| entries at unphysical mass after vertex fit [message #17625] |
Thu, 04 December 2014 10:17  |
Albrecht Gillitzer
Messages: 49
Registered: March 2004
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continuous participant |
From: *ikp.kfa-juelich.de
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Hi all,
When comparing the Lambda mass spectrum evaluated before and after the vertex fit using PndKinVtxFitter, one can see that the fwhm of the Lambda mass distribution improves by the vertex fit but in contrast larger tails in the mass distribution are created. On the low mass side the tails extend far into the unphysical region at masses below the sum mass of the daughter particles M_sum = m_pi + m_p.
The attached plot has been generated from 10000 events 4 GeV/c pbar p --> Xibar+ Xi- (PHSP) --> Lambdabar pi+ Lambda pi- --> pbar pi+ pi+ p pi- pi- with the oct14 release. Upper row: Lambda mass before vertex fit / Lower row: after vertex fit // Left: lin y axis / Right: log y axis.
The corresponding plot for Lambdabar shows the same feature. Simulation and analysis of the same reaction with the scrut14 release also shows the same feature.
My interpretation: Can it be that in the vertex fit the 3-momenta of the daughter particles are modified according to the modified track parameters as a result of the fit, but their energies are left unchanged? As a consequence the energy momentum relation would be destroyed and particles no longer would have the correct mass value.
Albrecht
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| Re: entries at unphysical mass after vertex fit [message #18422 is a reply to message #17625] |
Mon, 17 August 2015 15:16  |
Jennifer Pütz
Messages: 47
Registered: April 2015
Location: FZ Juelich
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continuous participant |
From: *ikp.kfa-juelich.de
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Hi everyone,
I found the same probelm as Albrecht. I ploted the difference of the masses of the daughters before and after the fit and saw that they are not exact needles. So I had a closer look into the PndKinVtxFitter and found that the four-momentum of the dauhgters after the vertex fit is set to (p_vtx, E_vtx). p_tx and E_vtx are the 3-momentum-vector and the energy of the daughter particle comming out of the vertex fit.
But E_vtx is not taking the mass hypothesis of the dauhgter particle into account. So I replaced E_vtx with sqrt(p_vtx^2 + m^2). (This was already implemented in the code but commented out.)
After doing that, the unphysical masses for Lambda0 have disappeared and the mass differences show now exact needles (see attached files).
Best,
Jenny
[Updated on: Mon, 17 August 2015 15:16] Report message to a moderator |
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