Diapositive 1

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Associated SM Higgs searchin H?bb,WW() final
states _at_ ATLAS
Huaqiao Zhang IHEP/CPPM On Behalf of the ATLAS
experiment
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Outline

• Introduction
• ttH,H?bb analysis
• Signature and preselection
• Event reconstruction and methods comparison
• Systematics and results
• ttH,H?WW() analysis
• Signature and event selection
• Systematics and results
• WH,H?WW() analysis
• Signature and event selection
• Systematics and results
• Summary

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SM Higgs searching in experiments
http//lepewwg.web.cern.ch/LEPEWWG/

• Higgs particles is the only particle that
  predicted in SM but not found yet experimentally
• LEP direct search exclude light Higgs below 114
  GeV
• With recent Tevatron results, SM electroweak fit
  prefers Higgs less than 190 GeV (including LEP)
  and 160 GeV without.
• Higgs Coupling to top quarks and W boson are
  important properties of Higgs

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The SM Higgs at the LHC

• Associate production
• Possible discovery channel ttH,H?bb
• Coupling measurement for Higgs mass 120, 200
  GeV gt SM Higgs or not
• Decay modes
• For mH gt 135 GeV/c2 H ? WW() dominates (BR
  0.91 _at_ 160 GeV/c2 )
• H?bb is the main channel for mH lt 135 GeV/c2 (BR
  0.68 _at_ 120 GeV/c2 )

Difficult for this mass region we need to
associate more than one channel
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The LHC and ATLAS

• LHC
• Proton-Proton collisions _at_ 14 TeV
• First run _at_ 10 TeV expected at the end of this
  summer
• Luminosity
• Low luminosity regime 1033cm-2s-1
• 30 fb-1 between 2008 and 2011
• High Luminosity regime 1034cm-2s-1
• 300 fb-1 by 2014/2015

• ATLAS
• General purpose experiments
• Classic detectors composed mainly by 3
  sub-systems
• Inner tracker
• Calorimeter system
• Muon spectrometer
• Very good Trigger/DAQ System
• Good e/g/m/tau/missEt/b-jets identification

ATLAS detector
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ttH,H?bb Signature and preselection

• ttH,H?bb potential discovery channel for light
  Higgs boson, top quark Yukawa Coupling
  measurement
• H?bb, t ? bjj, t ? blv??
• t ? blv, t ? blv?
• t ? bjj, t ? bjj
• Main backgrounds
• ttjets reducible background using b-tagging
• ttbb (EW/QCD) irreducible background
• Strategy Reconstruct the tt system to look at
  the rest 2 b-jets at the final state
• b-tagging is one of the most important keys for
  this channel
• Jet pairing and JES are also very important
• Efforts mainly for lepton jets channel
• Isolated electron or muon trigger
• One isolated lepton is required, Tau is not
  considered
• Electron, muon Acceptance,
• identification and isolation cuts
• At least 6 jets, of which 4
• tagged as b-jets

LO cross section (only ttjets NLONLL)
cut ttH ttbb(EW) ttbb(QCD) ttjets
1 Lepton (fb) 56.9 141 1356 63710
6 jets (fb) 36.2 76.7 665 26214
4 b-jets loose (fb) 16.2 23.4 198 2589
4 b-jets tight (fb) 3.76 4.2 29.6 50.7
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Cut-Based Reconstruction
Invariant mass of reconstructed hadronic W
boson (red for good combination)

• Leptonic W reconstruction
• Force ln mass to the W mass
• Solve 2nd degree equation to get neutrino pz
• 28 no solution ? neglect imaginary part

ATLAS preliminary

• Hadronic W reconstruction
• After requesting 4 b-tag jets, remaining jets
  are considered as light jets
• W boson candidates are formed of all combinations
  of light jet pairs

Invariant mass of reconstructed top quarks (red
for good combination)

• top quarks candidates are formed using one W
  boson candidate and one b quark
• Combination with m(jj) m(W) gt 25 GeV/c2 or
  m(treco) m(ttrue) gt 25 GeV/c2 are removed
• Combination minimizing a c2, based on the top
  quark masses, is chosen
• The two remaining b jets used to reconstruct the
  Higgs candidates

ATLAS preliminary
ATLAS preliminary
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Multivariate Based Reconstruction

• Pairing likelihood
• Using tt system kinematic properties to build a
  pairing likelihood
• 6 variables are used
• b-jets and light jets are treated separately
• Choose the combination that maximize the
  likelihood output

ATLAS preliminary
Invariant mass of reconstructed Higgs boson using
likelihood for signal sample (red for good
combination)

• Constrained fit
• Fit jet pT and ETmiss to give the mass of the top
  quarks
• Pairing likelihood is formed using the c2 output
  of the constrained fit together with b-tagging
  and kinematic quantities
• 14 variables are used
• 3D likelihood is used to take into account the
  correlations
• Final selection likelihood is used to separate
  signal and physics background

f ijet scale factor for jet momentum
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Comparison of Reconstruction Algorithms

• S/sqrt(B) differences relatively small among the
  3 methods
• Multivariate techniques increase Higgs bb pair
  purity by 5
• Analyses suffer from low purity mainly coming
  from b exchange between top and Higgs
• Wide distribution for reconstructed Higgs mass
• No clear signal peak on top of background
  distribution
• Combinatorial background dilutes differences
  between ttbb and ttH sig

