Observation of Z+b, Z → ee, μμ, with CMS at √s = 7 TeV

EWK-10-015

Abstract

Events coming from the associated production of the Z boson with at least one bottom-quark jet are studied using proton-proton collisions delivered by the LHC at a centre-of-mass energy of 7 TeV and recorded by the CMS detector in 2010. The selection of a final state with two leptons, electrons or muons, from the Z, and an additional b-tagged jet is described. The event yield and shapes of kinematic variables in 36 inverse-picobarns of data recorded by CMS using single lepton triggers are compared with Monte Carlo predictions from MadGraph. In the data sample, 29 (36) events pass the dielectron (dimuon) and high-purity based b-tagging selections, with a measured purity of 0.83 0.10 of Z+b events. For the same b-tagging selection, the ratio σ(Z+b)/σ(Z+jet) is found to be 0.054 0.016 (0.046 0.014) in the data for the Z → ee (Z → μμ) selection, compared to 0.043 0.005 (0.047 0.005) estimated from NLO theory predictions with the same kinematic selections on the leptons and jets than the selections applied to the data.

Approved plots

Click on plots to get PDF versions

The data are represented by black dots with error bars (statistical uncertainties). The signal Z+b(b) component is shown as a red-filled histogram.Plots are shown for both high efficiency and high purity simple-secondary vertex b-tagging discriminant selections before and after b-tagging.

Control distributions before b-tagging

bestzmassEle.png bestzmassMu.png

Invariant mass of the electron and muon pairs at the Z+jet level, before b-tagging

jet1pt.png jet1eta.png

p_T and &eta of the leading jet at the Z+jet level, before b-tagging

bTagDiscr_HE_Zjsel.png bTagDiscr_HP_Zjsel.png

Distribution of the output variable of the high-efficiency and high-purity b-tagging discriminant used in this analysis, before b-tagging. The fraction shown on the plot represents the fraction of events with a secondary vertex in the data.

Distributions after b-tagging

bestzmassHE.png bestzmassHP.png

Dilepton invariant mass distribution in the range 60 to 120 GeV

bjet1pt_ZbHPsel.png

Transerse momentum of the leading b-tagged jet

dphiZbj1.png

Angle [rad] between the directions of the Z-candidate and the leading b-tagged jet transverse momenta.

Extraction of the ratio σ(Z+b)/σ(Z+j)

vtxfit_HE.png vtxfit_HP.png

Template likelihood fit to the mass of the secondary vertex to extract the purity. Final results for the purity are obtained by averaging the results of the fit of the secondary vertex mass and of the b-tag discriminant.

Ratio R =σ(pp→Z+b+X)/σ(pp→Z+j+X) extracted from data, and expected from NLO theory (MCFM) and MADGRAPH+PYTHIA samples. Due to different distributions in pT and eta of the leptons in Z+b and Z+j events, the ratio is dependent on the lepton selection and acceptance cuts should be quoted together with the ratio results.

Ratio is obtained for pT(jet)>25 GeV and η(jet)<2.1.

MCFM ratio : exactly same acceptance cuts on leptons and jets as data selection.

Madgraph ratio: ratio of events selected in MC at reco level, normalising Z+b sample to MCFM NLO cross-section for Z+b process, and Z+j sample to inclusive NNLO cross-section.

Sample R(Z→ee) (%), pT(ele) > 25 GeV, η(ele) <2.5 R(Z→μμ) (%), pT(μ) > 20 GeV, η(μ) < 2.1
Data HE 4.3 +/- 0.6 (stat) +/- 1.1 (syst) 5.1 +/- 0.6 (stat) +/- 1.3 (syst)
Data HP 5.4 +/- 1.0 (stat) +/- 1.2 (syst) 4.6 +/- 0.8 (stat) +/- 1.1 (syst)
MADGRAPH 5.1 +/- 0.2 (stat) +/- 0.2 (syst) +/- 0.6 (theory) 5.3 +/- 0.1 (stat) +/- 0.2 (syst) +/- 0.6 (theory)
MCFM 4.3 +/- 0.5 (theory) 4.7 +/- 0.5 (theory)

Approved Event Displays

completeView_White_148031_517672013_656.png

Event display of a typical well-balanced Z+b event, Z→μμ

completeView_White_149003_47711105_97.png

Event display of one of the 2 remaining events with 2 high-purity b-tagged jets, Z+b+b, Z→μμ

completeView_White_148029_236121538_307.png

Event display of the second of the 2 remaining events with 2 high-purity b-tagged jets, Z+b+b, Z→ee

