difference in cross-section between MG5 value and analytical value

Asked by Deepanjali Goswami on 2018-09-06

Dear experts ,

 I have a query in the model (Type II seesaw model + Inert Doublet model ). When I generate process in MG5 ,

 p p > D++ D- , (D++ > H+ H+, H+ > j j H0, H+ > l+ vl H0), ( D- > H- H0, H- > l- vl H0),

 cross section comes out to be very small ~ 10^{-45} fb by using MG5. But analytical value of cross section is a few fb.

May I ask you what may be the reason for the error.

Thanks and sincerely,
Deepanjali

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Last query:
2018-09-18
Last reply:
2018-09-18

Did you check the associated FAQ?

I would bet here that the width of all the particles are not consistent in your param_card, but this might be one of the other points obviously

Cheers,

Olivier

FAQ #2442: “why production and decay cross-section didn't agree.”.

Hi Olivier,

Thanks for your suggestion and reply.
Yes, the width of the particle decay (D++ > H+ H+) ~ 28 GeV. I think it is the problem .

Thnaks and sincerely,
Deepanjali

Hi Olivier,

I am sorry to write to you again, I still could not resolve my issue.
I have checked the process with small decay width DD+ (DD +> H+ H+) i~ 5 GeV, but I can see the same problem of having a small cross-section for the pocess p p > D++ D- , (D++ > H+ H+, H+ > u d~ H0, H+ > l+ vl H0 QED<=3), ( D- > H- H0, H- > l- vl H0 QED<=3 ), is of order 10^(-26).

I am not getting any clue for the same, I have also checked all the points mentioned in FAQ #2442.

It will be great if I can have any more help.

Thanks and sincerely,
Deepanjali

What you need to do is to understand which decay kills your cross-section.
For that reason, you have to compute each stage one after each other and compare the cross-section that you get numerically with what you expect.
so you need to compute:
1) p p > D++ D-
2) p p > D++ D- , (D++ > H+ H+)
3) p p > D++ D- , (D++ > H+ H+, H+ > u d~ H0)
4) p p > D++ D- , (D++ > H+ H+, H+ > l+ vl H0 QED<=3)
and the same type of process for the decay of D-

This should allow you to spot where your cross-section drops in a dramatic way

Cheers,

Olivier

> On 17 Sep 2018, at 16:42, Deepanjali Goswami <email address hidden> wrote:
>
> Question #673420 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/673420
>
> Status: Answered => Open
>
> Deepanjali Goswami is still having a problem:
> Hi Olivier,
>
> I am sorry to write to you again, I still could not resolve my issue.
> I have checked the process with small decay width DD+ (DD +> H+ H+) i~ 5 GeV, but I can see the same problem of having a small cross-section for the pocess p p > D++ D- , (D++ > H+ H+, H+ > u d~ H0, H+ > l+ vl H0 QED<=3), ( D- > H- H0, H- > l- vl H0 QED<=3 ), is of order 10^(-26).
>
> I am not getting any clue for the same, I have also checked all the
> points mentioned in FAQ #2442.
>
> It will be great if I can have any more help.
>
> Thanks and sincerely,
> Deepanjali
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier,

I have checked all the steps before as you mentioned and found that all these (1-4) are fine but
when I put all the three sub decay processes at a time including D- decay , then cross-section decreases abruptly. but with two sub decay process there is no issue.

I hope it wil help you now.

Thanks and sincerely,
Deepanjali

So did you try the same for the D- case?

1) p p > D++ D-
2) p p > D++ D-, D- > H- H0
3) p p > D++ D-,( D- > H- H0, H- > l- vl H0 QED<=3)

Cheers,

Olivier

> On 17 Sep 2018, at 19:17, Deepanjali Goswami <email address hidden> wrote:
>
> Question #673420 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/673420
>
> Deepanjali Goswami posted a new comment:
> Hi Olivier,
>
> I have checked all the steps before as you mentioned and found that all these (1-4) are fine but
> when I put all the three sub decay processes at a time including D- decay , then cross-section decreases abruptly. but with two sub decay process there is no issue.
>
> I hope it wil help you now.
>
> Thanks and sincerely,
> Deepanjali
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier,

I have found the problem starts with this decay process,
(2) p p > D++ D-, D- > H- H0 (xsec ~ 10^{-14} pb ) and continues with the decay of H- too,
(3) pp > D++ D- , (D- > H- H0, H- > l- vl H0 QED<=3 )( xsec ~ (10^{-46} pb) .

