b quark decay

Asked by Michele on 2012-02-19

Hi,

I'm trying to force a semileptonic b decay from t t~ decay in MG5 1.4.1.
In other words I would like to generate a process like this

generate p p > t t~ , t > W+ b, (t~ > b~ W-, W- > j j, (b~ > j W+ > j mu+ vm))

but I get a message like this:

InvalidCmd : No amplitudes generated from process Process: b~ > g u c d s u~ c~ d~ s~ w+> g/u/c/d/s/u~/c~/d~/s~ mu+ vm WEIGHTED=6. Please enter a valid process

Could you help me please?
Thank you.

Michele.

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
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Last query:
2012-02-20
Last reply:
2012-02-20

Hi Michele,

First note that in order to have it working, you need to ask for a non diagonal ckm (which is the default approximation).
so you need to import a model either
import model sm-full
or
import model sm-ckm.

then the correct syntax is the following:
generate p p > t t~ , t > W+ b, (t~ > b~ W-, W- > j j, b~ > j mu+ vm)

Cheers,

Olivier

Michele (michele-gabusi) said : #2

Hi Olivier,

the reported example was fully SM indeed.
But if your process consist of a SM decay and an exotic one (loaded by a
different model), must you load your sm-full model before and your
customized one in sequence?
Or are there any other parameters to change by hand?

Thank you.

Michele.

2012/2/19 Olivier Mattelaer <email address hidden>

> Your question #188237 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/188237
>
> Status: Open => Answered
>
> Olivier Mattelaer proposed the following answer:
> Hi Michele,
>
> First note that in order to have it working, you need to ask for a non
> diagonal ckm (which is the default approximation).
> so you need to import a model either
> import model sm-full
> or
> import model sm-ckm.
>
> then the correct syntax is the following:
> generate p p > t t~ , t > W+ b, (t~ > b~ W-, W- > j j, b~ > j mu+ vm)
>
> Cheers,
>
> Olivier
>
> --
> If this answers your question, please go to the following page to let us
> know that it is solved:
>
> https://answers.launchpad.net/madgraph5/+question/188237/+confirm?answer_id=0
>
> If you still need help, you can reply to this email or go to the
> following page to enter your feedback:
> https://answers.launchpad.net/madgraph5/+question/188237
>
> You received this question notification because you asked the question.
>

--
Michele Gabusi
Università degli Studi di Pavia
Dipartimento di Fisica Nucleare e Teorica
via A. Bassi 6, Pavia
Tel. +39 0382 987593 (Office)
Tel. +39 0382 987637 (Lab)

Johan Alwall (johan-alwall) said : #3

Hello Michele,

Note to begin with that in the models that come with MadGraph 5, the b quark has no width and it is therefore not possible to simulate processes where the b quark decays. It is in general not meaningful to attribute a unique width to the b quark, since the b quark hadronizes before it decays, and the lifetime depends on the properties of the individual B hadrons. With this in mind, you can of add a width to the b quark in your model (e.g. at FeynRules level), and generate any physical process you want. The syntax is, as Olivier points out,

generate p p > t t~ , t > W+ b, (t~ > b~ W-, W- > j j, b~ > j mu+ vm)

Only the last model loaded is used for a process.

All the best,
Johan

FabioMaltoni (fabio-maltoni) said : #4

Ciao Michele,

also b's are normally decayed by Pythia and therefore I would
suggest not to decay them with mg.

Fabio

On 20 Feb 2012, at 09:45, Johan Alwall wrote:

> Question #188237 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/188237
>
> Status: Open => Answered
>
> Johan Alwall proposed the following answer:
> Hello Michele,
>
> Note to begin with that in the models that come with MadGraph 5, the b
> quark has no width and it is therefore not possible to simulate
> processes where the b quark decays. It is in general not meaningful to
> attribute a unique width to the b quark, since the b quark hadronizes
> before it decays, and the lifetime depends on the properties of the
> individual B hadrons. With this in mind, you can of add a width to the b
> quark in your model (e.g. at FeynRules level), and generate any physical
> process you want. The syntax is, as Olivier points out,
>
> generate p p > t t~ , t > W+ b, (t~ > b~ W-, W- > j j, b~ > j mu+ vm)
>
> Only the last model loaded is used for a process.
>
> All the best,
> Johan
>
> --
> You received this question notification because you are a member of
> MadTeam, which is an answer contact for MadGraph5.

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