Double_t turnon(Double_t *x, Double_t *par) { 
	double halfpoint = par[0];
	double slope = par[1]; 
	double plateau = par[2]; 
	//double offset = par[3]; 
	//double plateau = 1.0; 
	double offset = 0; 
	double pt = TMath::Max(x[0],0.000001); 
	double arg = 0; 
	arg = (pt - halfpoint)/(TMath::Sqrt(pt)*slope); 
	double fitval = offset+0.5*plateau*(1+TMath::Erf(arg));
	return fitval; 
} 
Double_t polynom2(Double_t *x, Double_t *par) {
   double p0 = par[0]; 
   double p1 = par[1]; 
   double p2 = par[2]; 
   double pt = TMath::Max(x[0],0.000001); 
   double fitval = p0+p1*pt+p2*pt*pt; 
   return fitval; 
} 
Double_t ftotal(Double_t *x, Double_t *par) {
	return turnon(x,par) + polynom2(x,&par[3]);
}
void plot_turnon() { 
   TFile *f = new TFile("presel.root");
   TH1F *sht20_seta = (TH1F*)f->Get("sht20_seta");
   TH1F *allem_seta = (TH1F*)f->Get("allem_seta");
   gROOT->SetStyle("Plain"); 
   gROOT->ForceStyle(); 
   gStyle->SetOptTitle(1); 
   gStyle->SetOptStat(0); 
   sht20_seta->Rebin(2); 
   sht20_seta->Sumw2(); 
   allem_seta->Rebin(2); 
   allem_seta->Sumw2(); 
   TH1F *sht20_res = new TH1F("sht20_res","|#eta|<1.6, E*_SHT20",80,0.,80.); 
   sht20_res->Rebin(2); 
   sht20_res->Divide(sht20_seta,allem_seta,1,1,"B"); 
   TCanvas *c1 = new TCanvas("c1","c1",700,400); 
   c1->Divide(); 
   sht20_res->Draw(); 
   TF1 *f1 = new TF1("f1",turnon,0,27,3); 
   TF1 *f2 = new TF1("f2",polynom2,28,60,3); 
   TF1 *total =new TF1("total",ftotal,0,70,6); 
   f1->SetParameters(14.,0.9,0.9); 
   f2->SetParameters(7.44003e-01,7.50543e-03,-6.55469e-05);
   Double_t par[6]; 
   sht20_res->Fit("f1","R"); 
   sht20_res->Fit("f2","R+"); 
   f1->GetParameters(&par[0]); 
   f2->GetParameters(&par[3]); //par[5] *= 0.9;
   //cout << par[0] << endl; 
   total->SetParameters(par); 
   sht20_res->Fit(total,"R+"); 
}