| Spin | Fundamental Particles | High-Energy Particles | Hadrons | Conclusion |  

Important Note: Numbers in the following sections may be expressed in scientific notation. This means that you may find a number followed by ×10ⁿ. In other words, the first number is multiplied by 1 followed by n zeroes. If n is negative, that means that you are dividing instead of multiplying. In the Advanced Topics activities, you may find an "e" sitting in the middle of a number. The "e" means that the value of the number is really the number before the "e" times 10 raised to the power of the number after it. Maybe you'll understand it better with some examples:

All particles have a property called "spin". Spin is measured in multiples of (1.055×10^-34 Joule-seconds), with the same as the units for the angular (rotational) momentum for normal objects. Nevertheless, it is not quite the same. One way of describing spin without resorting to college-level mathematics is that spin is a sort of rotational symmetry. Spin 0 particles are completely symmetrical, like a circle. Spin 1 particles are completely unsymmetrical, so you have to turn it around one full revolution for it to look the same, like an arrow. Spin 2 means that the particle look the same after half a revolution, like a double-headed arrow. Spin ¹/₂ particles must be turned around twice to look the same. This phenomenon is quite difficult to describe, but Richard Feynman's explanation in QED did an excellent job.

One of the consequences of spin is that particles with odd-half-integer spin (¹/₂,³/₂,...) and identical values for their other properties (flavor, position, and velocity) can not exist at the same time (the Pauli Exclusion Principle). These particles are called "fermions". The particles with integral spin (0,1,...) do not follow this rule are called "bosons". Bosons can be made from composite particles containing even numbers of fermions (Mesons are one example).

In addition, the spin number determines the number of "polarizations" (sort of like directions) that the particle's spin can have. Spin 0 particles have one polarization, spin ¹/₂ particles have two, spin 1 particles have three, and so on. Massless particles may not be able to occupy all possible polarizations for their spin value. The polarization is one of the properties of a particle, so two electrons can occupy the same place at the same time if their polarizations are opposite. This phenomenon causes electrons in atoms to arrange themselves so that the periodic table is an accurate description of an element's properties.

The following particles are fundamental (have no internal structure) in the "Standard Model", but there are theories being developed that give them structure. None of these is complete or widely accepted yet. Also, these particles may not have structure after all.

There are two general types of particles in the universe, particles of matter (and anti-matter) and particles that carry forces. The force-carriers are all special types of bosons called "gague bosons". Matter particles are further broken down into matter and anti-matter particles. The fundamental matter particles come in two types, leptons and quarks. Quarks carry color charge and leptons do not.

The fundamental particles of matter (all spin ¹/₂)
Each has an anti-particle with opposite electric charge

	Name ("Flavor")	Mass*	Responds to Force(s)**	Electric Charge	Color Charge
Leptons	electron	0.511		-1
	electron neutrino	<1.5×10-5		0
Quarks	up	10		+2/3
	down	5		-1/3

Particles that carry the forces (all spin 1)

Name	Mass*	Responds to Force(s)**	Carries Force(s)**	Electric Charge	Color Charge
photon	0	none		0
W+,W-	80,000			+1,-1
Z0	91,000			0
gluons (8 kinds)	0			0
mesons (spin 0,1,2,...) not fundamental	varies		***	+1,0,-1

*-	Rest mass in MeV (1 MeV=1.78×10-30kg); quark masses are extremely inaccurate (up to 60% uncertainty).
**-	- Electromagnetic Force - keeps nuclei and electrons together, causes particles and anti-particles to annihilate each other (except for neutrinos), and causes chemical reactions and most other non-gravitational phenomena we see - Weak Force - causes the conversion of one flavor of particle to another (resulting in radioactive beta decay), the decay of higher energy particles to low energy particles, and is the only way that neutrinos and anti-neutrinos can annihilate each other - Strong Force - binds quarks into hadrons and could bind gluons into hypothetical "glueballs"
***-	Mesons (usually the pi+, pi0, and pi-) carry the Residual Strong Force, which keeps protons and neutrons together in the nucleus.

Gravity is not included, since the force is so much weaker than the other three and Einstein's theory of relativity is extremely difficult to combine with quantum mechanics. However, the force acts on all massive particles and may be carried by a spin 2 particle called the "graviton".

The fundamental particles of matter listed above have two groups of counterparts with identical properties except for mass. The higher-energy particles also have anti-particle counterparts. Each of these three groups are called "generations" and are outlined below. The generation III particles are more massive than the generation II particles which are more massive than the generation I particles.

There seem to be complicated mathematical reasons for there being three generations, but no one is 100% sure that the reasoning is correct. Therefore, the search continues for an even more massive fourth generation. Masses in MeV are given in parentheses. Masses for the quarks are extremely inaccurate. (See above for more information)

Hadrons are particles made up of quarks bound together with gluons. Here, only hadrons made of up, down, and strange quarks will be discussed. Many of the so-called "charmed" hadrons have not yet been observed, and naming schemes are still under discussion.

Quarks and gluons come in three "colors", red, green, and blue. These labels have nothing to do with the colors of light, they were just a good analogy for the three types of charges of quarks and gluons. The anti-quarks have opposite color to the quarks. Anti-red is also called cyan, anti-green is called magenta, and anti-blue is called yellow. All hadrons must be colorless, or white, and there are five possible configurations of colors which meet this requirement:

1. red + green + blue, making a baryon
2. anti-red + anti-green + anti-blue, making an anti-baryon
3. red + anti-red, making a meson
4. green + anti-green, making a meson
5. blue + anti-blue, making a meson

We sincerely hope that this page did not scare you. And don't think that we are the experts, too. We had to look this information up in many books, all of which can be found on the Individual Exploration page. (We found the masses at the PDG's site.) Even the people who study this stuff for a living need to look up some of this information, and we don't expect you to memorize it all either. We just want you to understand a few principles about particle physics:

1. Particles with spins of ¹/₂,³/₂,... obey the Pauli Exclusion Principle; those with spins of 0,1,... do not.
2. The four fundamental forces each have gague bosons which carry the forces.
3. There are two groups of fundamental matter particles called leptons and quarks. Each of the four "normal" ones have two higher-energy "cousins".
4. The color charges in all combinations of quarks and anti-quarks must add up to white to make a stable particle (a hadron).
5. A quark and an anti-quark make a meson; 3 quarks make a baryon; 3 anti-quarks make an anti-baryon.
6. Over 95% of the matter you see every day is composed of electrons, protons, and neutrons.
7. Over 95% of the phenomena you see every day are a result of the electromagnetic force and gravity.
8. Over 100% of all particle physics makes no sense.

If this page wasn't enough for you to soak up all in one day, head on over to Advanced Topics and blow your brains out! A number of different activities will let you explore the results of various equations. The final activity is a test with a secret prize for those who get it right.

  | Spin | Fundamental Particles | High-Energy Particles | Hadrons | Conclusion |  

©1997 Shaheen Gandhi and Rahul Jain, All Rights Reserved. This site is a ThinkQuest 1997 Contest Project. Click here for comments.