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This was the language used by Langacker in his textbook (The Standad Model and Beyond). He says there are two big groups of solutions to the current SM problems, "bottom up" and "top down", but I don't get what he means by that. Could anyone explain this?

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I don't consider this an answer, (because it's copy and paste, for one thing, and way out of my understanding league) and also it's just as a source of one particular use of the term. But the comments box is too small.

It's particular to String Theory, but please bear with me and I will try to refine it to the Standard Model. I have a feeling the terms relate to the energy scales involved.

Source: Bottom up and top down

Since a realistic string theoretic model is, by design, a unification of the standard model of particle physics with quantum gravity aspects and hence at least with aspects of the standard model of cosmology, there are more constraints on such a model than are usually imposed on model building in particle physics alone: the model is not only supposed to reproduce the fundamental particle content but also address moduli stabilization, the cosmological constant and dark matter.

Accordingly one strategy to build models is to first aim for the correct fundamental particle content, and then incrementally adjust to account for the global gravitational constraints.

This approach is known as the bottom-up approach to string model building (AIQU 00).

Contrary to this is the historically older top-down approach (usually attributed to (Candelas-Horowitz-Strominger-Witten 85)) in the heterotic string theory compactification models (see below).

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If anyone's still wondering, McGill states it quite nicely on their website:

...a bottom-up approach (phenomenology) of trying to deduce the new laws from the latest experimental observations; the top-down approach, using the mathematical consistency of string theory to understand quantum gravity...

Source: http://www.physics.mcgill.ca/research/thep.html

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