ttbb and ttZ proc card settings

Asked by Kenichi Hatakeyama on 2021-04-04

Dear experts,

Can I ask you some question about generating ttbb events?

I am tryting to study EFT effects in ttH and ttZ events with a focus on H/Z->bb decays using the dim6top model [1], and also would like to check EFT effects on the main background process, ttbb. I am hoping to generate (1) ttZ, (2) ttH, and (3) ttbb, separately, in a way we can add them in a most seamless way.

The ttH and ttZ are generated with these proc card settings [5,6]. In order to avoid ttH with H->bb and ttZ with Z->bb decays in the ttbb sample, for the ttbb proc card, I was using "/h z" [3], but after checking [2] I thought it may be better to do "$z h" [4] for potential off-Z contributions and interference effect, but I am not completely sure. In run card, I use Breight-Wigner cut off of 15.0 GeV. I just wanted to check these settings make sense with experts.

And, for ttZ for example, is there a way to to get relative contributions of Z coming from ISR vs Z coming from FSR [7] in madgraph? And for ttZ proc card, I often see "/h" and I am adding it in [6], but I am not sure what's the motivation/benefit of it. I again wanted to get experts' inputs on it.

Thank you for your help.

Cheers,
Ken

[1] https://feynrules.irmp.ucl.ac.be/wiki/dim6top
[2] https://indico.cern.ch/event/796134/contributions/3560246/

[3] proc card for ttbb, 4 flavor scheme, option 1
+++++
import model dim6top_LO_UFO
generate p p > t t~ b b~ /h z FCNC=0 DIM6=1 @0
add process p p > t t~ b b~ j /h z FCNC=0 DIM6=1 @1
+++++

[4] proc card for ttbb, 4 flavor scheme, option 2
+++++
import model dim6top_LO_UFO
generate p p > t t~ b b~ $z h FCNC=0 DIM6=1 @0
add process p p > t t~ b b~ j $z h FCNC=0 DIM6=1 @1
+++++

[5] proc card for ttH, 5 flavor scheme
+++++
import model dim6top_LO_UFO
define p = p b b~
define j = p
generate p p > t t~ h FCNC=0 DIM6=1 @0
add process p p > t t~ h j FCNC=0 DIM6=1 @1
+++++
and higgs is decayed with Pythia.

[6] proc card for ttZ, 5 flavor scheme
+++++
import model dim6top_LO_UFO
define p = p b b~
define j = p
define ell+ = e+ mu+ ta+
define ell- = e- mu- ta-
generate p p > t t~ z / h FCNC=0 DIM6=1, (z > j j FCNC=0 DIM6=0) @0
add process p p > t t~ z / h FCNC=0 DIM6=1, (z > vl vl~ FCNC=0 DIM6=0) @1
add process p p > t t~ z / h FCNC=0 DIM6=1, (z > ell+ ell- FCNC=0 DIM6=0) @2
add process p p > t t~ z j / h FCNC=0 DIM6=1, (z > j j FCNC=0 DIM6=0) @3
add process p p > t t~ z j / h FCNC=0 DIM6=1, (z > vl vl~ FCNC=0 DIM6=0) @4
add process p p > t t~ z j / h FCNC=0 DIM6=1, (z > ell+ ell- FCNC=0 DIM6=0) @5
+++++

[7] http://cds.cern.ch/record/2236668/files/ttVFeynmanDiagrams.png

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
2021-04-04
Last query:
2021-04-04
Last reply:
2021-04-04

Hi,

Yes using the "$" syntax is better since this forbids the onshell contribution
and therefore you do have
generate p p > t t~ b b~ $z h
add process p p > t t~h , h > b b~
add process p p > t t~Z , Z > b b~
which is very close (in term of cross-section and distribution) to
generate p p > t t~ b b~ (which includes all effect)
by separating the contribution you neglect the interference in the phase-space area where the Higgs or Z is onshell so this is not a 100% match.

> I use Breight-Wigner cut off of 15.0 GeV

This is not 15 GeV this is 15 times the width. This is in general a good value (sometimes a bit too large). I have never made a dedicated study on what is the optimal value (or a correct range) but I doubt that 15 will be problematic here.

> And, for ttZ for example, is there a way to to get relative contributions of Z coming from ISR vs Z coming from FSR [7] in madgraph?

Not really no. This is likely a gauge dependent statement anyway.

> And for ttZ proc card, I often see "/h" and I am adding it in [6], but I am not sure what's the motivation/benefit of it. I again wanted to get experts' inputs on it.

You should ask the people using that syntax, I always discourage to use the "/" syntax due to the issu of gauge invariance leading to unphysical prediction.

Cheers,

Olivier

Kenichi Hatakeyama (hatakek) said : #2

Hi Olivier,

Thank you for your quick response and clarification.

Ken

Kenichi Hatakeyama (hatakek) said : #3

Thanks Olivier Mattelaer, that solved my question.