Help on likleihwood ratio PDF

Dear experts,

Suppose I know that a certain substance is substanceA or substanceB. I know the density of substanceA and substanceB. I do a meausurement and I suppose gaussian distribution, with a fixed and known sigma. So my models are:

P(x|A) = G(x | densityA, sigma)
P(x|B) = G(x | densityB, sigma)

and I define them as RooGaussian. Now I want the distribution of the likelihood ratio. I can solve this problem on paper, but which classes can I use with RooStats? Can you give me some hints?