# Tau parameter in class TRolke

Hi,
I am using the TRolke class in the case of Poisson background and known efficiency and I have one doubt about the “tau” parameter: In the reference paper W. Rolke et al., Nucl.Instrum.Meth.A551:493-503,2005 the parameter called tau is the ratio between the background and the signal region.
On the other side, in the TRolke class reference tau is defined as the ratio between the signal and the background region.
Is the definition of tau in the root manual correct?
Thank you
Rosanna

I have some problem with the tau parameter 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 understand, tau is 5/3=1.7, right? If it is not right, how could I get the value of tau? Thanks!

I ask a brother who had published a paper by using the method of TRolke, he told me the tau in Rolke.C is wrong, we should use the ratio between background and signal region. You can also see TRolke.cxx in math/physics/src, which exists in the new version of root-6.06.04. One can find a formular “(x-y/tau)”, so from this, we think the tau = size of background/signal region is right. Thanks!

Perhaps what I understand the meaning of tau was wrong before. tau is the ratio of the window of background region and that of the signal region. For example, if the signal region is [1.865, 1.875] GeV, of which the signal window is 10 MeV (1.875-1.865= 0.01 GeV = 10 MeV), and the background region is [1.842, 1.854] || [1.878, 1.886] GeV, of which the window is 20 MeV, the value of tau is 2 （= 20 MeV / 10 MeV）. Thank you!

So, can we consider this thread as solved ?

I think so. Thank you