Tau parameter using the model SetPoissonBkgGaussEff in TRolke

In Rolke.C, I use the mode SetPoissonBkgGaussEff(x,y,em, tau, sde), where x is the events in the signal region, y is the events observed in the background region, and tau is defined as the ratio between the size of signal/background region. I want to know how to get the value of tau, and could I use the formula “x/y” directly? For example, I get x to be 5 in the signal region [1.865, 1.875]GeV, y to be 3 in the sideband region [1.842,1.854] || [1.876,1.886]GeV, as I understanf, tau is 5/3=1.7, right? If it is not right, how could I get the value of tau? Thanks!

Tau is the probability that a background events false into the signal region. So depending on your background model, if it is flat it depends on the size of the side band comparing to the size of the signal region.

As a side note, I would recommend to model your problem using RooFit/Roostats which give you much more flexibility and then obtain the corresponding asymptotic limit rather than using TRolke

Best Regards