bbH/bH Associated Production Process

Goal of the group

  • The goal of the group is to provide inclusive and exclusive cross-sections for bbh production and Monte-Carlo generation tools. The cross-section calculation in BSM is the shared responsibility with WG3/MSSM.

Group Conveners

  • Lei Zhang (ATLAS. Frieburg), Alexandre Nikitenko (CMS, IC), Michael Spira (TH. PSI), Marius Wiesemann (TH, CERN)

Inclusive 4FS+5FS matched cross-sections

Santander matching (currently used in the LHC analyses; to be replaced; see Recommendation item)

NLO+NNLLpart+ybyt matching

FONLL-B matching

Recommendation for the Run II ATLAS and CMS analyses with 2017-2018 data

* Use the numbers from NLO+NNLLpart+ybyt matching, by mentioning that the latter have been cross-checked against the FONLL ones and citing both works.

Available bbH NLO MC generators

bbh in 4FS in MG5_aMC@NLO

  • 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.
      

POWHEG-BOX Version 2 bbh in 4FS

  • find instructions on this link with CMS specific for the showering and hadronization with PY8

Sherpa 4 FS and 5FS

Recommendations for the acceptance uncertainty calculations in the experimantal analysis (Preliminary)

  • Use one of the recommended NLO generators above for the signal modelling in your analysis
    • set the central values of QCD scales and shower scale (Q_sh for aMC@NLO, hdamp for POWHEG and mu_Q for Sherpa) as described in the bbH Section of YR4
    • use pdf set, PDF4LHC15_nlo_nf4_30 (92000) if you generate in 4FS, which is the case for aMC@NLO and POWHEG; Sherpa has both 4FS and 5FS implementations
    • Evaluate the acceptance uncertainies of the used generator varying the factorization, renormalization and shower scales as described in the bbH Section of YR4 and add them linearly
    • add PDF uncertainty of the acceptance from the used generator to above uncertainties
    • To account for the difference in acceptance given by the different generators, evaluate (at least at particle level and at least for a few Higgs mass points) the acceptance with aMC@NLO, POWHEG and Sherpa and add it linearly to the acceptance uncertainty due to QCD and shower scales and PDFs.

Proposal for an analysis of the bbZ process as a benchmark for the bbH production process

MC studies presented in YR4/bbH section show the differences in the acceptance of the bbH process evaluated with different NLO generators. The difference is not a small, ~ 25-30 %. We propose to use the bbZ process as a benchmark for bbH and compare data with predictions of NLO generators, aMC@NLO, POWHEG and Sherpa. The plan is to:
  • measure with 13 TeV data the inclusive bbZ cross-section (>=1b, >=2b) and kinematical distributions:
    • pT/eta of Z, pT/eta of the leading and subleading b-jets and non-b-tagged jet, di-b-jet mass, DRbb, ...
    • the interesting possibility would be to separate the qq_bar->Z+g, g->bb process (which is not present in bbH production, while gg->H+g, g->bb is)
  • compare with predictions from aMC@NLO, POWHEG, Sherpa
  • tune generator parameters like Q_sh (in aMC@NLO), hdamp (in POWHEG) to data
  • propagate knowledge from data/MC comparison for the bbZ process to CMS/ATLAS searches for BSM H->mumu, tautau. bb; in particular for the signal generation settings

The bbH group conveners are responsible for the initiation and coordination of this project with the ATLAS/CMS/Theory teams. This proposal was reported during July 2017 HLHCXWG meeting in this talk

bbZ CMS information

Documents

LHCHXSWG YR4, bbH Section

Meetings

References

Useful Links

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Compressed Zip archivetar output-HXSWG.tar r1 manage 250.0 K 2017-07-05 - 11:43 AlexandreNikitenko bbH cross-section from NLO+NNLLpart+ybyt matching
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