Fit polynomial exponential function in RooFit

[paulan]

Hi experts,
I have a spectrum over which I usually fit a 4th degree chebychev polynomial. I am now trying to fit a polynomial exponential function [i.e, exp(-poly)] instead. I was wondering how could I go about this? Would I have to use RooGenericPdf to define my own function or is there a way to use RooExponential and RooChebychev for this? Some help with how to proceed would be greatly appreciated, thanks!

[jonas]

Hi, sorry for the late reply!

You can definitely use RooChebychev for the polynomial, but then you can’t put it into a RooExponential because this one strictly represents an exp(c*x) and implements analytic integration only for this case.

Indeed you should use a RooGenericPdf here, just with the exponential of the Chebychev:

RooGenericPdf("mypdf", "std::exp(cheb)", RooArgList(cheb));

I hope this works for you, if you have any other trouble let us know!

Cheers,
Jonas