F. Ahmadov

Joint Institute for Nuclear Research, Dubna, RussiaInstitute of Physics, ANAS, Baku, Azerbaijan

41th ITEP Winter School of Physics12-19 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 analysisResults from Monte-Carlo and experimental data Conclusion

The ATLAS Detector

The track of ParticlesIn ATLAS Detector

Photon

Electron or positron

Muons

π± or protons

neutron

Trackingdetector

Hadroniccalorimeter

Muonchamber

SM Higgs BosonThe 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) andproduction (II) channels

pp→WH(bb) process

Feynman diagram for associative production Higgs and W bosons

Distribution of the invariant massof the Higgs candidate (ATL-PHYS-PUB-2009-088)

Background processes

WHW+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 selectionLoose 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 Calo iso: sum_ETcone30/ET < 0.14 Other cuts: ID: tight++.

Number of electrons after each cutsElectron Sel. CERN/

Ed.Liv/Brum Wisconsi

nBonn Dubna

No cut 334614 334614 334614 334614 334614Author 1 or 3 125600 125600 125600 125600 125600Loose++ 28646 28645 28646 28646 28646ET > 10 GeV 98799 98777 98740 98722 98709|Eta|< 2.47 228739 228739 228739 228739 228739OQ 229216 229216 229216 229216 229216Track iso 87888 87884 87879 87943 87894|D0| < 0.1 120915 120915 120915 120915 120915Loose Electrons

8276 8276 9400 8277 8276

Medium++ 13088 13088 13088 13088 13088ET > 25 GeV 34190 34188 34191 34170 34211Signal Electrons

6285 6283 6284 6282

Tight++ 9744 9744 9744 9744 9744Calo iso 28329 28313 28314 28329 28324Signal WH Electrons

5655 5655 5653 5656 5653

Loose muons: Combined Tight Muid muon: 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; Calo muon: 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 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: ET > 25GeV, |η|<2.5Calo iso: sum_ETcone30/ET < 0.14

Number of muons after each cuts (1)Muon Selection CERN/

Ed.Liv/Brum Wisconsin Bonn Dubna

No cuts but |eta|<2.7

32934 32934 32934 32934 32934

Tight MuID Muons 22331 23150 22331 22331 22331Muons in the muid cont. with an ID track

30424 30521 30424 30424 30424

Blayer Hits 29555 29651 29555 29555 29555Pixel Hits 29759 29856 29759 29759 29759SCT Hits 30112 30113 30112 30112 30112Silicon Holes 30424 30521 30424 30424 30424TRT Hits 29955 30052 29955 29955 29955MCP Hits Cuts 28266 28267 28266 28266 28266pT > 10 GeV 13517 13576 13521 13528 13524|Eta| < 2.7 29819 30005 29998 30424 29998|D0| < 0.1mm 19933 19968 19933 19933 19933|Z0| < 10mm 24065 24138 24065 24065 24065Track iso 18067 18562 18485 18483 18484MuID Muons 9348 9696 9288 9350 9286

Number of muons after each cuts (2)Muon Selection CERN/

Ed.Liv/Brum Wisconsin Bonn Dubna

Standalone muon 2510 2413 2510 2510 2510Standalone tight muid

819 819 819 819

2.5 < |eta| < 2.7 799 709 423 799 799pT > 10 GeV 424 371 799 424 423Standalone Muons 250 198 249 250 249Author 6/16 24939 24939 24939 24939 24939Hit cuts 25727 25727 25727 25727 25885pT > 20 GeV 10394 10396 10396 10394 10396|eta| < 0.1 1260 1260 1260 1260 1260Quality 27535 27535 27535 27535 27535OR(calo-selec) 22101 22156 22156Track iso 23939 23944 23944 23939 23944Calo Muons 179 143 179 179 179Loose Muons 9777 10083 9716 9779 9714pT > 25 GeV 8712 8705 8705 8712Signal ZH Muons 7303 7416 7414 7303 7258Calo iso 15584 15584 15582 15584Signal WH Muons 7159 7113 7159 7114

Veto jets: Algorithm: AntiKt4TopoEMJets, Kinematics: pT > 20GeV & |η|<2.5 or

pT > 30GeV & 2.5<|η|<4.5 Signal jets: Kinematics: pT > 20GeV &|η|<2.5OR loose electrons: OR(jet-el): Veto and Signal Jets with ΔR<0.4 to

loose electron with pT

elec >15 GeV are removed.

Jet selection

Number of jets after each cutsJet Selection CERN/Ed. Liv/Brum Wisconsi

nBonn Dubna

Initial 198474 198474 198474 198474

198474

E > 0 198474 198474 198474 198474Not Bad 198229 198229 198229 19822

9198229

pT > 20 & |eta|<2.5 orpT >30 & 2.5<|eta|<4.5

99576 99639 99576 101154

99629

|eta|<4.5 195467 195467 195467 195467

195522

|JVF|<0.5 130283 130325 130283 130283

130283

Loose Jets 80753 80804 80753 81380 78958|eta|<2.5 159952 159952 159952 15995

2160188

Signal Jets 75174 75215 75174 75705 73718B-tagged Signal Jets

29655 29659 29655 29745 29443

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

lνbb selection

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.

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 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.  

Local p0 value

Higgs boson candidate event

ConclusionThe 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 variablesContinue analysis with full 2012 ATLAS

data.

THANK YOUFOR ATTENTION!