Particle Physics Overview

Overview

Particle Physics is a constantly changing science. It seeks to understand the fundamental building blocks of everything - the particles that cannot be broken down into anything else. Over the decades, it has spawned many new sciences based upon what it once thought was fundamental.

Here you will find a relatively complete overview of particle physics. This page will not delve too much into the mathematics nor physics of the subject - there are many college and graduate school classes that teach this - but you will be able to learn about the basics, and you will be able to find enough information here in order to understand the terminology in the rest of the site.

This page starts from the ground up, starting with the fundamental forces and then building up with Bosons and Fermions. Then, the page talks about heavier particles that are made of parts of quarks, continuing with a brief discussion of atomic physics, and culminating with a discussion of antimatter.

Fundamental Forces

There are four "fundamental" forces that we know of. Again, by fundamental, this is what current physicists believe every force is a representation of, and can be fundamentally be broken down into. They are:

Chart information from http://cpepweb.org/.

Bosons

Bosons go hand-in-hand with the Fundamental Forces discussed above, for they are the actual particles that carry the forces. They have an integer spin (0, 1, 2, ...).

The photon, symbolized by "γ," is probably the particle that most people are familiar with. Usually, it is thought of as a particle of light, but it is also the carrier of the electromagnetic force. It has 0 electric charge, and 0 mass. It has a spin of 1.

The gluon, symbolized by "g", is the carrier of the Fundamental Strong Nuclear Force. It has 0 electric charge and 0 mass. It has a spin of 1.

The carriers of the Weak Nuclear Force are W⁺, W^-, and Z⁰ particles. They have a spin of 1, as well, and electric charges of +1, -1, and 0, respectively. Their masses are 80.4 GeV, 80.4 GeV, and 91.187 GeV, respectively.

The carrier of Gravity is the theoretical particle of the graviton. It has not yet been proven to exist; its theoretical spin is 2 and electric charge 0.

Fermions

Fermions are the constituents of matter as we know it. What classifies them is that they have a half-spin (1/2, 3/2, 5/2, ...). The two sub-categories of fermions are Leptons and Quarks. Leptons have integer electric charges while quarks have third electric charges.

Leptons

The Standard Model currently holds six different leptons, all called "flavors," all of which have a lepton number of 1. There are the electron, muon, and tau particles, along with their associated neutrinos. Theoretically, neutrinos are massless. If you note above, there are not masses noted for them -- it is just known that they do not have a mass above that stated.

Several experiments are currently attempting to determine if the neutrinos have mass by determining the precise number of each type that is received from the sun. If the numbers differ from those predicted, then either models of solar neutrino generation are wrong or neutrinos have switched flavor on the way to Earth. If they have switched flavor, then theory demands that they have mass.

The muon and its antiparticle are formed naturally by the decay in the upper atmosphere of pions that are produced by cosmic rays:

The muon is a very unstable particle that can decay from either an electron or a positron in the following fasion:

Quarks

As with leptons, there are six flavors of quarks that fall into three pairs, all of which have a baryon number of 1/3. They are the up and down, charm and strange, and top and bottom quarks. All particles that are made of quarks are called "Hadrons." Particles made of two quarks are called "Mesons," while particles made of three are called "Baryons." The Standard Model holds that there are no other combinations of quarks, and no quarks have ever been produced that are not in a pair or triplet.

Murray Gell-Mann was the man to label the quark, and he got it from the book "Finnegan's Wake" by James Joyce. The line "three quarks for Muster Mark..." appears in the book. Gell-Mann won the 1969 Nobel Prize for his work in classifying elementary particles.

Up and down quarks are the most common types, for they make up protons and neutrons - the bulk constituents of atoms.

Hadrons

Even though hadrons are technically a sub-category of fermions, for hadrons are combinations of quarks, they are such a large category that they are listed seperately here.

