Is the Standard Model a theory or a model?

The words model, hypothesis, and theory are each used quite differently in science. Their use in science is also quite different that in everyday language.

To scientists, the phrase "the theory of ..." signals a particularly well-tested idea. A hypothesis is an idea or suggestion that has been put forward to explain a set of observations. It may be expressed in terms of a mathematical model. The model makes a number of predictions that can be tested in experiments. After many tests have been made, if the model can be refined to correctly describe the outcome of all experiments, it begins to have a greater status than a mere suggestion.

Scientist do not use the term "the theory of .." except for those ideas that have been so thoroughly tested and developed that we know there is indeed some range of phenomena for which they give correct predictions every time. (But, language being flexible, scientists may use "a theory" as a synonym for "a hypothesis", so listen carefully.)

Today, any set of scientific ideas referred to as "the theory of ..." is a well-tested and well-established understanding of an underlying mechanism or process. Such a theory can never be proved to be complete and final -- that is why we no longer call it a "law." However, it is the same kind of well-tested set of rules, with an established area of applicability, as the older ideas called "laws".

These theories will always remain part of our understanding of matter, even when new discoveries take us beyond them in certain ways. For example, Einstein's Special Theory of Relativity replaced Newton's Laws of Mechanics. But, Newton's version is retained as an excellent approximation to the Einstein theory when objects are moving at velocities that are small compared to the velocity of light. (Actually Newton's Laws remain true in Einstein's theory provided you use the correct form and the correct relativistic definitions of energy and momentum.)

Similarly, the theory of atoms is not invalidated, but rather extended, by discoveries of structure within protons and neutrons. If we later discover that the particles now treated as fundamental in the Standard Model do indeed have substructure, or that there are additional types of more massive particles that mediate interactions not included in the Standard Model, this will similarly lead to extensions of the Standard Model. However, we have done enough testing to be convinced there will always be a range of phenomena for which the Standard Model provides accurate predictions.

The fact that we have a theory with the name "Standard Model" is a bit peculiar. There were a number of similar competing models. The one that kept passing the experimental tests became the Standard Model and eventually this became the theory of fundamental particles and their interactions. Physicists continue to use the name Standard Model, but add capital letters to denote its status as more than just a model!

Particle physics research today seeks to go beyond the Standard Model, to answer questions that it does not address, or to explain relationships that are simply arbitrarily fit by parameters in the Standard Model description of nature.  It also seeks to test parts of the model that have yet to be tested, and to explore whether its applicability extends to new higher energy events.