P(n,μ)=e^-μμⁿ/n!
μ=N_theory=εLσ+b
n=N_observed

To set a limit on a parameter p entering in N_theory at a certain CL:
P(N_observed,N_theory(p))<1-CL

Example - limits when no event is observed:
P(0,μ)=e^-μ
i.e. μ=-log[P(0,μ)]>-log(1-CL)

Useful number: -log(0.05)~3.