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Problem with Xi(1820) in EvtGen
https://forum.gsi.de/index.phpindex.php?t=rview&goto=18558&th=4714#msg_18558
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
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 ]]>Jennifer Pütz2015-10-14T11:49:13-00:00Re: Problem with Xi(1820) in EvtGen
https://forum.gsi.de/index.phpindex.php?t=rview&goto=18559&th=4714#msg_18559
StefanoSpataro2015-10-14T13:30:58-00:00Re: Problem with Xi(1820) in EvtGen
https://forum.gsi.de/index.phpindex.php?t=rview&goto=18560&th=4714#msg_18560
I'm using a beam momentum of 4.6 GeV/c. The .dec file is attached.]]>Jennifer Pütz2015-10-14T13:35:05-00:00Re: Problem with Xi(1820) in EvtGen
https://forum.gsi.de/index.phpindex.php?t=rview&goto=18561&th=4714#msg_18561
I suppose this is a problem of EvtGen, we should see if they answers. Whom did you contact?]]>StefanoSpataro2015-10-14T15:25:11-00:00Re: Problem with Xi(1820) in EvtGen
https://forum.gsi.de/index.phpindex.php?t=rview&goto=18562&th=4714#msg_18562
I contacted Anders Ryd, one of the guys who wrote the EvtGenGuide.
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.]]>Jennifer Pütz2015-10-15T07:26:37-00:00Re: Problem with Xi(1820) in EvtGen
https://forum.gsi.de/index.phpindex.php?t=rview&goto=18564&th=4714#msg_18564
StefanoSpataro2015-10-15T12:10:38-00:00Re: Problem with Xi(1820) in EvtGen
https://forum.gsi.de/index.phpindex.php?t=rview&goto=18565&th=4714#msg_18565
Jennifer Pütz2015-10-15T12:18:03-00:00Re: Problem with Xi(1820) in EvtGen
https://forum.gsi.de/index.phpindex.php?t=rview&goto=18621&th=4714#msg_18621
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?]]>StefanoSpataro2015-11-01T12:50:45-00:00Re: Problem with Xi(1820) in EvtGen
https://forum.gsi.de/index.phpindex.php?t=rview&goto=18622&th=4714#msg_18622
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
]]>Elisabetta Prencipe (2)2015-11-01T16:03:35-00:00Re: Problem with Xi(1820) in EvtGen
https://forum.gsi.de/index.phpindex.php?t=rview&goto=18623&th=4714#msg_18623
StefanoSpataro2015-11-01T16:11:27-00:00Re: Problem with Xi(1820) in EvtGen
https://forum.gsi.de/index.phpindex.php?t=rview&goto=18625&th=4714#msg_18625
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]]>Jennifer Pütz2015-11-02T08:52:11-00:00Re: Problem with Xi(1820) in EvtGen
https://forum.gsi.de/index.phpindex.php?t=rview&goto=18626&th=4714#msg_18626
StefanoSpataro2015-11-02T09:49:09-00:00Re: Problem with Xi(1820) in EvtGen
https://forum.gsi.de/index.phpindex.php?t=rview&goto=18627&th=4714#msg_18627
Jennifer Pütz2015-11-02T09:50:45-00:00