a
down quark. Therefore one would conclude that the proton would want to
disperse in separate directions due to electromagnetic forces. Although
this is not the case since the strong force is able to overcome the electromagnetic
force. The strong force is what keeps hadrons together (a compilation of
quarks and anti-quarks, depending on which kind). In the strong force,
the force carriers or mediators are particles in themselves, gluons. Gluons
operate through color charge and they have a color charge as well. There
are three different color charges for the three different families of quarks.
There are as well, three different color charges for the three corresponding
anti-quarks. A gluon has two different color charges, one color charge
for the two quarks that they are in contact with and the anti-color for
the particular quark. When the colors that correspond to the charges of
the gluon and the quarks they are in contact with are combined the color
is white (this is why all hadrons have a net color charge of zero). This
is how the model of color charge works through the strong force. The reader
must realize that color charge is a model to describe the strong force
and that there are really no colors that truly exist in the gluons and
quarks. Now that the color charge of the strong force has been explained
the gluons themselves need to be made clearer. Gluons work like a spring
between two quarks. Therefore, the closer the two quarks are the less energy
the spring or gluon has to put forth. Since gluons are used to bind protons
together the gluons are constantly being stretched (since the two up quarks
oppose each other). Although when there is so much energy being expended
by the gluon, the gluon will snap and form two new particles, a quark and
an anti-quark. Therefore the electromagnetic and strong forces work together
in a repeating cycle.
Another force that governs all of matter is an offshoot
of the strong force, the residual strong force. The residual strong force
operates according to the same principle as the strong force. Now that
the constituents of protons are bound together through the strong force,
there has to be a force that binds the nucleus. The nucleus is composed
of protons and neutrons. Therefore according to the electromagnetic force
alone the nucleus would want to repel each other in opposite directions.
The residual strong force holds the nucleus together. Even though protons
have no net color charge they still have polar color charges. The up quarks
in each hadron (a compilation of quarks) attract each other through gluons,
causing the nucleus of all atoms to be a dense and together.
The next force in particle physics is the electromagnetic (EM) force.
This force is the second strongest force in nature behind the strong force.
The EM force operates through the charges of particles. A positively charged
particle attracts a negatively charged particle, and two like charged particles
repel. The mediator in this case is a photon.
The third strongest force in nature is the weak
force. The weak force is responsible for the decay of all particles. The
mediators for the weak force are W (positively charged), W (negatively
charged), and Z(neutrally charged) bosons. In this interaction a particle
decays and emits one of the three bosons, some quarks or leptons (depending
on what particle is decaying), and an electron neutrino. At first, scientists
had trouble attributing how energy was escaping from the decay reaction.
They found the answer in the electron neutrino. The electron neutrino has
no charge, has no structure (we think), and has very little mass. Therefore
this is the phantom particle carrying the extra energy away. (An electron
neutrino does actually exist; it is not simply a means of explaining some
unknown phenomena.)
The next force that exists in nature is gravity.
Gravity is the weakest of the forces and it has not been integrated into
the standard model. The effects of gravity on the fundamental particles
are negligible. Therefore, for now, the theory can get away without accounting
for it. One of the main problems with gravity is that there is no known
force carrier. Even though the mediator is not known there is a name just
waiting for it, the graviton.
The reader must realize that all of the interactions
work together, not independently. And through all of these forces the movement
of particles can be predicted through the Standard Model.
The Standard Model makes many conclusions about fundamental particles
and forces in nature, but it also has many missing parts. These problems
include: the mystery of why there are three families of fundamental particles,
the Higgs Boson, dark matter, and supersymmetry.
As stated previously, the first family of quarks
and leptons is the only group that has been naturally observed in nature.
The two heavier flavors of quarks and leptons are extremely similar to
the first families except for the fact that they are much more massive.
Therefore, it is a mystery why there are three flavors instead of just
one. Although, we understand why these particles in the last two families
are not able to exist in the matter (that is observed on this planet).
If these particles existed naturally, the matter would be reconfigured
very quickly since the life of these particles before they decay is merely
a millionth(s) of a second.
According to quantum theories of gravity, there
is not enough mass in the universe to account for the orbits of the planets.
Therefore, scientists have theorized that there is an elusive substance
called dark matter. Dark matter is thought to have a mass but it is not
illuminated. With dark matter, there would be enough mass in the universe
to account for the planet’s orbits. Another factor that supports the existence
of dark matter is the fact that this universe would make more sense if
it were closed. A closed universe does not expand forever. To achieve a
closed universe there must be more mass giving rise to more gravitational
forces, ultimately yielding a gravitationally closed universe. If dark
matter’s existence were verified, it would be an extension of the Standard
Model.
Another mystery in particle physics is the Higgs
Boson. Under Quantum Electrodynamics it is required that the mediator (a
photon) have no mass. Likewise, the Electroweak force (an integrated force
compiled of the weak force and the electromagnetic force) requires that
the W and Z bosons have zero mass. Although, through experience one can
clearly see that these bosons do have mass. To make the electroweak force
work, the W and Z particles must have no mass alone but have a mass when
they are interacting with other particles. The Higgs Field is the mechanism
that gives the weightless bosons (and everything) mass. To show how a Higgs
Field operates an analogy must be employed. A massive particle can move
through empty space and have no resistance. In this sense, this massive
particle is behaving like a mass less object. Although, as soon there is
some sort of resistance (Higgs Field) that the particle must combat, the
particle will have to behave as a massive object. In other words, a person
can walk through an empty room without resistance, but as soon as there
is a crowd (the Higgs Field) the person will feel the effects of being
massive. If the Higgs Field/Boson exists than the electroweak force works
because the W and Z bosons have no mass (they only have resistance due
to a undetectable field). Although the Higgs Boson attributes mass to everything,
what attributes mass to this boson? This is one of the fundamental problems
associated with the Standard Model. When the Higgs Boson is discovered
the Standard Model will be one step closer to being complete.
Supersymmetry is another unknown in the Standard
Model. It adds three favorable elements. In the Standard Model there is
a problem in “tweaking” the mass. The mass of a fundamental particle, without
symmetry, approaches infinity. Under supersymmetry, there would be a “susy”
particle counterpart that is similar to the original particle through mathematical
transformations. The tweaking of the susy particle counterpart would come
out to be a very large negative number. If the masses are subtracted, the
final mass of the particle would be reasonable (in accordance with nature).
This process is called renormalization.
Another welcomed factor that supersymmetry adds
to the Standard Model is unifying the four forces (strong, weak, gravity,
and electromagnetic). In the past, two or more forces in nature have been
unified into a single force that describes both ( e.g. Newton’s theory
of gravity that explains why an apple falls to the earth and why the planets
orbit the sun). At relatively low energy levels, the four forces of fundamental
particles appear independent. With supersymmetry, the four forces of nature
diverge into a single force at high energy levels that explains all of
the forces at a lower energy level.
The last element that supersymmetry adds to the
Standard Model is predicting the graviton. The weak force has the W and
Z bosons, the strong force has gluons, and the electromagnetic force has
photons. Like the other forces, gravity needs a mediator for the Standard
Model to be symmetrical. Therefore, supersymmetry completes the picture
by adding the graviton.
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Last Update: June 26, 2001
By: AR