# ttbar generation problem

Hi everyone

I don't know where to ask a question about a generation problem. Please direct me to the right place if this is not.

I want to generate a ttbar process decaying hadronically, lepton+jets and dilepton. For this I wrote:

generate p p > t t~, (t~ > W- b~, W- > l- vl~), (t > W+ b, W+ > l+ vl)

add process p p > t t~, (t~ > W- b~, W- > j j), (t > W+ b, W+ > j j) @2

add process p p > t t~, (t~ > W- b~, W- > l- vl~), (t > W+ b, W+ > j j) @3

add process p p > t t~, (t~ > W- b~, W- > j j), (t > W+ b, W+ > l+ vl) @4

I am not if this is correct, because I am analysing the branching ratio of this process and it doesn't fit. I want to be sure if I am doing the right thing.

Thanks for your help.

Alejandro

## Question information

- Language:
- English Edit question

- Status:
- Solved

- Assignee:
- No assignee Edit question

- Solved by:
- Johan Alwall

- Solved:
- 2011-09-15

- Last query:
- 2011-09-15

- Last reply:
- 2011-09-14

Johan Alwall (johan-alwall) said : | #1 |

Hi Alejandro,

Did you include the tau in the definition for leptons?

define l = l ta+ ta-

If not, that might explain your branching ratio disagreement.

Johan

Johan Alwall (johan-alwall) said : | #2 |

Sorry, I meant

define l+ = l+ ta+

define l- = l- ta-

of course.

alefisico (alefisico) said : | #3 |

Thanks Johan for your answer.

It doesn't seem to change anything. So, in principle it is correct the way I am generating this process?? Because it also could be a problem of my analyser, but before start to change my analyser I want to know if I am doing the right think.

Thanks!!!!

Alejandro

Johan Alwall (johan-alwall) said : | #4 |

Hi Alejandro,

Using your process generation, I get results in perfect agreement with

BR(t->b e/mu/ta nu)= 0.33, BR(t->bjj)=0.67. Note that you need to run

without cuts on the decay products (e.g. by setting cut_decays=F) to

get the expected cross section ratios.

Cheers,

Johan

alefisico (alefisico) said : | #5 |

Hi Johan and thanks for your help.

I didn't change cut_decays to false but instead I changed the minimum and maximum values of standard cuts, it is not the same?

If yes, this is what I was afraid, that perhaps the generation is correct but my analyser is wrong. It is an easy way to calculate the BR in MG5 or I should create an "external" analyser (like a python script or something like that) to do this?

Best,

Alejandro

Johan Alwall (johan-alwall) said : | #6 |

Hello Alejandro,

You get the cross sections for the different process groups (specified by

the process identifier @N) directly in the generation html page. It's

trivial to calculate the BRs directly from the different cross sections:

2l: BR(t->lnu)

0l: BR(t->jj)

1l: xsec(1l)

Cheers,

Johan

alefisico (alefisico) said : | #7 |

Hi Johan

Thanks for your help and patience. That is what I needed. I really appreciate it.

Cheers,

Alejandro

alefisico (alefisico) said : | #8 |

Thanks Johan Alwall, that solved my question.