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Small positive chi2 values in PndKinFitter [message #14682]

Tue, 30 April 2013 11:13

Lu Cao
Messages: 77
Registered: February 2013

continuous participant

From: *ikp.kfa-juelich.de

Dear all,

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.

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Re: Small positive chi2 values in PndKinFitter [message #14683 is a reply to message #14682]

Tue, 30 April 2013 14:13

StefanoSpataro
Messages: 2736
Registered: June 2005
Location: Torino

first-grade participant

From: *to.infn.it

Hi,
we have seens the same problem analysing the D*+D*- channel, chi2 too small from the fitter.

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Re: Small positive chi2 values in PndKinFitter [message #14684 is a reply to message #14683]

Tue, 30 April 2013 17:41

Klaus Götzen
Messages: 293
Registered: June 2006
Location: GSI

first-grade participant

From: *adsl.alicedsl.de

Hi,

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

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Re: Small positive chi2 values in PndKinFitter [message #14685 is a reply to message #14684]

Tue, 30 April 2013 18:44

StefanoSpataro
Messages: 2736
Registered: June 2005
Location: Torino

first-grade participant

From: 2.235.190*

In our case, the used fitter was the 4C and the channel was completely charged (2 kaons + 4 pions).
I believe it is a problem of tracking errors.

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Re: Small positive chi2 values in PndKinFitter [message #14686 is a reply to message #14685]

Tue, 30 April 2013 20:33

Klaus Götzen
Messages: 293
Registered: June 2006
Location: GSI

first-grade participant

From: *adsl.alicedsl.de

Ralf actually is/was taking a close look to the fitters. Maybe that helps to solve the problem soon...

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Re: Small positive chi2 values in PndKinFitter [message #14816 is a reply to message #14684]

Mon, 27 May 2013 13:14

Lu Cao
Messages: 77
Registered: February 2013

continuous participant

From: 114.138.133*

Hi,

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?

index.php?t=getfile&id=7381&private=0

Attachment: Ds_mcf.png
(Size: 80.47KB, Downloaded 438 times)

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