The Standard Model of Particle Physics

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Menu: Ingredients of the Universe

So, we used to think that ordinary, atomic matter dominated the mass inventory of the universe. Even then, we knew that other forms of matter were possible. A zoo of other particles have been created in the lab and have been produced by cosmic rays. You may have heard of some of them: mesons, positrons, muons...

Ordinary matter is almost the only stable known form of matter.

So, how do those other particles relate to ordinary atomic matter, and why don't they contribute to the mass of the universe?

First, the easy answers:
Of all the possible types of particles from the zoo (the zoo is made of members and combinations of members of the "Standard Model"), the only stable particles (stable meaning they don't decay very rapidly) are:

So, to summarize: The combinations of particles which form ordinary atomic matter happen to be the only particles (except for the ghostly neutrinos) which are long-lived enough to be important in considerations of the cosmological mass scales.

But, what of the others and what are their relationships to ordinary atomic matter?

The Standard Model

Let's see if we can cover the entire Standard model as simply and succintly as possible. According to the Standard Model, there are two types of things (besides empty space, which may not be really 'empty'):

Let's first talk about the force carriers. An example of a force carrier is the photon, which is a packet of light. It actually carries the electromagnetic force. There are others which correspond to the other forces:

These particles mediate interactions between the fundamental fermions, the particles we usually think of as "matter". These come in two basic types:

Many different kinds of particles may be constructed from the quarks, though all except the proton and neutron are extremely unstable. The strong force holds quarks together into nucleons and nucleons together into nuclei. The electrons which orbit the nucleus are leptons, held in place by the electromagnetic force. The weak force allows some types of radioactive decay, as well as providing neutrinos with their very feeble interactions with the visible world.

For vastly more detail on this subject, check out The Particle Adventure.


Last updated April 28, 2007