This is probably very basic but I can't quite find the answer in other questions.
As I understand it we are hoping to create supersymmetric particles at the LHC (in this second run) and these particles could be candidates for dark matter. What I am finding confusing is: If we have to create these particles in the LHC, then that means they aren't normally stable? So if they are not stable in the universe now, how could they account for missing mass?
Hope that makes sense, thank you
The fact that the neutralino (the common SUSY DM candidate) can be created in a high-energy collisions means that it interacts with other particles. Specifically, it must interact with the protons in the incoming beams, at least indirectly.
However, just because it has interactions, it doesn't mean that it isn't stable. By stable, we mean that it has a very long (possibly infinite) life-time so that it won't spontaneously decay. We don't mean that it's completely inert.
The stability of the neutralino in SUSY is guaranteed by two facts:
Together these facts forbid the neutralino ($\chi$) from decaying as, $$ \begin{align} \chi &\to \text{SUSY} + \cdots &\text{Forbidden by conservation of energy: $\chi$ is LSP}\\ \chi &\to \text{ordinary particles} &\text{Forbidden by $R$-parity: LHS has $R$-parity, RHS does not} \end{align} $$ Thus all decays are prevented.
Just because they are created at the LHC does not mean they are not stable. The LHC is used to create anti-protons, for example, and an anti-proton is just as stable as a proton. Left alone either are expected to last forever (or at least almost). Just because they annihilate if brought into contact does not mean they are not stable.
Theories of supersymmetry suggest how they might be created and experiments can test these theories (even though the created particles might not be detectable via other means). The signature would be the missing energy observed in the experiment.
