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Problem with Xi(1820) in EvtGen [message #18558]

Wed, 14 October 2015 13:49

Jennifer Pütz
Messages: 47
Registered: 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.

index.php?t=getfile&id=8545&private=0

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

index.php?t=getfile&id=8548&private=0

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

Attachment: XiMinus1820_Spin_3half.png
(Size: 32.24KB, Downloaded 809 times)
Attachment: XiMinus1820_Spin_1half.png
(Size: 21.95KB, Downloaded 689 times)
Attachment: XiMinus1820_Spin_5half.png
(Size: 22.21KB, Downloaded 795 times)
Attachment: evt.pdl
(Size: 75.49KB, Downloaded 407 times)
Attachment: XiMinus_1820_lambda0_K.dec
(Size: 0.27KB, Downloaded 389 times)

[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: 2736
Registered: 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?

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Re: Problem with Xi(1820) in EvtGen [message #18560 is a reply to message #18559]

Wed, 14 October 2015 15:35

Jennifer Pütz
Messages: 47
Registered: 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.

Attachment: XiMinus_1820_lambda0_K.dec
(Size: 0.27KB, Downloaded 322 times)

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Re: Problem with Xi(1820) in EvtGen [message #18561 is a reply to message #18560]

Wed, 14 October 2015 17:25

StefanoSpataro
Messages: 2736
Registered: 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?

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Re: Problem with Xi(1820) in EvtGen [message #18562 is a reply to message #18558]

Thu, 15 October 2015 09:26

Jennifer Pütz
Messages: 47
Registered: 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.

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Re: Problem with Xi(1820) in EvtGen [message #18564 is a reply to message #18562]

Thu, 15 October 2015 14:10

StefanoSpataro
Messages: 2736
Registered: 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.

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Re: Problem with Xi(1820) in EvtGen [message #18565 is a reply to message #18558]

Thu, 15 October 2015 14:18

Jennifer Pütz
Messages: 47
Registered: 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.

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Re: Problem with Xi(1820) in EvtGen [message #18621 is a reply to message #18565]

Sun, 01 November 2015 13:50

StefanoSpataro
Messages: 2736
Registered: 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?

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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: 214
Registered: 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

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Re: Problem with Xi(1820) in EvtGen [message #18623 is a reply to message #18622]

Sun, 01 November 2015 17:11

StefanoSpataro
Messages: 2736
Registered: 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.

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Re: Problem with Xi(1820) in EvtGen [message #18625 is a reply to message #18558]

Mon, 02 November 2015 09:52

Jennifer Pütz
Messages: 47
Registered: 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).

index.php?t=getfile&id=8562&private=0

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

Attachment: XiMinus1820_Spin_3half_higher_pbarpSys_mass.png
(Size: 26.28KB, Downloaded 509 times)

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Re: Problem with Xi(1820) in EvtGen [message #18626 is a reply to message #18625]

Mon, 02 November 2015 10:49

StefanoSpataro
Messages: 2736
Registered: 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?

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Re: Problem with Xi(1820) in EvtGen [message #18627 is a reply to message #18558]

Mon, 02 November 2015 10:50

Jennifer Pütz
Messages: 47
Registered: April 2015
Location: FZ Juelich

continuous participant

From: *ikp.kfa-juelich.de

yes, sure.

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