Branching ratio for H+ (charged Higgs)

Asked by Sergei Chekanov on 2018-06-21


 I'm creating a MG5 simulations for ATLAS studies using HepSim repository. I need to make 2 samples:

  import model MSSM_SLHA2
 define p = g u c d s u~ c~ d~ s~ b b~
 generate p p > h+ t~, ( h+ > h01 w+)
 add process p p > h- t, ( h- > h01 w- )

How does MG5 know about the branching ratio for a given H+ mass and tan(beta). I see MG5 applies the branching ratio, but not clear how and what parameters are used. Can I set it to 1, for example?

I havea similar question about this process:

generate p p > h+ t~, ( h+ > t b~ )
add process p p > h- t, ( h- > t~ b)

Where this branching ratio is set, and is it trustful ?

best, Sergei

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Dear Sergey,

If you use that decay-chain syntax, MG5aMC does NOT use the branching ratio information.
You can set it if you want but MG5aMC will NOT use it anyway.

For such syntax we perform the phase-space integration over the invariant mass (up to some cut-off defined in the run_card)
of the Full matrix-element. We therefore have an explicit dependencies in the total width.

If you want to rely on the BR for your computation.
You should use MadSpin.

Here is an example of script:

 import model MSSM_SLHA2
 define p = g u c d s u~ c~ d~ s~ b b~
 generate p p > h+ t~
 add process p p > h- t
decay h+ > h01 w+
decay h- > h01 w-

in your ./path_to_param_card_with_BR_information.dat

you can specify your BR with the conventional format:
DECAY 15 2.630571e-17
# BR NDA ID1 ID2 ...
   6.918270e-01 3 -2 1 16 # 1.81990004322e-17
   2.430157e-01 3 -12 11 16 # 6.39270052965e-18
   3.732688e-02 3 -2 3 16 # 9.81910080485e-19
   2.783046e-02 3 -14 13 16 # 7.32100009927e-19

Note that MadSpin requires that ALL particles have the BR information (more exactly all which are present as decayed in the madspin_card) in order to not recompute the width himself. So be careful to either remove the default line of the madspin_card and/or to include the BR for all those particles.



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