Question: what is the purpose of supersymmetry and the "sparticles" in it?

David Freeborn answered on 27 Jun 2013:

Hi alexlw18,

I guess it depends what you mean by a purpose? Do you mean why do so many physicists think/want it to be true?

The first point is that it represents one of the last symmetries that nature could take which hasn’t been already excluded. This symmetry, called R-parity, gives every fermion a boson partner, and every boson a fermion partner, and these partners are the “sparticles” of supersymmetry. Nature seems to take on every possible symmetry it can, so if supersymmetry isn’t true, there must be some law we need to discover which somehow excludes this symmetry.

Supersymmetry would also give a very neat way to explain dark matter. Supersymmetric particles would (in most theories) only very rarely interact with ordinary particles (including photons- the particles of light). But it would still have a mass (a very heavy mass, in fact), so it would still interact with gravity. So if supersymmetry is true, we have a really good explanation of what dark matter might be.

Supersymmetry is also useful in explaining the mathematics of a low of processes, and why these processes aren’t much more probable than we see in nature (or even infinitely probable in some cases). That is the result of a lot of the equations of quantum field theory, and it’s quite troubling. Supersymmetric processes could “destructively interfere” with these processes via the laws of quantum mechanics. That would, in effect, cancel out the probabilities in our equations, and lead to the results we actually see.

So there are tantalising reasons to believe supersymmetry might be true!