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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The Lambda is a neutrlly charged particle. Our fitters cannot create avalid Helix & Covariance from that.

Since the Xi does fly some distance my only suggestion is you try and make an analytical helix-line point-of-closest-approach finder by hand.

Cheers

Ralf]]>

I don't see how you can fit a second secondary vertex (of the Xi-), since this vertex is not constraint by at least two charged tracks. A decay X -> pi+ pi- Lambda could deliver something like that, but not one with X -> pi- Lambda.

Best,

Klaus]]>

Cheers,

/Karin]]>

"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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