The Present Theory of Fundamental Particles and Forces

An Article Written Originally for Midlevel Teachers

 

Before World War II, it was known that the nucleus was composed of closely packed protons and neutrons, but little was known about the "strong force" that kept them together.

From 1950 to 1970, accelerators were built which were designed to probe nuclei with higher speed and more energetic, charged particles such as electrons and protons. The result was that hundreds of new particles were discovered and their properties defined.

In 1963, a theory was proposed that a major group of these particles, called hadrons, could be thought of as made from a few, more fundamental particles, called quarks. Protons and neutrons are members of the hadron group.

Quarks are proposed to be the simplest, irreducible, structureless building blocks of hadrons. The Quark Hypothesis states that quarks in combinations of two or three, make all the observed hadrons.

In 1963, the three quarks were named: up (u), down (d), and strange (s). In 1974, the existence of Charm quark (c) was revealed and in 1977, Leon Lederman and his collegues at Fermilab uncovered the fifth quark, bottom (b). A neutron is composed of three quarks: u d d; a proton is u u d; and a lambda is u d s. One more quark, top (t), has been found at Fermilab in 1995.

Electrons, neutrinos, and a few other particles make up another group of particles called leptons. Leptons are not considered divisible and are not made up of quarks.

The results of particle physicists' theoretical and experimental work up to 1985 might be summarized this way:

All matter is thought to be made up of quarks and leptons and the forces through which they interact. There are six quarks (each comes in three "colors" making 18 particles and each has an antiparticle making 36 quarks in total.) The six quarks are named up (u), down (d), strange (s), charm (c), bottom (b), and top (t). (The last two are sometimes fancifully referred to as "beauty" and "truth.") The six quarks have been confirmed through indirect observations, but not isolated as individual particles.

The other six particles (also appearing in antiparticle form, making 12 total) are the leptons. These include electrons (e), electron neutrinos (ue), muons (m), muon neutrinos (um), tau particles (t), and tau neutrinos (ut).

The twelve particles (48 in all if you include colors and antiparticles) are subject to the four fundamental forces of nature. These forces are gravity, electromagnetic, strong, and weak. Each force is defined by the way it interacts with particles to build up composite form of matter: protons, neutrons, nuclei, atoms, molecules, planets, stars, and so on.

Each of the forces has a strength, a range, and a "carrier" particle as outlined in the table below.

One of the fundamental quests of the Fermilab scientists is to find an underlying link to unify the four basic forces. This Unification Theory would link all particles and forces into a coherent and simple description of nature.
In order to "observe" the basic particles of matter and collect data which may be of use toward theory development and perhaps the Unification Theory, particle probes with great amounts of energy are needed. The protons with 1000 GeV (1 TeV) energy now available in Fermilab's accelerator will help in this quest. By creating head-on collisions between these protons and 1000 GeV antiprotons (generated earlier in stationary target collisions in a nearby storage ring) circulating in the opposite direction, 2000 GeV collision data will be generated.

The main purpose of Fermilab and other large particle accelerators is to collect data that will support or refute theories. The need for new and better data is continuous. Numerous experiments remain to be done and each new theory and the related attempts at experimental verification inevitably lead to new insights as well as new questions about the most fundamental particles and forces that form all matter.

http://ed.fnal.gov/tchrbkground/theory.html