# Protected operators by SUSY

People say that

In superconformal field theories with extended susy (say 4d, $\mathcal{N}=2$) we have protected local operators annihilated by some supercharges $Q_I$. Also, upon breaking conformal symmetry there is a well defined particle spectrum which contains in particular BPS states.

I am not sure I understand the term "protected". Protected operators against what? Also, how and why these operators lead to BPS states? I know that the latter are annihilated by half or quarter the supersymmetries, but again why protected?

Is there a simple example of such an operator $\mathcal{O}$ where it is obvious that $$Q \mathcal{O} =0?$$

• What people? Source? – JamalS Dec 21 '17 at 15:20
• Heard it today in a talk. But I think it is written in many papers. – Marion Dec 21 '17 at 15:21