Calculating decay width for Higgs for a new model.

Hi Shankha,

If m_h > 2m_w and m_h > 2M_z, then the 2 body decay calculator should be enough.
But Note that with this method, you will always loose the branching ration of Higgs going in two photons/gluons.
Since they are not part of the model (Don't take into account this comment if they are in yours)

If m_h < 2 m_w, then indeed the current code is not enough.
Three solutions:
1) you compute the width by hand:
generate h > all all
generate h > w+ l v~
add process h > w- l~ v
...
output
launch

This creates a param_card with the valid width BR.
2) you use the online calculator:
madgraph.phys.ucl.ac.be
click on Tools
then on Calculator
then on SM (MG5)
Fill the form

3)You can use the version of MG5 which is able to compute the three body decay. But this is still in alpha:
bzr branch lp:~maddevelopers/madgraph5/decay_calculator

Cheers,

Olivier

Thank you very much. But there's still a confusion on my part. When I am calculating the decay width of higgs by process 1 and 2, the results are different. I checked the values of all the parameters yet they are somehow coming to be different.

With method number 2, on choosing the "Calculation Scheme" under "Yukawa couplings" to "w/ evolution upto mH", I am getting a decay width which matched with the one given in LHC working group.

But in any way, if I follow these steps (for any model sm, heft or even my own model) :

generate h > all all
add process h > w+ l- vl~
add process h > w- l+ vl
add process h > z l+ l-
add process h > z vl vl~
output
launch

I am somehow getting a somewhat different decay width.

I will try method 3 as suggested.

Yours sincerely,

Shankha Banerjee.

Hi Shanka,

> Thank you very much. But there's still a confusion on my part. When I am
> calculating the decay width of higgs by process 1 and 2, the results are
> different.

In fact method 2 computes as well loop induced decay. Decay mode which are not accessible for the two other method.
Is this the difference that you observe? Or is it something else?

Cheers,

Olivier

On May 12, 2013, at 6:06 AM, Shankha Banerjee <email address hidden> wrote:

> Question #228667 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/228667
>
> Status: Answered => Open
>
> Shankha Banerjee is still having a problem:
> Thank you very much. But there's still a confusion on my part. When I am
> calculating the decay width of higgs by process 1 and 2, the results are
> different. I checked the values of all the parameters yet they are
> somehow coming to be different.
>
> With method number 2, on choosing the "Calculation Scheme" under "Yukawa
> couplings" to "w/ evolution upto mH", I am getting a decay width which
> matched with the one given in LHC working group.
>
> But in any way, if I follow these steps (for any model sm, heft or even
> my own model) :
>
> generate h > all all
> add process h > w+ l- vl~
> add process h > w- l+ vl
> add process h > z l+ l-
> add process h > z vl vl~
> output
> launch
>
> I am somehow getting a somewhat different decay width.
>
> I will try method 3 as suggested.
>
> Yours sincerely,
>
> Shankha Banerjee.
>
> --
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> MadTeam, which is an answer contact for MadGraph5.

Hi Olivier,

                 When I try to compute the decay width h -> Z Z* by including all the sub processes like h > z l+ l-, h > z ta+ ta-, h > vl vl~, h > q \bar{q}, the partial decay width I obtain is ~ 0.00008 whereas the SM value (for a higgs mass of 125 GeV) is 0.0001. So my result is off by about ~ 27%. Similarly, for the decay width h -> W W*, the result I am getting using MG5 is 5.4% off from its SM value (even after considering the CKM matrices properly).

I even checked it using the online decay calculator. There also the decay widths are about this much different from the SM values.

Can you please help me out to resolve this issue?

Thank you.

Yours sincerely,

Shankha Banerjee.

Hi Olivier,

                 Thank you very much. You are right because there will be loop contributions for the h -> Z Z* which will not be negligible. I somehow need to include that in my model file as an effective vertex like the h -> gamma gamma or g g -> h.

Thanks again.

Yours sincerely,

Shankha.

Thanks Olivier Mattelaer, that solved my question.