The results of particle accelerator experiments have led scientist to postulate the existence of three types of forces important in the nucleus: The strong force, the weak force, and the electromagnetic force. These forces are though to account for all types of interaction found in matter. The fourth force found in nature, but not in the nucleus, is the gravitational force. These forces are believed to be generated by the exchange of particles between the interacting pieces of matter. For example, the gravitational force is thought to be carried by particles called gravitons. The electromagnetic force is assumed to be exerted through the exchange of photons. The strong force, not charge related, and only effective at very short distances ( 10^-13 cm), is postulated to involve the exchange of particles called gluons. The weak force is 100 times weaker than the strong force and seems to be exerted through the exchange of two types of large particles, the W (has a mass 70 times the proton's mass) and the Z (has a mass 90 times the proton's mass).

The particles discovered have been classified into several categories. Three of the most important classes are as follows:

1. Hadrons are particles that respond to the strong force and have internal structure. Hadrons consist of Baryons and Mesons. A Baryon is composed of three quarks. A Meson is composed of a quark and an antiquark.
2. Leptons are particles that do not respond to the strong force and have no internal structure.
3. Quarks are particles with no internal structure that are thought to be the fundamental constituents of hadrons. Neutrons and protons are hadrons that are thought to be composed of three quarks each.

   Each of these main classes also contains antiparticles. For example the electron, which is a Lepton, has an antiparticle called a positron ("electron" with a positive charge).

The world of particle physics appears mysterious and complicated. For example, particle physicists have discovered new properties of matter they call "color", "charm", and "strangeness" and have postulated conservation laws involving these properties. This area of science is extremely important because it should help us to understand the interactions of matter in a more unified way.

Nucleus
Hadrons		Leptons
Baryons	Mesons
Protons Neutrons Other, short- lived particles	Pions Kaons Other, short- lived particles	Tau Muon Electron	Tau neutrino Muon neutrino Electron neutrino

Elementary Particles: Leptons and quarks
Quarks

Bottom	Top
Strange	Charm
Down	Up

The Four Forces of Nature

1. Gravatational Force
   weakest of the four
   acts at long range, varying by
   force transmitted by gravitons

2. Electromagnetic Force
   originally two forces, electric and magnetic until they were unified into the electromangetic
   holds atoms, molecules, liquids, and solids together
   acts at long range, varying by
   force transmitted by photons
   studied in quantum electrodynamics (QED)

3. Weak Nuclear Force
   this second weakest force allows matter to disintegrate and transmutate,
   - decay from neutrons, + decay from protons
   acts at very short range
   independant of electric charge
   acts between leptons and hadrons
   acts between hadrons and other hardons

4. Strong Nuclear Force
   holds the quarks together to form mesons and baryons
   strongest of all the forces
   acts at very short range
   force transmitted by gluons (coors of quarks)
   studied in quantum chromodynamics (QCD)

This page was made by Erik Epp.