In the simulation (#18621) of pbar p -> Ds+ Ds-; Ds- -> K+ K- pi-; Ds+ ->eta e+ nue; eta -> pi+ pi- pi0, I did the mass constraint fitting to the pi0 candidates with PndKinFitter. All the chi2s are pretty small with the mean value of ~0.001. This also happens in the case of eta.

I noticed the previous topic discussed by Albrecht about the negative and extremely small chi2 in PndKinVtxFitter.

https://forum.gsi.de/index.php?t=tree&th=3385&start=0&rid=19 79&S=7cee6febef5676d17dc9e3154e1269e2

Considering the formula for chi2, the small values may be due to a large sigma in the denominator.

Does anybody have some solutions or hints to improve it?

or is there any way else to understand this problem?

Thanks in advance.]]>

I don't know the reason for the problem, I just can imagine, that the covariance matrix for the neutrals/gammas might be incorrect. I actually don't know how to check.

Would be a question for the EMC experts...

Best regards,

Klaus]]>

In the simulation of Ds- -> K+ K- pi-, I found the chi2 values are too small in the mass constraint fitting of Ds meson, as that in the Ds+ (eta,e+,nu_e) case I mentioned before. And it seems impossible to filter out the background completely with chi2 cutting to get a delta function shaped histo for the Ds mass.

If the problem related the fitter can not be solved at this moment, how can I improve the selection in the present situation?

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