Particle Physics Overview
Overview
Particle physics is one of the frontiers of science where theory is meeting the test and hopeful confirmation of high-energy experiments. It seeks to understand the fundamental building blocks of everything - the particles that cannot be broken down into anything else - in how they work, how they interact, and other characteristics about them. Over the decades, it has spawned many new sciences based upon what it once thought was fundamental (for example, chemistry was once effectively "particle" physics).
Here you will find a relatively complete overview of basic 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 and concepts on the rest of this website.
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, and then culminating with a discussion of antimatter.
Note: One convention often used in particle physics is to describe the mass of an object in terms of its energy. They are interchangeable through Einstein's famous equation 
. They are usually expressed in terms of an eV (electron-Volt, where 1 eV = 1.6·10-19 Joules).
Fundamental Forces
At its most basic level, a "force" is a way of communicating something. For example, in order for a star to "tell" a planet that it is present, it has to communicate, and it does that through the force that we call "gravity." As another example, for an electron to communicate to a proton that it is present, it uses the force that we call "electromagnetism."
To-date, physics has broken down every fundamental communication mechanism into four "fundamental forces" of nature: Gravity, Electromagnetism, Weak Nuclear, and Strong Nuclear. The first two operate at all scales - from atoms to galaxies - while the second two only have ranges that operate inside of atoms.
The following table summarizes some of the basic properties of these forces:
| Force Name: | Strong Nuclear |
Weak Nuclear |
Electromagnetic |
Gravitational |
||
Fundamental |
Residual |
|||||
| Mediating Particle: | Gluons |
Mesons |
W+, W-, and Z0 |
Photon (γ) |
Graviton |
|
| Affects What? | Color Charge of Quarks and Gluons |
Hadrons |
Flavor of Quarks and Leptons |
Electric Charge |
Matter and Energy |
|
| Relative Strength | 2 u quarks at 10-18 m |
25 |
N/A |
0.8 |
1 |
10-41 |
2 u quarks at 3·10-17 m |
60 |
10-4 |
1 |
10-41 |
||
2 protons in nucleus |
N/A |
20 |
10-7 |
1 |
10-36 |
|
This table has a fairly large amount of information, but for conceptual purposes, the relative strength of the forces is worth paying significant attention to: Relative to the Electromagnetic, the only force that is stronger is the Strong Nuclear, while the Gravitational force is significantly weaker.
So far, it is unknown why any of these forces have the relative strength they do, why a certain particle is what communicates the force, why they act on what they act on, nor why they have the effective ranges they do. That is still an are of theoretical study, and we may determine the answer in the future.
For information on what each of these particles that convey the forces do, and what they convey, see the next section on Bosons:
Bosons
In particle physics, the next-up category one encounters from the fundamental forces are bosons, since these are the particles that carry the forces between objects.
A basic property of bosons is that they have a given spin to them that is described by an integer (0, 1, 2, 3, ...).
The gluon (g) communicates the Strong Nuclear force. It has 0 electric charge, 0 mass, and a spin of 1. Its function in a larger sense is to keep protons and neutrons bound together in atoms.
The Weak Nuclear force is carried by the W+, W-, and Z0 bosons. They all have a spin of 1, and they have an electric charge of +1, -1, and 0 (as indicated by their superscripts). Experiments have shown their masses to be 80.4 GeV, 80.4 GeV, and 91.2 GeV, respectively. Its function in a larger sense is responsibility for the process of beta decay, which is how most radioactivity works.
Moving on to the electromagnetic force, the photon (γ) is the mediator and is probably the boson with which people are most familiar. Usually, it is thought of as a particle of light, but light is really just a manifestation of the Electromagnetic force (see the page "Light (The Electromagnetic Spectrum)" for more information). The γ has an electric charge of 0, 0 mass, and a spin of 1.
The graviton - the carrier of the Gravitational force - is the only boson that is generally included in the Standard Model of Particle Physics that has so far eluded experimental detection. Its theoretical spin is 2 and electric charge is 0.
