Poles do not cancel in tt + pseudoscalar production

Asked by Rafael Coelho Lopes de Sa on 2021-03-11

Dear all,

I read a lot of topics in this webpage about generating top processes at NLO and finding "poles don't cancel" problem.

The process I am trying to generate is ttbar + pseudoscalar in 2HDM+a model (for DM mediator studies... I am using this model here https://github.com/LHC-DMWG/model-repository/tree/master/models/Pseudoscalar_2HDM)

Initially, I tried the following syntax

generate p p > t t~ > b b~ w+ w- h4 [QCD]

and this does not work (poles don't cancel). Reading the several topics, here are the things I have already tried:

* Run in NF=4 (that's the default anyway). Does not work.
* Try to remove ww contribution doing:

generate p p > t t~ > b b~ w+ w- h4 $$ w+ w- [QCD]

These don't work either (still poles don't cancel)

Do you have a suggestion of what I could try to generate this process at NLO?

Thanks in advance!

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Is there a particular reason of not using madspin for the top decay?

Did you try
generate p p > t t~h4 [QCD]
decay t > w+ b
decay t~ > w- b~

Otherwise, you will have a lot of issue witn EW loop that can contribute which are now computable in 3.x branch but this will slow down the code by huge factor for a very minimal gain.



Hi Olivier,

Thanks! The reason I was running this particular code is because I saw a claim in a paper that the contribution of the "3-body decay"

t -> b w h4

to the process was important. I understand that if I do

generate p p > t t~ h4 [QCD]

and then use madspin, then the top that will be decayed later will be on-shell and there will be no 3-body decays. Am I interpreting it wrongly?

Thanks in advance again.


Here I do not have any good answer for you. MadSpin is limited to two body decay. In principle you could you the special mode spinmode=onshell to handle three body decay here. But then you loose the breit-wigner distribution.
If you are interested in MC implementation this can be a small little project to extend the special mode spinmode=onshell
to also allow off-shell effects. This should not be too difficult to do.



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