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

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

The ATLAS Detector

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

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.

Higgs Boson Production

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

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

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

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 muons pT 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

Electron selection Loose electrons:  Kinematics: |η| 10GeV,  Track iso: sum pT tracks/pT < 0.1 within cone ΔR=0.2,  Impact par: |d 0 |<0.1mm,  Other cuts: Author = 1 or 3, ID: loose++, Object cleaning: (el_OQ&1446)==0; OR loose electrons: OR(el-jet)- Loose withp T <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: E T > 25GeV  Calo iso: sum_ETcone30/ET < 0.14  Other cuts: ID: tight++.

Number of electrons after each cuts Electron Sel.CERN/Ed.Liv/BrumWisconsinBonnDubna No cut Author 1 or Loose ET > 10 GeV |Eta|< OQ Track iso |D0| < Loose Electrons Medium ET > 25 GeV Signal Electrons Tight Calo iso Signal WH Electrons

Loose muons:  Combined Tight Muid muon:  ID track cuts: pass MCP recommendation,  Kinematics: |η| 10GeV,  Track iso: sum pT tracks/pT < 0.1 within cone ΔR=0.2,  Impact par: |d 0 |<0.1mm & |z 0 |<10mm  Quality: Tight MuID;  Standalone muon:  Kinematics: GeV  Quality: Tight MuID;  Calo muon:  Author == 16,  Kinematics: |η| 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 muid muon candidate. Muon selection (1)

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: E T > 25GeV, |η|<2.5  Calo iso: sum_ETcone30/ET < 0.14

Number of muons after each cuts (1) Muon SelectionCERN/Ed.Liv/BrumWisconsinBonnDubna No cuts but |eta|< Tight MuID Muons Muons in the muid cont. with an ID track Blayer Hits Pixel Hits SCT Hits Silicon Holes TRT Hits MCP Hits Cuts pT > 10 GeV |Eta| < |D0| < 0.1mm |Z0| < 10mm Track iso MuID Muons

Number of muons after each cuts (2) Muon SelectionCERN/Ed.Liv/BrumWisconsinBonnDubna Standalone muon Standalone tight muid < |eta| < pT > 10 GeV Standalone Muons Author 6/ Hit cuts pT > 20 GeV |eta| < Quality27535 OR(calo-selec) Track iso Calo Muons Loose Muons pT > 25 GeV Signal ZH Muons Calo iso Signal WH Muons

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

Number of jets after each cuts Jet SelectionCERN/Ed.Liv/BrumWisconsinBonnDubna Initial E > Not Bad pT > 20 & |eta|<2.5 or pT >30 & 2.5<|eta|< |eta|< |JVF|< Loose Jets |eta|< Signal Jets B-tagged Signal Jets

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

Invariant mass of two b-jets The signal shown is for m H = 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.

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.

The local probability p0 for a background-only experiment to be more signal-like than the observation as a function of m h 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. Local p0 value

Higgs boson candidate event

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 ‏.

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