Dear all,
I’m trying to fit a histogram with a reversed crystal ball function (signal) and a polynomial function (background).
The crystal ball has the “n” and the “alpha” parameters.
If I set “n” 's limits as [1,8] after the fit I get n=2.3
If I set “n” 's limits as [1,5] after the fit I get n=4.49.
The problem is every time I change the upper limit I get a completely different fit (N_signal events …) results.
I’m not familiar with the crystal ball function so I don’t know what are the best limits to choose on these parameters!!
1-Should I assume that for n [1,8] I will have the best fit for my histogram?
2-I don’t want to fix my parameters however is there’s any way to not limit them?
I hope someone will be able to shed a light on my problem. I’m a Roofit newbie and I still did not figure everything out.
Thank you in advance.
RooRealVar mean1("#mu","",3.0962,3.0962,3.0979);
RooRealVar sigma1("#sigma","",0.0048,0.0046,0.0056);
RooRealVar alpha("#alpha","",-1.54,-2,-1);
RooRealVar n("n","",1,1,8);
RooCBShape sig1("sig1", "sig1", x, mean1, sigma1, alpha,n);
RooRealVar a0("a0","a0",0.0005,-2.,1.) ;
RooRealVar a1("a1","a1",-0.0002,-2.,1.) ;
RooRealVar a2("a2","a2",0.0005,-1.,1.) ;
RooChebychev bkg("bkg","background p.d.f.",x,RooArgList(a0,a1,a2));
RooRealVar nsig("N_{SIG}","signal events",500,0,100000000);
RooRealVar nbkg("N_{BKG}","signal background even0ts",500,0,100000000);
RooExtendPdf esig("esig","esig",sig1,nsig) ;
RooExtendPdf ebkg("ebkg","ebkg",bkg,nbkg) ;
RooAddPdf all("all","all",RooArgList(esig,ebkg)) ;