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F. Ahmadov Joint Institute for Nuclear Research, Dubna , Russia

Search for the SM Higgs boson production in association with a W boson and decaying to the bb pair with the ATLAS. F. Ahmadov Joint Institute for Nuclear Research, Dubna , Russia Institute of Physics, ANAS, Baku, Azerbaijan 41th ITEP Winter School of Physics 12-19 February 2013.

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F. Ahmadov Joint Institute for Nuclear Research, Dubna , Russia

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  1. Search for the SM Higgs boson production in association with a W boson and decaying to the bb pair with the ATLAS • F. Ahmadov • Joint Institute for Nuclear Research, Dubna, Russia • Institute of Physics, ANAS, Baku, Azerbaijan • 41th ITEP Winter School of Physics • 12-19 February 2013

  2. Outline • The ATLAS Detector • Standard Model Higgs Boson • Higgs boson production and decays • Cut-flow analysis • Results from Monte-Carlo and • experimental data • Conclusion

  3. The ATLAS Detector

  4. The track of Particles In ATLAS Detector Tracking detector Hadronic calorimeter Muon chamber Photon Electron or positron Muons π± or protons neutron

  5. SM Higgs Boson The search for the SM Higgs boson is a central component of the physics program at the LHC. Recently, the observation of a boson decaying to a pair of photons or massive vector bosons has been reported by the ATLAS and CMS collaborations. Determining the nature of this boson - whether it is indeed the SM Higgs boson - is now one of the most important questions in particle physics. An SM Higgs boson with a mass of about 125 GeV would be accessible in both bosonic and fermionic decay channels at the LHC. Observing the decay into fermions is vital in testing whether the new boson is compatible with a SM Higgs boson. In particular, the decay to b-quarks plays an important role since this is expected to be the dominant decay mode at this mass (BR(H → b¯b) ≈ 58%). Therefore an observation in this channel is crucial in order to provide a direct constraint on the largest decay mode.

  6. Higgs Boson Production

  7. Higgs Boson Decay Higgs decay branching ratio (I) and production (II) channels

  8. pp→WH(bb) process Feynman diagram for associative production Higgs and W bosons

  9. Background processes • WH • W+jets, • WW, • WZ, • ttbar • and single top Distribution of the invariant mass of the Higgs candidate (ATL-PHYS-PUB-2009-088)

  10. Event selection • HFor Rejection: remove double counted events • Lar Error: remove events with error in LAr calorimeter • Triggers: for electrons pt with 24GeV isolated or w. 60GeV, • for muonspT with 24GeV & isolated or w. 36GeV • Vertex: require that the first primary vertex contain at least 3 tracks • MET cleaning: reject events where at least one jet with pT>20 GeV & |η|<4.5 is characterized as bad by the loose criteria • Pileup reweighting: Weight applied to MC • MC truth bug: reject MC events that have no truth particles

  11. Electron selection • Loose electrons: • Kinematics: |η|<2.47, ET > 10GeV, • Track iso: sum pT tracks/pT < 0.1 within cone ΔR=0.2, • Impact par: |d0|<0.1mm, • Other cuts: Author = 1 or 3, ID: loose++, Object cleaning: (el_OQ&1446)==0; OR loose electrons: OR(el-jet)- Loose withpT <15 GeV with ΔR<0.4 to a veto jet are removed, OR(el-mu)- Remaining (loose) electrons with ΔR<0.2 to a (loose) muon are removed • Signal electrons: OR loose electron and • Kinematics: ET > 25GeV • Caloiso: sum_ETcone30/ET < 0.14 • Other cuts: ID: tight++.

