Dear All
I have an alpha peak need to apply a proper fitting function, and because alpha peak is not well Gaussian peak. but rather a Gaussian + tail on the left.
I have created two fitting functions
Power for the tail range and Gaussian for the peak as follows

TF1 *fPower = new TF1("fPower","[0]*TMath::Power(x,(1/[1]))", 80,250);
fPower->SetParameters(10000, 68);
fPower->SetLineColor(kBlue);
f->Fit("fPower", "R");
TF1 *gausBi= new TF1("gausBi","gaus",200,1300);
gausBi->SetLineColor(kBlack);
f->Fit(gausBi, "R+");

Next step I am trying to formulate this in one function for the full range, or what refer to convolution.
so I build this new function and called the initial parameters as follows

Thank you for this suggestion, I tried crystalball function, but it did not work and because it is difficult to control the parameters I choose to do it manually by convolution between two functions.