Getting decay chains to work with Pythia8?

Asked by Martin John White on 2011-06-23

I am trying to get decay chains working in Pythia8. If I do this for example:

generate p p > t t~, t > b j j, t~ > b~ l- vl~
output pythia8

I should in theory be able to build and run the pythia code. The problem is that it won't compile because the cross-section inherits from a baseclass called Sigma6Process which doesn't exist in Pythia (it has Sigma2Process and Sigma3Process for 2 -> 2 and 2->3 processes, but of course won't handle higher multiplicity).

Looking at this in more detail, it seems that MadGraph is simply counting the multiplicity in the final state and assigning SigmaNProcess for N final state particles, regardless of whether the chain happened via a succession of 2->2 or 2-> 3 processes.

What am I doing wrong? Is there a way to adjust the syntax to get this working? Using "add process" breaks the output because pythia won't take in generic flavour combinations.

Question information

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Answered
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MadGraph5_aMC@NLO Edit question
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Last query:
2011-06-27
Last reply:
2011-06-27

Hi Martin,

>What am I doing wrong? Is there a way to adjust the syntax to get this working? Using "add process" breaks the output because >pythia won't take in generic flavour combinations.

Well, we never thought about this (at least I never). So at current stage, this is not possible to do in a "easy way".
I'll keep this as a suggestion.

One solution should be to generate each square matrix element separately and then modify the pythia analysis file in order to make the correct decay. Johan should know more about this.

Cheers,

Olivier

Johan Alwall (johan-alwall) said : #2

Hello Martin,

Pythia 8 can only handle matrix elements with up to 3 final state particles - it doesn't matter if they are in the form of a decay chain or not. We are planning to include matrix element-improved decay chains in Pythia 8 in the future, but this is still work in progress. In the meantime, we will make sure to clarify the error messages in MadGraph 5 to avoid confusion.

You can of course always simulate decay chains using MadEvent and then feed the resulting event file into Pythia. This also allows for matrix element-parton shower matching.

All the best,
Johan

Martin John White (mwhi) said : #3

Hi,

Thanks to both of you for your very prompt answers. If I understand you correctly, there are two obvious ways to proceed:

1) Run Madevent and put the events into Pythia to simluate the parton shower. This looks straightforward enough, though I am wondering how ISR and FSR will be handled here. Presumably Madevent will simply generate the hard scatter without radiation, and then Pythia will simply do the shower, and thus ISR/FSR aren't included? Or am I wrong here?

2) Put the 2->2 processes into Pythia and go from there. This is what we have been looking at over the past week. In our toy example, we have a doubly charged scalar which can decay to leptons. Trying to output pythia8 information for delta++ -> e+ e+ doesn't work since Pythia won't accept the incoming flavour combination. However, it is happy with e+ e+ -> delta++ as you would expect. We have added production for delta++ via qqbar -> (Z/gamma) -> delta++ delta--, and we can then get the delta++ to decay by telling Pythia in the command file that the delta is a resonance and by specifying the decay modes. My question here is this: having specified e+ e+ -> delta++ in Madgraph, this gets a matrix element code in Pythia. We don't use it for production since we assume a p p collider. But can we specify this matrix element code in the AddChannel Pythia command to correctly simulate the decay? It seems to work and doesn't break anything. For more complicated examples, would this ensure that, e.g., spin correlations were handled correctly?

Sorry for the barrage of questions.

Cheers,

Martin

Hi Martin,

>1) Run Madevent and put the events into Pythia to simluate the parton shower. This looks straightforward enough, though I am >wondering how ISR and FSR will be handled here. Presumably Madevent will simply generate the hard scatter without radiation, and >then Pythia will simply do the shower, and thus ISR/FSR aren't included? Or am I wrong here?

They are no problem for ISR/FSR. Indeed MadEvent generate the hard scattering without radiation but pythia takes those events and modifiy the kinematics of the events in order to simulate correctly the ISR/FSR.
Off course the most precise way to deal correctly with radiation is to use the matching.
http://cp3wks05.fynu.ucl.ac.be/twiki/bin/view/Main/IntroMatching

For your second question, I don't know the answer. I think that this question might be asked to the pythia team. But maybe that Johan knows the answer for that.

Olivier

Johan Alwall (johan-alwall) said : #5

> 2) Put the 2->2 processes into Pythia and go from there. This is what we have been looking at over the past week. In our toy example, we have a doubly charged scalar which can decay to leptons. Trying to output pythia8 information for delta++ -> e+ e+ doesn't work since Pythia won't accept the incoming flavour combination. However, it is happy with e+ e+ -> delta++ as you would expect. We have added production for delta++ via qqbar -> (Z/gamma) -> delta++ delta--, and we can then get the delta++ to decay by telling Pythia in the command file that the delta is a resonance and by specifying the decay modes. My question here is this: having specified e+ e+ -> delta++ in Madgraph, this gets a matrix element code in Pythia. We don't use it for production since we assume a p p collider. But can we specify this matrix element code in the AddChannel Pythia command to correctly simulate the decay? It seems to work and doesn't break anything. For more complicated examples, would this ensure that, e.g., spin correlations were handled correctly?

We have not yet started working on decays with Pythia 8, so for now, all you can do automatically with MadGraph 5 is to generate the matrix elements for 2->1/2/3 processes. Matrix element corrections of decays are in the plans. If you would like to do some implementation of this yourself, please refer to the Pythia 8 documentation or ask the Pythia 8 authors, and feel free to share updates about your progress with us.

All the best,
Johan

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