decay chain with EWdim6 model

Asked by Emanuele Di Marco on 2020-04-21

Dear all,

I wanted to generate events corresponding to the 5-points amplitude in Fig. 1 of PRL 123, 181801: A(bV -> tVV).
I.e., at LHC, for example,

pp -> W+W- t j

with the subsequent decay W+ > l+ vl (or others).

I have two questions:

1. how I force the five point amplitude correctly.
I tried with EWdim6 model to do

import model EWdim6
define p = p b b~
define j = p
generate p p > w+ w- t j / h z a NP==2

which does not exactly what I was looking for.
Sorry if this sounds trivial, but it's the first time I try to generate events myself.

2. how to force the decay of the w. In the SM everything works fine with the decay chain syntax, but when I require NP==2:

generate p p > w+ w- t j, (w+ > l+ vl), (w- > l- vl~) / h z a NP==2

I get this:
"MadGraph5Error : Decay processes cannot specify squared orders constraints."

I can still get this:

generate p p > l+ vl l- vl~ t j / h z a NP==2

However the decay chain would be really useful for other final states.



Question information

English Edit question
MadGraph5_aMC@NLO Edit question
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Solved by:
Olivier Mattelaer
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Last reply:


The paper that you quote does not seems to use the EWdim6 model.
The paper associated to EWdim6 model is 1205.4231
In this case they seems to use another UFO model.

"MadGraph5Error : Decay processes cannot specify squared orders constraints."

Yeah this means that we can not handle your syntax.
The decay chain is designed by block.
and you can specify the coupling for each block
so here you asked for the following three block
p p > w+ w- t j
w+ > l+ vl
w- > l- vl~ / h z a NP==2

Which obviously does not make sense (and is likely not what you asked for

Now it is not clear to me what you plan to do. Would you like to do something like this
generate p p > w+ w- t j / h z a NP==2, (w+ > l+ vl), (w- > l- vl~)
(i.e. pure SM decays)
or something like
generate p p > w+ w- t j / h z a, (w+ > l+ vl), (w- > l- vl~)
but requesting on the full diagram NP==2.
in that case the syntax should be like
generate p p > w+ w- t j / h z a, (w+ > l+ vl), (w- > l- vl~) @0 NP==2

I do not think that neither of the two above syntax do work,
But you can here have the same effect by using QED<=X QCD<=Y NP<=Z syntax which should have the same effect for some value of X,Y and Z



Hi Olivier,

thanks a lot for the prompt answer. It clarifies a lot.
For the model of the paper, I will ask the authors about the UFO model that they used.

Regarding the question on the decay chain. I see now the division in blocks.
If I want to do pure SM decays, but excluding QCD production, i.e. something like the diagrams that I get with

define p = p b b~
generate p p > w+ w- t j / h z a QCD==0

(in the SM). But then when I add the decay chain to force the subsequent decays of the 2 w e.g. in fully leptonic final state with

generate p p > w+ w- t j / h z a, (w+ > l+ vl), (w- > l- vl~) @0 QCD==0

I got again the error (which probably makes sense, but I don't see how).
I can produce the diagram that I want with

generate p p > l+ vl l- vl~ t j / h z a QCD==0

but this contains many others, while I would like to see the on-shell Ws.

Now, is there something bad in decaying the Ws later, adding:

decay w+ > l+ vl
decay w- > l- vl~

Thanks a lot (and sorry if some questions are naive...)


I think that with some more thinking I have the syntax right (in the SM):

generate p p > w+ w- j t QED=6 QCD=0 / z a h, w+ > l+ vl, w- > l- vl~

which makes (I hope) what I wanted.
Thanks a lot for the very useful hints.


Thanks Olivier Mattelaer, that solved my question.