# long process

Asked by NADY on 2011-11-06

Hi Olivir , Hi Johan
I have a probem
I know that Pythia8 can process 2 -> 3 Maximum
but I have a process in my implemented model 2 -->7 that is :

define p = u u~ g
generate p p > z , (z > Zp N1 N1 /ve , (N1 > w+ e- /ve , w+ > e+ ve ) , (N1 > z ve , z > mu- mu+))
where N1 is heavy neutrino Zp is Z prime

to solve this problem i divided this long process by :
generate p p > z @1
add process z > Zp N1 N1 /ve @2
add process N1 > w+ e- /ve @3
add process w+ > e+ ve @4
add process N1 > z ve @5
add process z > mu- mu+ @6
To export matrix elements code to Pythia8

THIS IS TRUE OR FALSE

Please help me
Cheers,
Nady

## Question information

Language:
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MadGraph5_aMC@NLO Edit question
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Last query:
2011-11-06
Last reply:
2011-11-06
 Johan Alwall (johan-alwall) said on 2011-11-06: #1

Hello Nady,

Pythia can only handle up to 2->3 processes, so there is no way to handle a long decay chain. The way this is done in Pythia is that you run the core process, p p > z (or possibly p p > z , (z > Zp N1 N1 /ve)), and then Pythia will use the BR information in the DECAY blocks of the param_card to perform the decays of N1. However, these decays will be simple isotropic decays, so if your analysis requires spin correlations within or between the decays, there is at present no way to reproduce these with Pythia matrix elements.

If you need spin correlations, you should use MadEvent (i.e., generate your full decay chains and then do "output madevent"), generate events (using the "launch" command) and then pipe the events files into Pythia.

Note that
generate p p > z @1
add process z > Zp N1 N1 /ve @2
add process N1 > w+ e- /ve @3
add process w+ > e+ ve @4
add process N1 > z ve @5
add process z > mu- mu+ @6
doesn't make any sense - this would ask Pythia to simultaneously generate events from the 6 processes you describe, but doesn't do anything to combine them. I'm also not sure if Pythia can generate decay processes (1 > N).

All the best,
Johan

 NADY (nady-bakhet) said on 2011-11-06: #2

Hi Johan
What do you meany by
" generate events (using the "launch" command) and then pipe the events files into Pythia "
you mean theat the result lhe file and then use it with Pythia or what?
I did this but the result is
the output of any event is z- > mu+ mu - only nothing
doesn't give N1 , Zp ,....
Cheers
nady

 Johan Alwall (johan-alwall) said on 2011-11-06: #3

Hello Nady,

Note that since you ask for z decay, you need M_{Zp}+2m_{N1} < m_Z (which is of course impossible, since you also ask for N1 decaying to Z). The correct process you want is probably

define p = u u~ g
generate p p > z > Zp N1 N1 /ve , (N1 > w+ e- /ve , w+ > e+ ve ) , (N1 > z ve , z > mu- mu+)
output
launch

although you need to be very careful about gauge invariance when you select a single set of diagrams in this way. I would therefore recommend to not specify the s-channel Z in the core process.

Best,
Johan

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