bbH/bH Associated Production Process

• aMC@NLO-MadGraph. Take V 2.6.1
• folders for yb^2 and ybyt terms
• message from Marius

     I have already set them up according to the YR4 with LHCHXSWG recommendations.
     In particular muR=muF=H_T/4 is set as default scales. Most importantly MG5_aMC 
     has changed its default shower scale from sqrt(s)-like to H_T/2. For bbH usually 
     a lower shower scale has proven useful. Since H_T/2 is already lower than the old 
     default we have decided to use H_T/2/Sqrt(2) as a central value, and then you can 
     do variations by Sqrt(2) up and down to get systematics of the matching uncertainties.
     Since our extensive studies have been done with the old s-like shower scale, you 
     may also consider a different central value. A possible measure to find it is to 
     consider NLO+PS and NLO pT_bbH distribution and choose an upper bound for shower 
     scale variations which shows a satisfactory matching between them at high pT. In 
     any case I reckon that the provided recommendation fullfils this.
  
     It would be very interesting to monitor whether with this choice the large differences 
     to POWHEG persist, or whether they are reduced for the acceptance and some observables.
  
     (It shall be noted that, as before, the ybyt folder supports only running matched 
     to the shower no fixed order alone.)
   

* instrictions from Marius

     You find there two folders ("bbH4FS_yb2","bbH4FS_ybyt") which determine the yb^2
     and the yb*yt terms. They have to be run separately, but the results can be simply
     added in the end (either the final distributions or already the event files).

     Extract the .tar file in your MG5_aMC main folder (Use MG5_aMC 2.6.1). The bbH folders
     created with v2.6.1 should be taken. First you compile MG5_aMC 2.6.1 with some dummy process:

     Setup slc6_amd64_gcc530 compiler going to CMSSW_8_0_3 area and issue "cmsenv"
     Go to MG5_aMC_v2_6_1 directory and run dummy process:     

     ./bin/mg5
     generate p p > h b b~ [QCD]
     output bbH_test
     exit

     Delete bbH_test folder, copy bbH folders under main MG5 directory and go to bbH4FS_yb2 or bbH4FS_ybyt folder.

     The following applies to both
     folders bbH4FS_yb2 and bbH4FS_ybyt, go inside one of the two and:

     First:

       a) adjust mg5_path path in Cards/amcatnlo_configuration.txt

       b) if want to use lhapdf (to access PDF4LHC_nlo_nf4 (92000)) add in amcatnlo_configuration.txt
       lhapdf = /cvmfs/cms.cern.ch/slc6_amd64_gcc530/external/lhapdf/6.1.6-giojec7/bin/lhapdf-config     
       (see location of lhapdf with scram tool info lhapdf; setenv SCRAM_ARCH slc6_amd64_gcc530 - setup compiler used for 6.1.6. CMSSW_8_0_3) 

       c) comment fastjet setting

     Run the process with 

        ./bin/generate_events

     Results (and event files) will be found in: Events/run_*

    Inputs can be set in:
       Cards/param_card.dat
       Cards/run_card.dat
      (Cards/shower_card.dat -- for shower inputs in case you run the shower via MG5_aMC)
 
       The bottom Yukawa is MSbar renormalized, the internal bottom mass is on-shell.
       mb(mb) input for Yukawa can be adjusted in param_card.dat. The particular choice of
       mb(muR) can only be changed in the code (SubProcesses/reweight_xsec.f function
       rwgt_muR_dep_fac, variable mbmuR) since it involves the running from mb(mb)->mb(muR).
       The given default values were chosen after a comparison with Michael Spira and
       should correspond to what he uses for his 4FS prediction. With these values we obtain
       agreement with his results at the level of 1% (which is the quoted numerical
       uncertainty of these numbers) when using a fixed scale (muF=muR=(mH+2mb)/4).

       By default, I have set all the parameters in run_card.dat and param_card.dat such
       that they should correspond to what we have chosen in our publication. The default
       total inclusive results (muF=muR=H_T/4) should be (modulo ~<1% numerical uncertainty):
         yb^2:   0.448  pb
         yb*yt: &#65293;0.0365 pb
       as can be read off table 1 of our paper.

       Changing shower scale:

       In the newer MG5_aMC versions there is no need anymore to hack the code. You can directly set the shower scale via the input param_card.dat:

            0.3535534 = shower_scale_factor ! multiply default shower starting
                                                                  ! scale by this factor

       I have set the default already to 1/Sqrt[2]/2 so that variations of it would imply setting it to 1/2 and 1/4.