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2015:groups:tools:vbb

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# A comparison of generators/fixed order predictions for Z+b(b) and W+b(b) among them and against data

Papers: http://arxiv.org/pdf/1107.3714.pdf (Campbell et al.)

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Gavin Hesketh, Vitaliano Ciulli, Rikkert Frederix, Marek Schoenherr, Paolo Francavilla, Gionata Luisoni, Davide Napoletano, Carlo Pandini, Frank Krauss, Carlo Oleari, Stefano Pozzorini, …

This study includes both comparison at particle level (after hadronization) and at parton level (either before or after the parton shower). One of the goal is comparing results in 5-flavour vs 4-flavour scheme. Some Rivet routine have been provided based on measurements of ATLAS and CMS. At particle level this plugins allows comparison to the results of the measurement on 7 TeV data, so we should use this center-of-mass energy.

### ATLAS Rivet routines:

Measurement of differential production cross-sections for a Z boson in association with b-jets in 7 TeV proton-proton collisions with the ATLAS detector arxiv:1407.3643

Measurement of the cross-section for W boson production in association with b-jets in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector arXiv:1302.2929

Center-of-mass energy: 7 TeV

Run the analyses over Z→ee or W→enu events. The .yoda file is also needed to define the binning for some plots, and for data comparison.

The routine will work with either a pure parton level hepmc file, or a full particle level file. It will reconstruct anti-kT 0.4 jets with rapidity < 2.4 (2.1 for W) and pT>20 GeV (25 for W), and label as b-jets based on matching Delta R(jet, quark/hadron) < 0.3. The quarks are taken at the end of the shower (no b-quark daughters), and the weakly decaying b-hadrons are used (no b-hadron daughters).

Makes the following plots:

• quarklev_b_quark_pT : pT of all b quarks with rapidity<2.7
• quarklev_b_quark_pT_matching_jet : pT of all b quarks with rapidity<2.7, matching a jet
• quarklev_bjet_number_matching_b_quarks : number of b-quarks matching each jet
• quarklev_bjet_number_matching_b_quarks_5GeV : number of b-quarks with pT>5 GeV matching each jet
• quarklev_bjet_pT_single_quark_0GeV : pT of bjets containing a single b quark (no cut on quark pT)
• quarklev_bjet_pT_double_quark_0GeV : pT of bjets containing at least two b quarks (quark pT>5 GeV)
• quarklev_bjet_pT_single_quark_5GeV : pT of bjets containing a single b quark (no cut on quark pT)
• quarklev_bjet_pT_double_quark_5GeV : pT of bjets containing at least two b quarks (quark pT>5GeV)
• quarklev_bjet_bb_over_b_quarks : ratio of jets with >=2/==1 matched b quarks, vs jet pT
• quarklev_bjet_bb_over_b_quarks_5GeV : ratio of jets with >=2/==1 matched b-quarks with pT>5GeV, vs jet pT.

If the full particle level is present in the events, the routine will also make the same plots at the hadron level (titles are “hadlev_…”), as well as the standard plots from the Z+b ATLAS analysis (with data comparison).

### CMS Rivet routine:

Cross-section and angular correlations in $Z$ boson with $b$-hadrons events at $\sqrt{s} = 7$ TeV arxiv:1310.1349

Center-of-mass energy: 7 TeV

Run the analysis CMS_2013_I1256943 over Z→ee and Z→mumu events. The relevant files have been uploaded for reference, but the routine is available as standard analysis in Rivet. Events with exactly two $b$-hadrons and an $e^+e^-$ or $\mu^+\mu^-$ lepton pair from the Z boson decay are selected. Each lepton has $p_T > 20$ GeV and $|\eta| < 2.4$ and the dilepton invariant mass is $81 < M_{\ell\ell} < 101$ GeV. The $b$-hadrons have $p_T > 15$ GeV and $|\eta| < 2$. The differential cross sections are measured for $p_T > 0$ and $p_T > 50$ GeV.

The analysis works only if full particle level is present in the events, because weakly decaying b-hadrons are used (no b-hadron daughters), without clustering jets. Which permitting the study of $b$-hadron pair production at small angular separation.

The routine will make the standard plots of the Z+BB CMS analysis with data comparison.

List of plots:

• _h_min_ZpT: total cross section for different Z boson $p_T$ thresholds

Histograms in non-boosted region of $Z$ $p_T$

• _h_dR_BB: Differential cross section as a function of the angular variable $\Delta R_{\mathrm{BB}}$
• _h_dphi_BB: Differential cross section as a function of the angular variable $\Delta \phi_{\mathrm{BB}}$
• _h_min_dR_ZB: Differential cross section as a function of the angular variable $\mathrm{min}\Delta R_{\mathrm{ZB}}$
• _h_A_ZBB:Differential cross section as a function of the angular variable $\mathrm{A}_{\mathrm{ZBB}}$

Histograms in boosted region of $Z$ $p_T$ ($p_T > 50$ GeV)

• _h_dR_BB_boost: Differential cross section as a function of the angular variable $\Delta R_{\mathrm{BB}}$
• _h_dphi_BB_boost: Differential cross section as a function of the angular variable $\Delta \phi_{\mathrm{BB}}$
• _h_min_dR_ZB_boost: Differential cross section as a function of the angular variable $\mathrm{min}\Delta R_{\mathrm{ZB}}$
• _h_A_ZBB_boost: Differential cross section as a function of the angular variable $\mathrm{A}_{\mathrm{ZBB}}$

### Ongoing work and planned contributions:

#### Sherpa (Davide, Frank)

Samples:

• 4F MC@NLO (pp→Zbb)
• 5F MEPS@LO (pp→Z + pp→Zj+ pp→ Zjj + pp → Z jjj) all processes at LO,
• 5F MEPS@NLO (pp→Z + pp→Zj+ pp→ Zjj) MC@NLO + (pp → Z jjj) at LO.