There is another boson - more theoretical than the graviton - known as the Higgs boson (H0). It is theorized to explain the origin of mass itself - a "particle field" that lends objects mass via their interaction with it. It is thought to have a mass greater than 112 GeV.
Fermions
Fermions are the constituents of matter as we know it. What classifies them as separate from Bosons is that, instead of an integer spin, they have a half-spin (1/2, 3/2, 5/2, ...). There are two sub-categories of Fermions - Leptons and Quarks. The difference between the two is that Leptons have integer electric charges while quarks have third electric charges. Each type of Lepton and Quark is called a "flavor."
Leptons: There are six Leptons in the Standard Model, as shown in the table below:
Flavor |
Mass (GeV) |
Electric Charge |
Lifetime (sec) |
Electron Neutrino (νε) |
< 2.2·10-9 |
0 |
Stable |
Electron (e-) |
0.000511 |
-1 |
Stable |
Muon Neutrino (νμ) |
< 0.00017 |
0 |
Stable |
Muon (μ-) |
0.1057 |
-1 |
2.197·10-6 |
Tau Neutrino (ντ) |
< 0.0155 |
0 |
Stable |
Tau (τ-) |
1.777 |
-1 |
2.906-13 |
All leptons have a lepton number of 1. There are the electron, muon, and tau particles along with their associated neutrinos. Theoretically, neutrinos were initially thought to be massless, but they are now known to have masses, but they are experimentally unmeasurable by today's equipment.
They are known to have masses because of the "solar neutrino problem." It is not the purpose of this page to discuss this issue, but there is good background information on it here.
Quarks: There are six Quarks in the Standard Model, as shown in the table below:
Flavor |
Mass (GeV) |
Electric Charge |
| Up (u) | 0.0024 |
2/3 |
| Down (d) | 0.0048 |
-1/3 |
| Charm (c) | 1.27 |
2/3 |
| Strange (s) | 0.104 |
-1/3 |
| Top (t) | 171.2 |
2/3 |
| Bottom (b) | 4.2 |
-1/3 |
All quarks have a baryon number of 1/3 (hence why 3 quarks make 1 baryon). 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
Technically, Hadrons are fermionic matter because they are composed of quarks. However, because hadrons are not as indivisible as fermions, they are presented here as a separate category.
Quarks are never found alone, 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 it 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 (two-quark combinations) would be created, and that is what is observed.
Besides there being no particles made of one quark, there are no particles made with more than three. Particles made of two quarks are made of a quark and an antiquark and are called "Mesons." Particles that are made of three quarks are called "Baryons."
Mesons: There is a very large list of mesons (external link) based on the various combinations of one of the six quarks with one of the six anti-quarks. They are defined to only be made of two quarks and to also have an integer spin and so can be classified as bosons.
Baryons: There is a very large list of baryons (external link) based on the various combinations of 3 of the 6 quarks and anti-quarks. They have half-integer spins and so can be classified as fermions. Another way to classify different types of baryons is by their "strangeness," which is given by the number of s or s quarks they are made of. In interactions, there is a conservation of baryon number that no known process can violate.
For a brief period of time, it was thought that a few exotic "pentaquark" baryons made of three regular and one anti-quark, but that experimental evidence is largely unconvincing to the scientific community at this time.
Two of the most "famous" baryons are the proton and neutron. The proton is made of a uud quark combination, while the neutron is udd. Unlike most baryons which are stable over timescales much shorter than 1 second, protons are thought to be stable over timescales of 1035 years. Neutrons, however, will decay after approximately 8.9·102 seconds if they are not bound in atomic nuclei.
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 sense and "looks good."
One important part of this is symmetry. To this effect, the Standard Model predicts that everything has an exact opposite, or antiparticle.
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.
It should be noted that antimatter is not all theoretical - it has been created in laboratories. In fact, if you have ever gotten a PET scan, you have been scanned by antimatter. "PET" stands for "Positron Emission Tomography" and works by observing gamma rays that are emitted from your body after consuming a positron-emitting radioactive isotope. The gamma rays are created when positrons are released and annihilate with electrons in your body.