In particle physics, the term model building refers to a construction of new quantum field theoriesbeyond the Standard Model that have certain features making them attractive theoretically or for possible observations in the near future. If the model building physicist uses the tools of string theory, he or she is called "superstring model builder". A model builder typically chooses new quantum fields and their new interactions, attempting to make their combination realistic, testable and physically interesting. In particular, an interesting new model should address questions left unanswered in the Standard Model which has, including three massive neutrinos, 28 free parameters. A model which extends the Standard Model should predict one or more of these parameters or shed light on some other issue such as why there are three quark-lepton families or, the most common motivation, the naturalness or hierarchy problem associated with the quadratic divergences appearing in the scalar sector.
Model builders constitute a group between experimentalists and "pure" theorists; model builders are theorists, but with an emphasis on using current tools to fit data, in addition to the more long-term pursuit of a more complete theory of nature. Model builders are one step closer to pure theorists than phenomenologists are, although the distinction is often blurred in practice. Model building is speculative because current particle accelerators can only probe up to a few TeV, where physics is well described by the Standard Model. One result of renormalization group theory is that at low energies, models flow toward universality classes and different models may flow to the same universality class so many models can coexist beyond the Standard Model. Only experimental data will distinguish between them. All experimental attempts to look for irrelevant couplings beyond the Standard Model, such as those that give rise to proton decay, flavor changing neutral currents, Peskin–Takeuchi parameters and nonuniversal couplings have so far come up only with upper bounds. However, the observation of neutrino oscillations could be explained by adding a Majorana neutrino mass, which is an irrelevant coupling that could arise, e.g., from the seesaw mechanism. There may be patterns among the marginal and relevant couplings so that among all the possible models which give rise to the Standard Model universality class, one of them has fewer free parameters than the others allowing it to make predictions. It is an article of faith amongst some model builders in particle physics that there exists an elegant high-energy theory or UV completion of the Standard Model.
