| PndKinVtxFitter [message #17159] |
Tue, 12 August 2014 08:48  |
Alexandr Zinchenko
Messages: 22
Registered: May 2009
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occasional visitor |
From: *jinr.ru
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Hello.
I was trying to apply a vertex constrained fit to a charmonium candidate (made of J/psi pi+ pi-)
The relevant analysis macro lines are as follows (based on tutorials/rho macros)
// *** combinatorics for J/psi -> mu+ mu-
jpsi.Combine(muplus, muminus);
// *** combinatorics for psi(2S) -> J/psi pi+ pi-
psi2s.Combine(jpsi, piplus, piminus);
// ***
// *** do VERTEX FIT (psi(2s))
// ***
for (j = psi2s.GetLength()-1; j >= 0; --j) {
PndKinVtxFitter vtxfitter(psi2s[j]); // instantiate a vertex fitter
vtxfitter.SetVerbose();
vtxfitter.Fit();
The printout from PndKinVtxFitter code (activated by the verbosity flag) is:
Initial vertex Position is -0.00141313 -0.0154373 -0.0183913
iteration Number 0
vertex Position is -nan -nan -nan
chi2 in iterartion -nan
iteration Number 1
vertex Position is -nan -nan -nan
...
Am I doing something wrong or the fitter does not work for such particle combination (1 neutral composite and 2 charged)?
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| Re: PndKinVtxFitter [message #17164 is a reply to message #17160] |
Tue, 12 August 2014 13:22  |
Alexandr Zinchenko
Messages: 22
Registered: May 2009
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occasional visitor |
From: *jinr.ru
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Maybe I got the logic behind the vertex constrained fit wrong.
In my understanding, this PndKinVtxFitter assumes the decay particles coming from the same vertex
(without assumption on the vertex position itself). Therefore, it can be useful, e.g., to reject background
combinations when building J/psi from 2 charged tracks (if the fit quality is not good enough).
If so, considering also neutrals when building more composite objects would help the same way to reject
background combinations. At least (in my understanding),
such an approach was proposed (and realized) during the PANDA Physics Book preparation - see Sect. 4.2.2.3, subsection
on ppbar-> J/psi pi+ pi-, item 3.
So, it looks like Rho package was able to do such a fit at that time.
Am I wrong?
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