Standard Model

What are the smallest constituents or basic building blocks of all matter? Or, in scientific terms, what are the elementary or fundamental particles of matter? The words "elementary" and "fundamental" mean that the particle has no substructure. It cannot be broken down further into its constituent parts. Originally, it was thought that the atom was a fundamental particle, but it was discovered that atoms are made up of electrons and a nucleus. And, the nucleus is made up of protons and neutrons. So is that it? Can protons, electrons and neutrons be broken down further? The electron is still considered an elementary particle. Protons and neutrons were found to have substructure. Presently, there are 12 fundamental particles divided into two classes - leptons and quarks.

Within the lepton class, there are 6 fundamental particles - electron, muon, tau, electron neutrino, muon neutrino, tau neutrino.

Within the quark class, there are 6 fundamental particles - up quark (u), down quark (d), charm (c), strange (s), top quark (t), bottom quark (b).

The Fermilab's site, "Searching for the Building Blocks of Nature", relates how they discovered the top and bottom quark. The site talks about the accelerators and detectors needed, other technologies such as MRI (Magnetic Resonance Imaging) developed as a result of this research and their ongoing search for fundamental particles and forces.


All matter within the world is made up of three of these particles - the electron, up quark and down quark. Electrons, protons (u,u,d quark triplet) and neutrons (d,d,u quark triplet) combine to form atoms and molecules. The siteAtom builder illustrates how electrons, up quarks, and down quarks combine to make an atom, in this case a carbon atom. Building an atom is not easy.


So, what holds it all together? There are four elementary forces or interactions between particles - strong (hold the quarks together to form neutrons and protons), weak (help heavy particles to decay), electromagnetic (holds electrons to atomic nucleus) and gravitational. The particles interact with one another via "force carrying particles" called bosons. There are 4 bosons.

1. Gluons (nuclear force)

2. Photons (electromagnetic force)

3. W and Z bosons (weak force)


This is the "Standard Model of Particle Interaction" (or Standard Model - as it stands) - 6 quarks, 6 leptons and 4 bosons. Particle Physics Timeline (or Timeline ) illustrates the development of ideas from 624 B.C. to the present that led to the Standard Model. But it is not the end of the story. The quest goes on.