RooFit fail to generate simple pdf

I’m trying a very simple generation/fit using the attached macro. I don’t understand why it fails to generate correctly (and fit) using the specified value of the slope for the RooExponential. If the value is set to, say -0.005, then it works fine.

Why can’t I use -0.8 as the slope of my exponential ?

roofitTest.C (2.2 KB)

Hi Stephane,

From the error messages that you get from RooFit, it is fairly clear what
goes wrong

[#0] ERROR:Eval – RooAbsReal::logEvalError(m_B_mass) evaluation error,
origin : RooExponential::m_B_mass[ x=mass c=slope_B ]
message : p.d.f normalization integral is zero
server values: x=mass=5210.72, c=slope_B=-0.5

Please note that you are requesting a function value of exp(5200*-0.5)
= ~6.8e-1130! This gets rounded to zero in double precision floats.
At a more modest value of the slope you will stay (just) in range of what can be held in a double precision float representation.


Oups ! I did not notice that… Sorry for the noise.