Notes:

• For the 5F_LO scheme different choices of mass treatment in the ME
• PDF variations: NNPDF30, CT14, MMHT14 (aS(mZ)=0.118)
• both parton and particle level
• Possibly Wbb too if enough time

4F Scheme Set Up:

• b-Mass = 4.72 GeV
• Z-Mass = 91.1876 GeV
• PDF_Set = NNPDF30_nlo_as_0118_nf_4
• alphaS(mZ) = 0.118
• Events = 400M (Weighted)

5F Scheme Set Up: (Both MEPS@LO and MEPS@NLO)

• PDF_Set = NNPDF30_nnlo_as_0118
• alphaS(mZ) = 0.118
• QCUT merging = 10.0 GeV
• Events = 200M (LO) ~ 1B (NLO) (Weighted)

Preliminary Plots (may still change):

#### Powheg (Gionata, Carlo)

• W+bb and W+bb+j

4F scheme set up + matching with alphas and PDF with 5 flavours

• b-Mass = 4.75 GeV
• Z-Mass = 91.1876 GeV
• W-Width = 2.1054 GeV
• CKM
1. Vud=0.97428
2. Vus=0.2253
3. Vub=0.00347
4. Vcd=0.2252
5. Vcs=0.97345
6. Vcb=0.0410
7. Vtd=0.00862
8. Vts=0.0403
9. Vtb=0.999152
• PDF_Set = MSTW2008 (21100 in the LHE numbering scheme)
• Results for W+bb+j:

#### Herwig (Graeme, Christian)

Preliminary results for Zbb production in the 5FS can be found on http://www.hep.man.ac.uk/u/graeme/public/LH_Vbb/preliminary/Zbb/

• Shown are the four matching-shower combinations of Herwig7, i.e.
1. NLO$\oplus$QTilde aka MCatNLO-based matching with the angular ordered shower,
2. NLO$\oplus$Dipole aka MCatNLO-based matching with the dipole shower,
3. NLO$\otimes$QTilde aka Powheg-based matching with the angular ordered shower,
4. NLO$\otimes$Dipole aka Powheg-based matching with the dipole shower.
• The uncertainty bands are from scale variations with respect to all scales in the hard sub-processes and in the shower, i.e. muR, muF (and the corresponding scales of alpha_s and pdf arguments in the shower) and muQ (shower start scale).
• The central scale choice is a fixed scale (Z mass) at the moment for muR=muF=muQ.
• The PDF sets being used are the default PDF sets to which the showers are tuned, i.e. MMHT2014lo68cl and MMHT2014nlo68cl.
• Statistics: 40k, unweighted.

Preliminary results for Zb production in the 5FS can be found on http://www.hep.man.ac.uk/u/graeme/public/LH_Vbb/preliminary/Zb/

• Description: see 1. through 4. above.
• Statistics: 100k, unweighted.

Preliminary results for Zbb production in the 4FS can be found on http://www.itp.kit.edu/~reuschle/.Plots_2016_LHVbb_Preliminary/Zbb4FS/plots_NNPDFnf4_BnotinJ_0Jet_MCatNLO-DefaultShower_WeightedOn_Central/

• Shown is the NLO$\oplus$QTilde aka MCatNLO-based matching with the angular ordered shower.
• No scale variations.
• The central scale choice is a fixed scale (Z mass) at the moment for muR=muF=muQ.
• The PDF set being used is NNPDF30_nlo_as_0118_nf_4.
• Statistics: 1M, weighted.

Preliminary results for Wbb production in the 4FS can be found on http://www.itp.kit.edu/~reuschle/.Plots_2016_LHVbb_Preliminary/Wbb4FS/plots_AllPDFs_BinJ_1Jet_MCatNLO-DefaultShower_WeightedOff_Central/ or similarly for on http://www.itp.kit.edu/~reuschle/.Plots_2016_LHVbb_Preliminary/Wbb4FS/plots_AllPDFs_BnotinJ_0Jet_MCatNLO-DefaultShower_WeightedOff_Central/

• The difference between the two is that for one we require one jet (which may contain a b quark) with a certain min pT (first link), for the other we don't cut on the b quarks at all (second link).
• Shown is the NLO$\oplus$QTilde aka MCatNLO-based matching with the angular ordered shower.
• No scale variations.
• The central scale choice is a fixed scale (W mass) at the moment for muR=muF=muQ.
• There is a choice of different PDF sets being used, i.e.
1. MMHTnf5: MMHT2014lo68cl and MMHT2014nlo68cl,
2. MMHTnf4: MMHT2014nlo68cl_nf4,
3. NNPDFnf5: NNPDF30_lo_as_0118 and NNPDF30_nlo_as_0118,
4. NNPDFnf4: NNPDF30_lo_as_0118_nf_4 and NNPDF30_nlo_as_0118_nf_4.
• Statistics: 100k, unweighted.

The remaining parts of the description of the run setups can be found in the vbb.pdf file in the draft for the proceedings.

### github instructions

1. go to github.com and get an account
2. In the top-right corner of the page, click Fork
3. open a shell where git is available
4. clone the repository with the command: git clone git@github.com:your_git_username/leshouches_vbb.git
1. if you have problems with ssh publickey try: git clone https://github.com/your_git_username/leshouches_vbb.git
5. enjoy

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