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Vertex fitter for two consecutive decays [message #17572] Tue, 25 November 2014 11:49 Go to previous message
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 2.2204e-16
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
 
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