  12. Number of electrons after each cuts

  13. Muon selection (1) • Loose muons: • Combined Tight Muidmuon: • ID track cuts: pass MCP recommendation, • Kinematics: |η|<2.7, pT > 10GeV, • Track iso: sum pT tracks/pT < 0.1 within cone ΔR=0.2, • Impact par: |d0|<0.1mm & |z0|<10mm • Quality: Tight MuID; • Standalone muon: • Kinematics: 2.5<|η|<2.7, pT > 10GeV • Quality: Tight MuID; • Calomuon: • Author == 16, • Kinematics: |η|<0.1, pT > 20GeV, • Track iso: sum pT tracks/pT < 0.1 within cone ΔR=0.2, • Quality: CaloMuonIDTag> 10 OR CaloLRLikelihood> 0.9 • OR(cb_mu-calo_mu): Reject calo mu. if it is within ΔR<0.1 of a selected muidmuon candidate.

  14. Muon selection (2) • OR loose muons: OR(mu-jet) - Loose muons with ΔR<0.4 to a veto jet (after jet-e OR) are removed • Signal muons: OR loose muon and • ID: tight combined/segment tagged muid, • Kinematics: ET > 25GeV, |η|<2.5 • Calo iso: sum_ETcone30/ET < 0.14

  15. Number of muons after each cuts (1)

  16. Number of muons after each cuts (2)

  17. Jet selection • Veto jets: • Algorithm: AntiKt4TopoEMJets, • Kinematics: pT > 20GeV & |η|<2.5 or pT > 30GeV & 2.5<|η|<4.5 • Signal jets: • Kinematics: pT > 20GeV &|η|<2.5 OR loose electrons: OR(jet-el): Veto and Signal Jets with ΔR<0.4 to loose electron with pTelec >15 GeV are removed.

  18. Number of jets after each cuts

  19. lνbb selection • Single lepton selection: Exactly one signal lepton (as defined above) • Di-Lepton Mass (MT): Required transverse mass of W < 120GeV and >40 GeV (if pT of W is less than 160GeV) • Missing ET: MET>25GeV • Number of jets: Two selected (as defined above) jets. Requirement on leading jet pT>45GeV and ΔR>0.7 for the 2 selected jets (if W pT<200GeV) . Remove events with more • than 2 loose jets • B-tagging: Two b-tagged jets weight with 70% efficiency

  20. Invariant mass of two b-jets The signal shown is for mH = 125 GeV. The background expectation is shown after the profile likelihood fit (solid) and compared to the predictions from the pre-fit Monte Carlo simulation (dashed). The size of the combined statistical and systematic uncertainty is indicated by the hashed band.

  21. Higgs Boson Exclusion Plots Expected (dashed) and observed (solid line) exclusion limits for the VH→lνbbar channel expressed as the ratio to the Standard Model Higgs boson cross section, using the profile-likelihood method with CLs. The green and yellow areas represent the 1σ and 2σ ranges of the expectation in the absence of a signal.

  22. Local p0 value The local probability p0 for a background-only experiment to be more signal-like than the observation as a function of mh for various progressive cases of combinations: H->γγ (red line); H->ZZ*->llll (green line); combination of H-> γγ and H->ZZ*->llll (blue line); combination of H-> γγ, H->ZZ*->llll and H->WW*->lvlv (magenta line) and the combination of all channels, including H->bb and H->ττ (black line). The dashed black curve shows the median expected local p0 under the hypothesis of a SM Higgs boson production signal at that mass for the combination of all channels. The horizontal dashed lines indicate the p-values corresponding to significances of 0σ to 7σ.  

  23. Higgs boson candidate event

  24. Conclusion • The search with the ATLAS experiment for the SM Higgs boson produced in association with a W boson and decaying to bb using 13fb-1 at √s=8TeV conducted by Cut-flow‏ analysis. • No significant excess is observed. • For mH=125 GeV, the observed upper 95% confidence level limit on the cross section times the branching ratio is found to be 1.8 times the SM prediction. • Going on: • Look around for other useful variables • Continue analysis with full 2012 ATLAS data‏.

  25. THANK YOU FOR ATTENTION!

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