Fwd: HDgeant4 and rare decays

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Mark Ito

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Sep 26, 2019, 8:14:14 AM9/26/19
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Question from Kevin Luckas...



-------- Forwarded Message --------
Subject: HDgeant4 and rare decays
Date: Wed, 25 Sep 2019 16:51:35 +0200
From: Kevin Luckas <klu...@jlab.org>
To: Mark Ito <ma...@jlab.org>


Dear Mark,

Is there an easy way to force Geant4 to use only certain decay channels of a given particle species, e.g. Lambda->p pi- ?

I was talking to Nick Zachariou about this and he mentioned, that I would have to copy the files from 

/u/group/halld/Software/builds/Linux_CentOS7-x86_64-gcc4.8.5/geant4/geant4.10.02.p02/source/particles/hadrons/barions/src

into my working directory and reinstall hdgeant4.

As I’m using the preinstalled versions, I would like to now if there is another way that you know of to fix decays of certain particles.

Many thanks,
Kevin

Richard Jones

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Sep 26, 2019, 8:29:07 AM9/26/19
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Hello Kevin,

Yes, it is easy to do, and no, you do not need to rebuild the G4 libraries or modify any source code. The capability to customize the decay tables for unstable particles is built into the standard user interface. You can read all about this by doing an online search, or get back to me for more details if you come up with nothing.

-Richard Jones

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Sean Dobbs

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Sep 26, 2019, 10:35:20 AM9/26/19
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Hi Richard,

Since this issue has come up several times, do you have some standard suggested webpages on this topic which we could link to from the wiki?

---Sean

Kevin Luckas

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Oct 1, 2019, 4:39:31 AM10/1/19
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Dear all,

I figured out a way to fix the existing branching fractions of certain particles. On startup hdgeant4 will print out all known particles and their decay tables. Such as:

--- G4ParticleDefinition ---
 Particle Name : lambda
 PDG particle code : 3122 [PDG anti-particle code: -3122]
 Mass [GeV/c2] : 1.11568     Width : 2.501e-15
 Lifetime [nsec] : 0.2631
 Charge [e]: 0
 Spin : 1/2
 Parity : 1
 Charge conjugation : 0
 Isospin : (I,Iz): (0/2 , 0/2 )
 GParity : 0
 MagneticMoment [MeV/T] : -1.93245e-14
 Quark contents     (d,u,s,c,b,t) : 1, 1, 1, 0, 0, 0
 AntiQuark contents               : 0, 0, 0, 0, 0, 0
 Lepton number : 0 Baryon number : 1
 Particle type : baryon [lambda]
G4DecayTable:  lambda
0:  BR:  0.639  [Phase Space]   :   proton pi-
1:  BR:  0.358  [Phase Space]   :   neutron pi0

To set the branching ratio of p pi- to 1 for example, one needs to put the following lines in the run.mac file above the /run/beamOn :

/particle/select lambda
/particle/property/decay/select 0
/particle/property/decay/br 1
/particle/property/decay/select 1
/particle/property/decay/br 0

Please note that the startup and printing out of the particles happens before the run.mac file is processed, so it will still show the lines above there. If you want to be sure the fix worked put an additional 

/particle/property/decay/dump

In the run.mac file, it will print out the no customised decay table before starting the simulation.

Best regards,
Kevin

Richard Jones

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Oct 1, 2019, 5:45:09 AM10/1/19
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Hello Kevin and all,

I dug through my old emails and found this message that I sent to the calorimetry group back in March, 2017. It looks like you found the same solution. Since this has come up more than once now, is there a place on the wiki where this should be posted?

Dear Elton,
Here are step-by-step instructions for selecting only the Dalitz decay of the pi0. Note that once this is done, ALL of your neutral pions will decay only through this channel, including any that are produced as secondaries in an electromagnetic shower from the first. You can also load these commands into a file and supply the name of the file as a command line argument to hdgeant4.

-Richard

[assumes you have done checkout and build steps described in my talk today]
$ cd $MY_HDGEANT4_HOME/test
$ vim control.in [here you should select the options you want, including to set INFILE to the name of the input hddm file containing events with pi0 that you want to simulate]
$ hdgeant4
[ several lines of printout suppressed here ]
Idle> /particle/select pi0
Idle> /particle/property/decay/select 0
Idle> /particle/property/decay/br 0
Idle> /particle/property/decay/select 1
Idle> /particle/property/decay/br 1
Idle> /particle/property/decay/dump
        G4DecayTable:  pi0
           0:  BR:  0  [Phase Space]   :   gamma gamma
           1:  BR:  1  [Dalitz Decay]  :   gamma e- e+
Idle> /run/beamOn 1000
Idle> exit

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Mark Ito

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Oct 1, 2019, 8:53:54 AM10/1/19
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