ttbar generation problem

Asked by alefisico on 2011-09-13

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.

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Solved by:
Johan Alwall
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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 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.



Best 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.


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?


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)=sqrt(xsec(2l)/xsec(tot))
0l: BR(t->jj)=sqrt(xsec(0l)/xsec(tot))
1l: xsec(1l)=2BR(t->lnu)BR(t->jj)


alefisico (alefisico) said : #7

Hi Johan

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


alefisico (alefisico) said : #8

Thanks Johan Alwall, that solved my question.