Thanks a lot for suggesting me to do these steps.

Thanks and sincerely,
Deepanjali

Hi Olivier,

I missed to mention the production cross-section for p p > D++ D- ~ 25 fb , and DW of (D- > H- H0 ) is ~ 3 GeV.

I hope these informations will help you now.

Thanks and sincerely,
Deepanjali

Ok,

Did you recheck all the point of the above FAQ for that decay?

If yes,
What is the mass of the D-, H- and H0?
What is the width of all such particles? (the one in your card and the one at LO accuracy)

Cheers,

Olivier

Hi Olivier,

Yes, I have checked the FAQ , but all the points seem to me as not coming under our decay process (D- > H- H0) where
mass_D- = 271 GeV, mass_H- = 123 GeV , mass_H0 = 63 GeV.
Total Width (D-) = 7.29 GeV
Total width (H-) = 7.77* 10^{-8} GeV
Total width (H0) = 0 GeV

These width's are found by putting "Auto" in param card.

Thanks and sincerely,
Deepanjali

What is the D- > H- H0 branching ratio computed by the Auto command?

Cheers,

Olivier

Hi Olivier,

Sorry for my late reply.
The BR for D- > H- H0 by Auto command is 45.3%, where as D- decays to H- A0 too with BR of 54.6%

Thanks and sincerely,
Deepanjali

Hi Olivier,

I think the issue with the process pp > D++ D- , (D- > H- H0, H- > l- vl H0 ) is solved now with QED<=9, it is satisfying the analytical value.

But I am not agree with other cross-section for the process p p > D++ D-, (D++ > H+ H+, H+ > u d~ H0 ) , where the analytical value is 5.6 fb, but MG5 is giving as 1.153 fb. Theproduction cross section is 24.9 fb. The DW (D++ > H+ H+ ~5.2 GeV), and DW(H+ > u d~ H0) is 2.15*10^{-4} GeV.

M-D++ = 291 Gev
M-D+ = 270 GeV
M-H+ = 123 GeV

Thanks in advance again!

Sincerely,
Deepanjali

What do you mean by DW? I do not know such accronym
Do you mean partial width? The important factor here is actually the Branching ratio.

But here again, you have to understand where the problem comes from.
Is
p p > D++ D-, (D++ > H+ H+)
gives you the expected cross-section or not?
Is it what you call production cross-section or is it p p > D++ D-?
Also note that for the syntax:
p p > D++ D-, (D++ > H+ H+, H+ > u d~ H0 )
BOTH H+ are decayed. so you should consider that in your analytical computation as well.
(including the combinatorial factor)

Cheers,

Olivier

> On 18 Sep 2018, at 16:47, Deepanjali Goswami <email address hidden> wrote:
>
> Question #673420 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/673420
>
> Status: Answered => Open
>
> Deepanjali Goswami is still having a problem:
> Hi Olivier,
>
> I think the issue with the process pp > D++ D- , (D- > H- H0, H- > l-
> vl H0 ) is solved now with QED<=9, it is satisfying the analytical
> value.
>
> But I am not agree with other cross-section for the process p p > D++
> D-, (D++ > H+ H+, H+ > u d~ H0 ) , where the analytical value is 5.6 fb,
> but MG5 is giving as 1.153 fb. Theproduction cross section is 24.9 fb.
> The DW (D++ > H+ H+ ~5.2 GeV), and DW(H+ > u d~ H0) is 2.15*10^{-4}
> GeV.
>
> M-D++ = 291 Gev
> M-D+ = 270 GeV
> M-H+ = 123 GeV
>
> Thanks in advance again!
>
> Sincerely,
> Deepanjali
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier,

I am sorry for using such acronym, yes it is partial decay width. The Branching ratio for D++ > H+ H+ is 100% and H+ > u d~ H0 is 22.47%.

I think the problem is solved now, I missed the point "BOTH H+ are decayed" to u d~ H0 unless we allow H+ decay to another channel.

Thanks very much!

Sincerely,
Deepanjali

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