h > bb bar Branching ratio

Dear Ushoshi,

I assume that you changed the Higgs mass from the default. Did you change the Higgs width in the param_card.dat to the appropriate new value? If you use the wrong width, you will get the wrong cross section for any process that includes an (on-shell) s-channel Higgs propagator, since the width is included in the matrix element expression. The easiest way to include the correct width is to use the SM width calculator available at http://madgraph.hep.uiuc.edu/.

All the best,
Johan

Hi Johan,
                   Thanks a lot. I have changed the Higgs mass to 125 GeV and I was using the Higgs width from LHC working group corresponding to 125 GeV Higgs mass i.e. 4.07 MeV.. Is there any problem with that?

Ushoshi

Hello Ushoshi,

If you want the correct cross section, you need to use the leading-order width, since the MadGraph matrix elements are calculated at leading order. If you don't care about the cross section but want to make sure that you get the correct Breit-Wigner distribution, then you should use the "real width". Note that this of course is irrelevant for a realistic study, since the Higgs width is much smaller than the resolution even for the 4-lepton final state. So I would suggest to use the leading order width as calculated by the MadGraph SM calculator, to get the correct (leading-order) cross sections and effective branching ratios.

Also please remember to turn off cuts on the decay products when comparing cross sections with Xsec*BR by setting cut_decays to F in the run_card.dat.

Cheers,
Johan

Hi Johan,
                      Thanks a lot. All the cuts in my run_card.dat are off. Actually for WW*, ZZ* and gamma gamma I'm getting correct branching ratio using "real width" i.e 4.07 MeV . I still don't understand why I'm getting off result for bb~ with this width. I have checked the Branching ratio with LO higgs decay width calculated by MG calculator. .For 120 GeV higgs mass I'm getting 0.70 instead of 0.64 as Branching ratio of h > b b~.

Ushoshi

Hello Ushoshi,

The BRs for WW* and ZZ*, and also for gamma-gamma, are given purely by well-determined electroweak parameters. However, for bbbar, you have a big dependence on the Yukawa coupling used. In particular, you can use a running Yukawa coupling or a fixed coupling at the b mass. You set the Yukawa in the param_card.dat. So in principle, you can tune the b Yukawa coupling you use in order to get the BR you wish for the b.

All the best,
Johan

Thanks a lot Johan.

Ushoshi