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The total cross section

Asked by Kirsty Moore

I have entered the process 'e,E->h,ne,Ne' with the higgs decaying to a bottom and antibottom quark 'h->b,B' and done a run over the energy from 150 GeV to 1TeV.

The results for cross section in the summary table are now different to the cross section for the process in the results below, does this mean that the decay is being taken into account and HEPMDB is now doing the calculation for the branching ratio etc for us and giving us the total cross section out for the process and decay?

If this is so, I have found that the value in the summary table for the total cross section, of production and decay, to be different to what I have calculated it to be manually.

an example of results I have got is shown below

En150.txt CalcHEP Numerical Details

 Done!

Processes sigma (fb) unc (%) PID Time (hr) N events
e,E->h,ne,Ne 3.0818e+01 5.01e-02 0 0.00 25/25
Total 3.0818e+01

Decays width (GeV) unc (%) PID Time (hr) N events
h->b,B 2.6509e-03 4.47e-05 0 0.00 3/5

Widths PID Time (hr)
Widths 0 0.00

Total 1.9250e+01 0.00 1/1

However when I calculate the total cross section manually using a branching ratio of 78.60% (which I calculated using the above width of 2.6509e-03 GeV and a total width of 3.37e-03 GeV which i got from finding the partial widths of all decays by entering the decay into the batch file as 'h->2*x) I get the value of total cross section for process and decay to be 2.42e+01 which is quite different to the 1.9250e+01 given in the summary table.

any help would be greatly appreciated! Many thanks.

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Alexander Belyaev (alexander.belyaev) said : 		#1

Dear Kirsty,
sorry for the delay with reply.
When you calculate production AND decay,
then decay Branching ratio is taken into account,
end
you will get result for
e,E->h (h->bb) ne,Ne
cross section which is equal to
sigma(e,E->h,ne,Ne) x Br(h->bb)

The question is what is the correct value of the Br(h->bb).
Indeed Br(h->bb) =0.78 from the Naive calculation when you evaluate h->2*x processes
However this approach does not take into account h decay to 3 and 4-body decays
which is not-negligible for h->WW* and h->Z* processes

So the actual Br(h->bb) is actually lower than 0.78 and is eqaul to 1.9250e+01/3.0818e+01
This Br takes into account all Higgs boson decays.
You can get the value of this Br(h->bb) as well as others calculated more accurately from the .lhe file
header -- the header of the file with the generated events when you request generation even a few events.

Hope this clarifies the situation.

Regards,
  Alex

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