As previously stated, hadrons are particles that are made of combinations of quarks. Quarks are never found singly, and no quark has ever been able to be isolated experimentally to date. Theoretically, it is actually impossible to isolate a quark due to quantum chromodynamics. The color force of chromodynamics is extremely strong at the level of quarks, and actually increases its strength with distance. Therefore, if you were to put enough energy into a quark system to try to pry it apart, the energy needed to separate them would be much greater than that needed to create new quarks. So, theoretically, new mesons would be created, and that is what is observed.

Besides there being no paticles made of one quark, there are no particles made with more than three. Particles made of two quarks are actually made of a quark and an antiquark. They are called "Mesons." Particles that are made of three quarks are called "Baryons."

Mesons

Mesons are made from combinations of a quark and an anit-quark. Out of place in the hierarchy that this page sets up, mesons are not actually fermions, but are classified as bosons (even though they are made of quarks and quarks are fermions). There are about 140 types of mesons. They have a spin of 0 or integers. The following is a table of some of the main mesons:

*These mesons are made of symetric and antisymmetric combinations of ds and ds quarks.

The pion is the lightest of all the mesons, and because mesons are the mediating particle of the Residual Strong Nuclear Force, they can be used to predict the maximum range of the strong interaction. The pion also shows that the masses of mesons (and hadrons) in general depend on the internal dynamics of the particle rather than the quarks within it because of the pion masses. Composed of u, d, u, and d quarks, one would expect them to have a mass of approximately 2/3 that of a proton, but they actually have masses of about 1/6. The main role of the pion is interaction with nuclei and transformation of a neutron to a proton or vice versa:

The J/Ψ meson's discovery in 1974 came as a surprize to experimentors, and was the first direct experimental evidence for the fourth type of quark, the charm quark.

The Υ meson's discovery at Fermilab in 1977 also did not fit into the then-standard framework. It was the first experimental evidence for the fifth type of quark, the bottom quark.

Baryons

Baryons are combinations of three quarks and / or antiquarks. They are categorized as hadrons and also as fermions. They have a charge and spin of integer multiples of 1/2, and they also have a baryon number of 1 and are assigned a strangeness number based on the number of s or s quarks that they are made of. Conservation of baryon number is an important part of reactions, and no known process can violate it. The following table lists some of th 120 baryons that are in the Standard Model:

Because baryons decay by the Strong Force, they typically should have decay rates on a time scale of 10^-23. However, many of the baryons listed are stable for much longer periods of time; this is because there is some conservation law that forbids their decay by the Strong Force, and so they decay via the Weak.

Atomic Physics

This section is designed to give a brief overview of what is involved in atoms and the sizes involved.

Normal atoms (e.g. not atoms) are approximately on the scale of 1 Å across, including the electron cloud. Protons and neutrons occupy a nucleus region that is on the scale of 0.1 mÅ, while electrons orbit in clouds whose shape and size are determined by the laws of quantum mechanics.

Atoms are generally classified by the number of protons in the nucleus. The Periodic Chart is a graphical classification system for categorizing atoms. Naturally existing atoms have a proton content of up to and including 92; laboratories have succeeded in creating atoms up to 116 protons, although they are very short-lived.

An important fact in nuclear physics is that fission or fusion results in a release of energy if the by-product(s) approach Iron (Fe) in terms of their atomic number. The reaction consumes energy if the by-product(s) go away from Fe.

Antimatter

Most scientists will admit that much of theoretical work is as much subject to aesthetics as it is to science. What this means is that while they seek to explain structure and observations, they are guided by the goal of explaining it in a manner that makes sence and "looks good."

One important part of this is symmetry. To this effect, the Standard Model predicts that everything has an exact opposite, or anitparticle.

Antiparticles have the exact same mass but the opposite, charge, spin, and other quantum numbers. They are usually represented by a bar over the symbol for the matter counterpart, such as u and u, although they are sometimes represented by the opposite superscript charge, such as e^- and e⁺ for the electron and its antiparticle the positron.