Decay withou branches

Asked by Yizhou Cai on 2021-03-06

Dear Experts,

I am wondering how MG5 works when I generate h2 > t tbar while only widths being set in param_card, without branches setting. Will it caculate branches or use a default ones. Thanks a lot in advance.

Regards,
Yizhou

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
2021-03-07
Last query:
2021-03-07
Last reply:
2021-03-07

If you do

> generate h2 > t tbar

Then the computation is the integral of the (constant) matrix-element this does not use the information of the BR, and return the partial width for that given process.

If you do
generate p p > h2 j , h2 > t tbar

then we do the phase-space integration over the mass of h2 from -15 times the width up to 15 times the width.
The only dependence is in the total width since it appears in the denominator.
This computation does not use the narrow-width approximation and therefore does not need to know the BR (which also allow to put cut on the final state). It is here important that the total width being physical.

if you do
generate p p > h2 j
output
launch
MadSpin=ON
decay h2 > t tbar

Then you will use MadSpin package which is based on the NWA, in that case no cut are possible on the top and the BR information need to be included. If they are not, the code will start by computing them thanks to auto-width computation module (1402.1178 <https://arxiv.org/abs/1402.1178>)

Cheers,

Olivier

> On 6 Mar 2021, at 15:55, Yizhou Cai <email address hidden> wrote:
>
> New question #695890 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/695890
>
> Dear Experts,
>
> I am wondering how MG5 works when I generate h2 > t tbar while only widths being set in param_card, without branches setting. Will it caculate branches or use a default ones. Thanks a lot in advance.
>
> Regards,
> Yizhou
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Yizhou Cai (yzcai) said : #2

Hi Olivier,

Thank you for your kindly reply.
We are working on the S+I pattern of heavy higgs decay, in which we generate g g > t t~ / h2 QED=99 QCD=99 and thenhack MG to remove the background part from the inclusive S+I+B. Would it work as the first situation?

Regards,
Yizhou

If you use

> g g > t t~ / h2

Then you do not have any h2 particle in any Feynnman diagram... so youur question is a bit weird.
But if you do not specify anything special then we do perform the phase-space integration over the full invariant mass, without any cut.

The potential issue is gauge invariance, obviously such syntax includes non resonant diagram and are therefore less sensitive to that but you might need to use the complex-mass-scheme to have a better handling of the gauge invariance if fix width is not enough

Cheers,

Olivier

> On 7 Mar 2021, at 02:41, Yizhou Cai <email address hidden> wrote:
>
> Question #695890 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/695890
>
> Yizhou Cai posted a new comment:
> Hi Olivier,
>
> Thank you for your kindly reply.
> We are working on the S+I pattern of heavy higgs decay, in which we generate g g > t t~ / h2 QED=99 QCD=99 and thenhack MG to remove the background part from the inclusive S+I+B. Would it work as the first situation?
>
> Regards,
> Yizhou
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Yizhou Cai (yzcai) said : #4

Sorry, I didn't make it clear. The process of h2 and h3 are both considered, and I am confused about the width of h3 in the g g > t t~ / h2 process.

I have got it generally. Let me discuss with group before I closing the problem.

Thank you very much.

Cheers,
Yizhou

Yizhou Cai (yzcai) said : #5

generate g g > t t~ / h1 h2 h3 for S+I pattern of h2, I mean.

Yizhou Cai (yzcai) said : #6

Thanks Olivier Mattelaer, that solved my question.