Single top production with VLQ_UFO model

hi,

We do not recommend to use the command
> define p = g u c d s b u~ c~ d~ s~ b~
> define j = g u c d s u~ c~ d~ s~
anymore since this is now done automatically when you import at 5 flavor model.
so your syntax is actually slightly inconsistent and it would be better to do

import model sm-no_b_mass

my guess is that VLQ_UFO is also a four flavor model
and therefore the definition of p/j are getting back to the 4 flavor definition?
If this is the case, I would advise to pass to a 5 flavor model.

Cheers,

Olivier

> On Dec 2, 2016, at 16:58, Didier Alexandre <email address hidden> wrote:
>
> New question #404579 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/404579
>
> Hi,
>
> I have come upon a problem when trying to generate single top quarks in Wb-fusion processes, when importing the VLQ_UFO model. Starting with the situation, in which I don't import the VLQ_UFO model and just use the SM, I can easily generate the process doing the following:
>
> [1]
> define p = g u c d s b u~ c~ d~ s~ b~
> define j = g u c d s u~ c~ d~ s~
> generate p p > j b~ t, t > w+ b QED=99 QCD=99
>
> However I want to produce the above process without explicitly indicating the intermediate top, i.e. :
>
> [2]
> generate p p > j b~ w+ b QED=99 QCD=99
>
> in which case only the final state particles are indicated. Up to this point, there is still no issue. The problem arises when I import the VLQ_UFO model
>
> [3]
> import model ./VLQ_UFO
>
> and use command [2]. In this case, the Wb-fusion process does not show up in the diagrams anymore.
>
> Any clues on what to try to understand why this process is absent would be greatly appreciated.
>
> Many Thanks in advance
> Didier
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier,

First of all, thank you very much for your reply and suggestions.

As far as I know, the VLQ model does not force 4 flavours. When I apply the definition of 'p' with 5 flavours as in my original question, which you do not recommend, MadGraph produces diagrams with b-quarks as colliding particles.

In addition, I have tried to set the mass of the b-quark to zero "manually" in the param_card.dat file.

With neither of these changes did I manage to generate the Wb-fusion diagrams I am trying to obtain.

Let me also point you to a reference of the model that I am using:
http://feynrules.irmp.ucl.ac.be/wiki/VLQ

Many Thanks
Best Regards

Didier

Hi Didier,

> As far as I know, the VLQ model does not force 4 flavors.

Except if you use a very old version of MG5aMC it should do it.
If I do
define p = p b b~
import model VLQ
I have the following:

> MG5_aMC>import model ./VLQ_UFO
> INFO: Change particles name to pass to MG5 convention
> Pass the definition of ‘j’ and ‘p’ to 4 flavour scheme. #Note that This line is in bold + colored to attract attention
> Kept definitions of multiparticles l- / vl / l+ / vl~ unchanged
> Defined multiparticle all = g u c d s u~ c~ d~ s~ a ve vm vt e- mu- ve~ vm~ vt~ e+ mu+ t b x tp bp y t~ b~ x~ tp~ bp~ y~ z w+ h w- ta- ta+

and you can indeed check that the p definition is what you expect.
> MG5_aMC>display multiparticles
> Multiparticle labels:
> all = g u c d s u~ c~ d~ s~ a ve vm vt e- mu- ve~ vm~ vt~ e+ mu+ t b x tp bp y t~ b~ x~ tp~ bp~ y~ z w+ h w- ta- ta+
> l- = e- mu-
> j = g u c d s u~ c~ d~ s~
> vl = ve vm vt
> l+ = e+ mu+
> p = g u c d s u~ c~ d~ s~
> vl~ = ve~ vm~ vt~

As you can see you are back to 4 flavor.

> When I apply
> the definition of 'p' with 5 flavours as in my original question, which
> you do not recommend, MadGraph produces diagrams with b-quarks as
> colliding particles.

We produce the diagrams that you ask for. Now the question is how much the computation that you asked for is consistent.
The problem is the interface with other code: PDF and parton-shower.
As long as you do not use PDF and parton-shower all the computation will be consistent.
But if you include PDF, then you need to use massive PDF (which,as far as I know, does not exists).
In general they are not problem of consistency with the Parton-Shower but since you are using a mixed 4/5 flavor computation, you
need to adapt the parton-shower to be consistent. (This is not handle with the automatic interface from madgraph)

> In addition, I have tried to set the mass of the b-quark to zero
> “manually" in the param_card.dat file.

That should solve the inconsistency. Note that you are not allowed to set mass of boson to zero in the param_card.
For quark this should however be ok.

Now I’m confused about what you are trying to achieve.
If i do:

> import model sm
> define p = p b b~
> define j = p / b b~ # this line is irrelevant since this is the default. This is just to stress that b quark is NOT in the j definition
> generate p p > j b~ t, t > w+ b QED=99 QCD=99

I get
> Total: 6 processes with 294 diagrams

> import model sm
> define p = p / b b~ # this line is irrelevant since this is the default. This is just to stress that b quark is NOT in the p definition
> define j = p / b b~ # this line is irrelevant since this is the default. This is just to stress that b quark is NOT in the j definition
> generate p p > j b~ t, t > w+ b QED=99 QCD=99
I get
> Total: 6 processes with 294 diagrams

i.e. you have the exact same diagram in both case. So I’m not sure why you include the b in the initial state for the sm model?
(It does make a difference for the VLQ_UFO model obviously.)

Now if you do
> import model VLQ_UFO
> generate c s~ > g b~ w+ b QED=99 QCD=99
> display diagrams

You will see the same diagram as the ones produced by the SM (for example look at diagram #8)
Which are the diagram that you are looking for right? I’m clearly missing a point in your question.
Could you clarify?

Cheers,

Olivier

> On Dec 5, 2016, at 16:02, Didier Alexandre <email address hidden> wrote:
>
> Question #404579 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/404579
>
> Status: Answered => Open
>
> Didier Alexandre is still having a problem:
> Hi Olivier,
>
> First of all, thank you very much for your reply and suggestions.
>
> As far as I know, the VLQ model does not force 4 flavours. When I apply
> the definition of 'p' with 5 flavours as in my original question, which
> you do not recommend, MadGraph produces diagrams with b-quarks as
> colliding particles.
>
> In addition, I have tried to set the mass of the b-quark to zero
> "manually" in the param_card.dat file.
>
> With neither of these changes did I manage to generate the Wb-fusion
> diagrams I am trying to obtain.
>
> Let me also point you to a reference of the model that I am using:
> http://feynrules.irmp.ucl.ac.be/wiki/VLQ
>
> Many Thanks
> Best Regards
>
> Didier
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.