Reverse Landau function in RooFit?!

[sinaotl]

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_ROOT Version: 6.13
_Platform: Centos7
_Compiler: PyRoot

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Hello Everyone,
I’m new to Roofit, and I’m trying different functions to fit my mass distribution. I have a low tail on the left side of the peak which is the mirror of what happens in Landau function. How can I get the inverse of Landau function in RooFit?!

Thanks,
Sina

P.S I found the solution later on myself. First, you need to change your x values to -x using TF1 and then use the TF1 in the function. i.e :
x = RooRealVar(“x”,“m_{tt} [GeV]”,0 , 100);
fa1 = TF1(“fa1”,"-x", 0, 100);
rfa1 = RooFit.bindFunction(fa1,x);# RooAbsReal

ml = RooRealVar(“ml”,“mean landau”,ml0,-6000,6000) ;
sl = RooRealVar(“sl”,“sigma landau”,sl0,0,3000) ;
landau = RooLandau (“landau”,“lx”,rfa1,ml,sl);

[StephanH]

Hello Sina,

that sounds like you are looking for the crystal ball function. It is the typical function to use if your mass distribution has a tail to the left because of e.g. losses in the reconstruction of the energy.

https://root.cern.ch/doc/master/classRooCBShape.html

Crystal Ball function

[sinaotl]

Hi,
Yes, I’m familiar with the Crystal Ball function, but I prefer to use the mirror of the Landau function, or in general any given function that is asymmetric.
Thanks.