# RooFit Fit Two Signal Peaks with Gaussians and Background with Exponential

Hello,

I am trying to fit two signal peaks with two gaussians, and the background with a decaying exponential. I have consulted the relevant tutorials: a couple of relevant examples illustrate how to fit a two-component signal PDF with two gaussians differing in `sigma` but not in `mean`. In addition, another example llustrates how to plot a model in a specific range. However, my issue differs from these cases in that I am trying to create a composite model with two gaussians with different parameters, so as to fit two different peaks.

Please find a snippet from my attempt below, in PyRoot

``````#ROOT file
fin = TFile('outfile_2CEP.root')
tdata = fin.Get('t1')

m_Psi_2S = 3686.09
m_JPsi = 3096

#//mass range
mass = RooRealVar("M","M",2500,4000)

## getting the data
data = RooDataSet("data","data",tdata,RooArgSet(mass))

#bkg
c_exp = RooRealVar("#tau_{bkg}","Exp arg constant",-0.01,0)
BkgPDF = RooExponential("BkgPDF","BkgPDF",mass,c_exp)

#//signals PDF (gaussian)
J_mass = RooRealVar("#mu1","#mu1", m_JPsi, 3096-65, 3096+65)
J_width = RooRealVar("#sigma1","#sigma1",30,0,200)

psi2s_mass = RooRealVar("#mu2","#mu2", m_Psi_2S, m_Psi_2S-65, m_Psi_2S+65) #mean of gaussian
psi2s_width = RooRealVar("#sigma2","#sigma2",30,0,100) #find width, 1 std dev

SigPDF1 = RooGaussian("SigPDF1","SigPDF1",mass, J_mass, J_width) #<----maybe a problem with mass?
SigPDF2 = RooGaussian("SigPDF2","SigPDF2",mass, psi2s_mass, psi2s_width) #see above

#only way the whole thing does not crach is using this below, probably bug!
NSig = RooRealVar("Signal yield","Signal yield",4000, 0, 10000)
NBkg = RooRealVar("Bkg yield","Background yield",500000, 0, 1000000)

sig1frac = RooRealVar("sig1frac", "sig 1 fraction", 0.9, 0., 1.)
sig2frac = RooRealVar("sig2frac", "sig 2 fraction", 0.1, 0., 1.)
SigPDF = RooAddPdf("sig", "sig composite pdf", RooArgList(SigPDF1, SigPDF2), RooArgList(sig1frac, sig2frac) )

bkg_frac = RooRealVar("bkg_frac", "bkg_frac", 0.3, 0., 1.)

model = RooAddPdf("model","model",RooArgList(SigPDF, BkgPDF), RooArgList(NSig, NBkg))

``````

I would love to understand where am I going wrong here, or even better, how to fix it.

Thanks,

Blaise

Hi,

I see a problem with the addition of the two signal pdf. If you want to define the two components as fraction you need to specify only one.

`SigPDF = RooAddPdf("sig", "sig composite pdf", RooArgList(SigPDF1, SigPDF2), RooArgList(sig1frac) )`

Otherwise you define two different components, Nsig1 and Nsig2 and have Nsig = Nsig1+Nsig2

Lorenzo

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