Simulation of particle decays in flight

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| Geant 3.16  |               GEANT User's Guide              | PHYS400  ##
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Author(s) : G.N.Patrick                                Submitted: 30.03.82
Origin    : Geant 2                                      Revised: 19.12.92
                                  

Subroutines

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                    |CALL GDECAY  |
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GDECAY is a control routine for the direct simulation of particle decays in flight. From a reference list of parent particles it selects a two- or three-body decay mode using the known branching ratios and calls all the necessary routines needed to generate the vertex and secondary tracks. It used the following input and output:

input:

via common blocks /GCTRAK/ and /GCKINE/

output:

via common block /GCKING/

GDECAY is called automatically by the tracking routines. It calls the subroutines GDECA2 for two-body decay, GDECA3 for three-body decay, GLOREN for Lorentz transformation and GDROT for rotation. For the documetation of GLOREN and GDROT, see PHYS 410.

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           | CALL GDECA2 (XM0,XM1,XM2,PCM) |
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GDECA2 simulates the two-body decay with isotropic angular distribution in the center-of-mass system. It is called from GDECAY.

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         | CALL GDECA3 (XM0,XM1,XM2,XM3,PCM) |
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GDECA3 simulates the three-body decay with isotropic angular distribution in the center-of-mass system. It is called from GDECAY.

Method

1. Upon entry to GDECAY a binary search is made through a reference list of parent particles. This list is stored in the JPART structure and currently contains the particles defined in GPART [CONS300]. If the track does not correspond to any of these particles, control is returned without any decay generation. Up to six decay modes and their corresponding branching ratios are then extracted from the JPART data banks. See Table 1 in CONS 310.
2. Using the branching ratios a decay is randomly selected. If, for a particle, the sum of the defined branching ratios is not equal 100%, it is possible that no decay is selected.
3. Depending on whether the two- or three-body decay is selected, either GDECA2 or GDECA3 is called to generate the four-momenta of the decay products with isotropic angular distribution in the center-of-mass system.
4. The momentum vectors of the decay products are finally transformed into the laboratory system and rotated back into the GEANT coordinate frame. The kinematics of the NGKINE products is stored in COMMON /GCKING/.
5. When a particle decays and no branching ratio is defined, then GDECAY calls the user routine GUDCAY.