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Problem with Xi(1820) in EvtGen Wed, 14 October 2015 13:49
 Jennifer Pütz Messages: 47Registered: April 2015 Location: FZ Juelich continuous participant From: *ikp.kfa-juelich.de
Hi everyone,

I have a problem with EvtGen.

I want to simulate pbar p → Xi(1820)- Xi+. For this I added Xi(1820) to the evt.pdl file with

`add  p Particle  Xi(1820)-   23314  1.8230000e+00  2.4000000e-02  2.0000000e-01    -3     3  0.0000000e+00      23314`

Xi(1820)- has a mass of 1.823 GeV/c^2 and spin 3/2.
But if I run EvtGen the mass distribution of Xi(1820)- has a rectangular shape.

If I change the spin to 1/2 or 5/2 the mass distribution is looking fine (see plots below).

For other particles with spin 3/2 EvtGen is running fine.

I wrote already to the person who is responsible for the mass generation in EvtGen. But he is not answering.
Maybe there is someone of you having an idea how to fix this.

Cheers,

Jenny

---------
I am using:

PandaROOT: trunk rev. 28555
FairSoft: mar15
FairRoot: v-15.03a

[Updated on: Wed, 14 October 2015 13:50]

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Re: Problem with Xi(1820) in EvtGen [message #18559 is a reply to message #18558] Wed, 14 October 2015 15:30
 StefanoSpataro Messages: 2736Registered: June 2005 Location: Torino first-grade participant From: *to.infn.it
Can you provide also the dec file and the beam momentum you are using?
Re: Problem with Xi(1820) in EvtGen [message #18560 is a reply to message #18559] Wed, 14 October 2015 15:35
 Jennifer Pütz Messages: 47Registered: April 2015 Location: FZ Juelich continuous participant From: *ikp.kfa-juelich.de
Hi Stefano,

I'm using a beam momentum of 4.6 GeV/c. The .dec file is attached.
Re: Problem with Xi(1820) in EvtGen [message #18561 is a reply to message #18560] Wed, 14 October 2015 17:25
 StefanoSpataro Messages: 2736Registered: June 2005 Location: Torino first-grade participant From: *to.infn.it
I can reproduce your problem. Moreover the centre of the square is not at the mass value but at higher position.
I suppose this is a problem of EvtGen, we should see if they answers. Whom did you contact?
Re: Problem with Xi(1820) in EvtGen [message #18562 is a reply to message #18558] Thu, 15 October 2015 09:26
 Jennifer Pütz Messages: 47Registered: April 2015 Location: FZ Juelich continuous participant From: *ext.kfa-juelich.de
Hi Stefano,

I contacted Anders Ryd, one of the guys who wrote the EvtGenGuide.

http://evtgen.warwick.ac.uk/static/docs/EvtGenGuide.pdf

He forwarded my message to David Lange who worked on the mass generation in EvtGen. This was 4 weeks ago.
I tried to contact David Lange again, but there is still no answer.
In the meanwhile I tried to contact the EvtGen guys from University of Warwick. But there is also no answer yet.
Re: Problem with Xi(1820) in EvtGen [message #18564 is a reply to message #18562] Thu, 15 October 2015 14:10
 StefanoSpataro Messages: 2736Registered: June 2005 Location: Torino first-grade participant From: *to.infn.it
It is interesting that, if you set the Xi(1820) particle to stable, it has the proper mass and width. It seems a problem of the decay itself.
Re: Problem with Xi(1820) in EvtGen [message #18565 is a reply to message #18558] Thu, 15 October 2015 14:18
 Jennifer Pütz Messages: 47Registered: April 2015 Location: FZ Juelich continuous participant From: *ikp.kfa-juelich.de
I tried already a different decay model for the Xi(1820)- decay. But this had changed nothing.
Re: Problem with Xi(1820) in EvtGen [message #18621 is a reply to message #18565] Sun, 01 November 2015 13:50
 StefanoSpataro Messages: 2736Registered: June 2005 Location: Torino first-grade participant From: *netrun.cytanet.com.cy
Could you please try to edit the mass of the pbarpsystem in evt.pdl:

`add  p Special   pbarpSystem                    88888  2.98           0.1            0                 0     0   0                 88888`

into

`add  p Special   pbarpSystem                    88888  5.5           0.1            0                 0     0   0                 88888`

and check if you obtain the same distribution?
Re: Problem with Xi(1820) in EvtGen [message #18622 is a reply to message #18621] Sun, 01 November 2015 17:03
 Elisabetta Prencipe (2) Messages: 214Registered: February 2013 first-grade participant From: *hsi15.unitymediagroup.de
Hi Stefano,

Jenny and I have been thinking what would go wrong in that decay model. As apparently it looked everything correct, we have contacted the EvtGen developers. Effectively, it looks there is a bug in the model.
Briefly, the problem occur at the level of the class src/EvtGenBase/EvtMassAmp.cpp::amplitude().
A quick fix suggested by the authors is the following: to comment out

if ( _vb->L() != 0 ) {amp=0.;}

for the birth vertex factor (_vb) on line 78 in src/EvtGenBase/EvtMassAmp.cpp::amplitude() for the
case when the sum of the pole masses of the daughters is less than the parent pole mass.
Maybe Jenny can give a more detailed update during the next seevogh meeting. Now it looks working correctly.

cheers, Elisabetta
Re: Problem with Xi(1820) in EvtGen [message #18623 is a reply to message #18622] Sun, 01 November 2015 17:11
 StefanoSpataro Messages: 2736Registered: June 2005 Location: Torino first-grade participant From: *nb.cytanet.com.cy
In any case I would like to see what happens setting the mass to 5.5, without the suggested patch.
Re: Problem with Xi(1820) in EvtGen [message #18625 is a reply to message #18558] Mon, 02 November 2015 09:52
 Jennifer Pütz Messages: 47Registered: April 2015 Location: FZ Juelich continuous participant From: *ikp.kfa-juelich.de
Hi Stefano,

I tried out what happens, if I use the mass of 5.5 without the suggested patch. Going to higher pbarpsystem masses seems to fix the problem (see plot below).

This is similiar to the observations of the EvtGen guys.
If I understood them correct, the code of src/EvtGenBase/EvtMassAmp.cpp::amplitude() checks first, if the pbarpSystem mass is bigger than the sum of the mass of the daughters. In my case the masses of Xi(1820)- and Xi+ are 1.823 + 1.322 = 3.145 which is much higher than the mass of the pbarpSystem. This seems to cause - together with L!=0 - that the mass distribution is flat.

Best,

Jenny
Re: Problem with Xi(1820) in EvtGen [message #18626 is a reply to message #18625] Mon, 02 November 2015 10:49
 StefanoSpataro Messages: 2736Registered: June 2005 Location: Torino first-grade participant From: *netrun.cytanet.com.cy
Could you please prepare a couple of slides for the next software SeeVogh, on Tuesday 10th October at 14?
Re: Problem with Xi(1820) in EvtGen [message #18627 is a reply to message #18558] Mon, 02 November 2015 10:50
 Jennifer Pütz Messages: 47Registered: April 2015 Location: FZ Juelich continuous participant From: *ikp.kfa-juelich.de
yes, sure.
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