MadGraph5_aMC@NLO

Total Xsec for Pair Light Higgs decay into bottom quarks shows Higher Xsec than Pair Higgs Production

Asked by Aya Ismail Hosny Beshr

Dear MadGraph experts,

I am trying to generate the following process using 2HDM Type-I (generated by SARAH);

g g > h1 h1 /ah2, h1 > d3 d3bar, (h1 > d3 d3bar)

where Mass of h1 is around 90 GeV

The xsec for the above process is around 0.1067 pb, which is higher than the production xsec for (g g > h1 h1 /ah2) which is nearly 0.05 pb.

So, how is this possible?

I tried another decay (b bar gamma gamma) and it shows lower xsec than production as expected of order 1.e-05

I am using a spectrum file (from SSP output) with the following light higgs decays BR (PDG 25 is assumed here for lighter Higgs than the SM one);

DECAY 25 9.15744048E-03 # hh_1
# BR NDA ID1 ID2
     2.22396411E-04 2 22 22 # BR(hh_1 -> VP VP )
     7.09358551E-02 2 21 21 # BR(hh_1 -> VG VG )
     9.05864317E-09 2 23 23 # BR(hh_1 -> VZ VZ )
     1.29733409E-05 2 24 -24 # BR(hh_1 -> VWm^* VWm_virt )
     8.33666263E-07 2 -1 1 # BR(hh_1 -> Fd_1^* Fd_1 )
     2.99862533E-04 2 -3 3 # BR(hh_1 -> Fd_2^* Fd_2 )
     8.01511493E-01 2 -5 5 # BR(hh_1 -> Fd_3^* Fd_3 )
     6.88852375E-09 2 -11 11 # BR(hh_1 -> Fe_1^* Fe_1 )
     3.07723333E-04 2 -13 13 # BR(hh_1 -> Fe_2^* Fe_2 )
     8.87466438E-02 2 -15 15 # BR(hh_1 -> Fe_3^* Fe_3 )
     1.59832874E-07 2 -2 2 # BR(hh_1 -> Fu_1^* Fu_1 )
     3.79620434E-02 2 -4 4 # BR(hh_1 -> Fu_2^* Fu_2 )

Any help will be much appreciated.

Thanks for your time,
Aya

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Olivier Mattelaer
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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Did you check the following FAQ already?
FAQ #2442: “why production and decay cross-section didn't agree.”.

Revision history for this message
Aya Ismail Hosny Beshr (ayaa) said : 		#2

Dear Olivier,

Thanks for your response!

I checked the FAQ and possibilities given,

1- The cut_decays parameter in the run_card is already set to False.

2- The bwcutoff = 15

3- I am not using width at NLO

4- The total decay width for light Higgs in param_card is of order O(10^-3) which is higher than (10^-12*mass)

5- The dynamical scale in run_card is set to -1, but actually I am not getting this point so much. I tried previously various syntax and getting almost the same cross-section.

Thanks for your time!
Aya

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#3

Ok I have update the FAQ to better explain this case.

The issue is that your total width is likely not correct at LO accuracy.

I have rephrase add information within the FAQ to make this point clearer:
In particular the FAQ, now include the following text:

MG5aMC will not use the branching information for the above syntax (even if provided), it will do the full phase-space integration. If narrow-width integration is valid, you can see such computation as.
- taking the production cross-section
- multiply by the phase-space integration associated to the decay (i.e. computing the partial width with your cut/...)
- divide by the total width (the one written in your param_card)
Therefore it is important that your total width is set to its physical value and not to a dummy value (remember that this is not a free parameter of your theory).

Does that help?

Cheers,

Olivier

Revision history for this message
Aya Ismail Hosny Beshr (ayaa) said : 		#4

Thanks a lot Olivier for the clarification!

Yes absolutely.

So in short if I understand correctly, the issue is incorrect total Wh in param_card, which seems to be less than the partial decay width for the given channel.

 Regards,
Aya

Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said : 		#5

Yes indeed.

Cheers,

Olivier

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Aya Ismail Hosny Beshr (ayaa) said : 		#6

Thanks a lot Olivier!

Regards,
Aya

Revision history for this message
Aya Ismail Hosny Beshr (ayaa) said : 		#7

Thanks Olivier Mattelaer, that solved my question.