-- GautierHamelDeMonchenault - 20-Mar-2011

Topic attachments
I Attachment History Action Size Date Who Comment
PDFpdf bTagDiscr_HE_Zjsel.pdf r2 r1 manage 34.3 K 2011-03-21 - 11:06 AnnemarieMagnan  
PNGpng bTagDiscr_HE_Zjsel.png r2 r1 manage 31.0 K 2011-03-22 - 15:07 ChristopheDelaere  
PDFpdf bTagDiscr_HP_Zjsel.pdf r1 manage 15.1 K 2011-03-21 - 10:58 AnnemarieMagnan  
PNGpng bTagDiscr_HP_Zjsel.png r3 r2 r1 manage 29.6 K 2011-03-22 - 15:07 ChristopheDelaere  
PDFpdf bestzmassEle.pdf r1 manage 17.9 K 2011-03-21 - 10:57 AnnemarieMagnan  
PNGpng bestzmassEle.png r2 r1 manage 35.6 K 2011-03-22 - 15:08 ChristopheDelaere  
PDFpdf bestzmassHE.pdf r1 manage 16.6 K 2011-03-21 - 10:57 AnnemarieMagnan  
PNGpng bestzmassHE.png r2 r1 manage 34.6 K 2011-03-22 - 15:11 ChristopheDelaere  
PDFpdf bestzmassHP.pdf r1 manage 15.9 K 2011-03-21 - 10:57 AnnemarieMagnan  
PNGpng bestzmassHP.png r3 r2 r1 manage 32.5 K 2011-03-22 - 15:12 ChristopheDelaere  
PDFpdf bestzmassMu.pdf r1 manage 17.9 K 2011-03-21 - 10:57 AnnemarieMagnan  
PNGpng bestzmassMu.png r2 r1 manage 36.2 K 2011-03-22 - 15:08 ChristopheDelaere  
PDFpdf bjet1pt_ZbHPsel.pdf r2 r1 manage 14.9 K 2011-03-21 - 10:58 AnnemarieMagnan  
PNGpng bjet1pt_ZbHPsel.png r3 r2 r1 manage 32.2 K 2011-03-22 - 15:09 ChristopheDelaere  
PNGpng completeView_Black_148029_236121538_307.png r1 manage 167.1 K 2011-03-21 - 10:58 AnnemarieMagnan  
PNGpng completeView_Black_148031_517672013_656.png r1 manage 132.6 K 2011-03-21 - 10:58 AnnemarieMagnan  
PNGpng completeView_Black_149003_47711105_97.png r1 manage 117.8 K 2011-03-21 - 10:59 AnnemarieMagnan  
PNGpng completeView_White_148029_236121538_307.png r1 manage 167.0 K 2011-03-21 - 10:59 AnnemarieMagnan  
PNGpng completeView_White_148031_517672013_656.png r1 manage 133.3 K 2011-03-21 - 10:59 AnnemarieMagnan  
PNGpng completeView_White_149003_47711105_97.png r1 manage 118.5 K 2011-03-21 - 11:26 AnnemarieMagnan  
PDFpdf dphiZbj1.pdf r1 manage 15.6 K 2011-03-21 - 11:00 AnnemarieMagnan  
PNGpng dphiZbj1.png r3 r2 r1 manage 32.9 K 2011-03-22 - 15:09 ChristopheDelaere  
PDFpdf jet1eta.pdf r1 manage 21.8 K 2011-03-21 - 11:00 AnnemarieMagnan  
PNGpng jet1eta.png r2 r1 manage 38.9 K 2011-03-22 - 15:09 ChristopheDelaere  
PDFpdf jet1pt.pdf r1 manage 17.5 K 2011-03-21 - 11:00 AnnemarieMagnan  
PNGpng jet1pt.png r2 r1 manage 36.6 K 2011-03-22 - 15:10 ChristopheDelaere  
PDFpdf vtxfit_HE.pdf r1 manage 15.9 K 2011-03-21 - 11:00 AnnemarieMagnan  
PNGpng vtxfit_HE.png r4 r3 r2 r1 manage 30.4 K 2011-03-22 - 15:22 ChristopheDelaere  
PDFpdf vtxfit_HP.pdf r2 r1 manage 14.8 K 2011-03-21 - 11:00 AnnemarieMagnan  
PNGpng vtxfit_HP.png r5 r4 r3 r2 r1 manage 25.5 K 2011-03-22 - 15:22 ChristopheDelaere  
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Topic revision: r5 - 2011-03-22 - ChristopheDelaere
 
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