Home » PANDA » PandaRoot » Analysis » Vertex fitter for two consecutive decays
Vertex fitter for two consecutive decays [message #17572] 
Tue, 25 November 2014 11:49 
Karin Schönning
Messages: 65 Registered: August 2012 Location: Uppsala University

continuous participant 
From: *physics.uu.se


Dear Pandaroot experts,
I am trying to use the vertex fitter to improve the resolution of the Xi mass, reconstructing Xi from the decay Xi > Lambda pi, Lambda > p pi .
I have used the vertex fitter for the lambda and it works. I take the fitted lambda candidates and combine with a pion which I have checked, using MC truth match, that it is a daughter of Xi and not Lambda. Then I try to apply the vertex fit again, see below:
 
Xi.Combine(goodlam,pim);
Xibar.Combine(goodlamb,pip);
for (j=0;j<Xi.GetLength();++j)
{
//PndVtxPRG vtxfitterx(Xi[j]);
PndKinVtxFitter vtxfitterx(Xi[j]); // instantiate a vertex fitter
bool checkx = vtxfitterx.Fit();
double chi2_vtx = vtxfitterx.GetChi2(); // access chi2 of fit
double prob_vtx = vtxfitterx.GetProb(); // access probability of fit
if (checkx) // when good enough, fill some histos
{
RhoCandidate *Xiv = Xi[j]>GetFit(); // access the fitted cand
TVector3 lVtx=Xiv>Pos();
double Ximassv=Xiv>M();
double xivz=lVtx.Z();
cout<<"Xibar mass : "<<Xi[j]>M()<<endl;
cout<<"Xibar mass vertex: "<<Ximassv<<endl;
cout<<"Xibar vertex z: "<<xivz<<endl;
Ximass>Fill(Xiv>M());}
}

But when running this I get into trouble, the printout is shown below. Is the vertex fitter supposed to work in this way or does it only handle single vertices?
Error in <TDecompLU::DecomposeLUCrout>: matrix is singular
Error in <TDecompLU::InvertLU>: matrix is singular, 0 diag elements < tolerance of 2.2204e16
Xibar mass : 0.186496
Xibar mass vertex: 0.167329
Xibar vertex z: 69.2357
Xibar mass : 1.32154
Xibar mass vertex: nan
Xibar vertex z: nan
Xibar mass : 1.31012
Xibar mass vertex: nan
Xibar vertex z: nan
Xibar mass : 1.32721
Xibar mass vertex: nan
Xibar vertex z: nan
Does anybody have an idea of what one can do about this?
Kindest regards,
/Karin







Re: Vertex fitter for two consecutive decays [message #17580 is a reply to message #17579] 
Tue, 25 November 2014 12:18 
Karin Schönning
Messages: 65 Registered: August 2012 Location: Uppsala University

continuous participant 
From: *physics.uu.se


In Erik Thomé's thesis he writes that
"Pairs of Λ¯ (Λ) and π+ (π−) were fitted to a common vertex under a ¯Ξ+
(Ξ−) hypothesis, requiring a χ2 probability > 0.001."
He used the old framework but since this is on the analysis level maybe there is only some cutting and pasting required. I will try to get a hold of him (he quitted academia and I am not sure if his email works) and check if he has some code on his own or if there was some standard part of pandora.
Cheers,
/Karin



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