ATLAS preliminary
ATLAS preliminary
Invariant mass of reconstructed Higgs boson using
constrained fit for all samples, a cut at -4.2 in
Ls/b for better stat. significance
90 lt mH lt 150 GeV/c2
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Systematic uncertainties and results _at_ 30 fb-1
main systematic errors, background sample

• Main systematic errors come from the JES, jet
  resolution and b-tagging efficiency
• Theoretical errors on background cross sections,
  especially top anti-top production, are sizable

Cut-based Likelihood Constrained fit
JES 5 14 8
Jet resolution 7 5.5 14
b-tagging efficiency 20 20 20
Light jet rejection 5 3 10
All contribution 22 25 28
ATLAS preliminary
S/sqrt(BdB2)

• Systematic errors on background higher than
  signal
• If nothing is done for the background extraction,
  the significance will decrease to 0.5
  (mH120GeV/c2)

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ttH,H?WW() Signatures

• ttH?2b4j2l2Vl
• ?6jets 2samecharge lep. MissEt
• Possible BKGs
• tt(1l), tt(2l), ttZ, ttW, tttt, ttWW, ttbb...
• ttH?2b2j3l3Vl
• ?4jets 3leptons MissEt
• Possible BKGs
• tt(2l),ttZ,ttW,tttt,ttWW,ttbb
• tau not considered in signal
• Complex final statesgtNumber counting experiment
• At least 2 neutrinos
• Multi jets Multi leptons MissEt
• Background control important
• Main BKG tt suppressed by lepton
  isolationgtLepton isolation is crucial
• ttZ suppressed by Z mass veto
• QCD BKGs neglectable

gt
b
P
2 leptons
4 light jets
P
b
b
P
3 leptons
2 light jets
P
b
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ttH,H?WW() events selection

• One high Pt Isolated lepton trigger
• Eff 82 (2L),91 (3L)
• offline significance impact lt 1
• Phys. Obj. kinematic region
• ?lt2.5 Ptgt15.GeV
• Electron identification
• Calorimeter seeded Alg.
• Matching to ID track
• Muon identification
• Seeded in Muon Spectrometer
• Combined with ID track
• Jet identification cone based, size 0.4
• Lepton Isolation
• Electron
• Muon
• Number of leptons gt 2/3, Number of jets gt 6/4
• Final tighten cut
• Zveto, MuonPtgt20GeV, Number of lepton2/3,
  sameCharged/null

fb sNLO 2L sel. 3L sel.
ttH2l 11.1 1.870.03 -
ttH3l 7.2 0.240.01 0.820.02
ttbar 833000 7.41.1 2.062.06
ttWnj 226 2.040.07 0.550.04
ttZ 1440 1.490.09 1.120.08
ttbb 2693 0.550.18 -
tttt 3.2 0.070.01 -
Eff70.5
Eff92.7
Cone Isolation
Tracker Isolation
Calorimeter Isolation
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Systematic uncertainties and results _at_ 30 fb-1
ATLAS preliminary

• Accuracy of sttH BRH?WW Best at 160 GeV
  47.4, if 10 uncertainties of BKG
• Biggest uncertainties from JES and BKG
  (especially ttbar) normalization.
• Without systematics, the statistical accuracy
  can achieve 28
• Systematics dominate the accuracy

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Multivariable methods for improvement

• Likelihood method on isolation
• Projective likelihood estimator
• Only 3 isolation variables used

ATLAS preliminary

• Mass constrained fit in ttH2L analysis
• A system with 2 neutrinos (one virtual W at Higgs
  mass lt160 GeV)
• 11 free parameters
• 8 enter chi2,3 from neutrino,5 Windows to
  determine combinatory
• 11 constraints
• Only 4 equality, other 7 domains
• Extra 8 implicitly 0.5 lt cali_E lt 1.

ATLAS preliminary
ttH,H?WW() 2L signal
More work needed to improve
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WH,H?WW() Signature and preselection

• Higgs physics
• Supplement to the Discovery
• Necessary confirmation for the property
• Couplings to gauge boson
• WH,H?WW() physics analyses
• One High Pt isolated lepton trigger
• Two and three lepton final states (2L/3L)
• WH?WWW()?lv lv jj
• WH?WWW()?lv lv lv
• Full analysis over WH against ttbar, WZ, ttW,
  ZZ
• Number counting experiments

• Number of jets and leptons
• Same charge(2L), Zveto
• Missing Transverse Energy(3L)
• Lepton isolation
• Calorimeter isolation
• Tracker isolation
• Cone isolation
• Impact parameter
• B-jet/jet veto
• W mass range
• Higgs-Spin cut

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Preliminary results _at_ 30 fb-1
fb WH2L WH3L WZ tt ZZ ttW wbb W-gtlvjet
sNLO 31.2 5.04 750 833000 72.5 73.3 562 3.4106
2L selection 0.68 - 1.2 0.43 0.05 0.02 0.0006 0
3L selection - 0.28 0.14 0.28 0.01 0.003 0 0
Xection S/B (fb) 0.68/1.7 0.28/0.4
If 20 systematics WH2L WH3L Combined
Significance 1.6 1.9 2.4
67 64 47
Suffer from the statistics of MonteCarlo
samples , BKG Systematics dominated by JES
19(2L), 17(3L) H?WW() coupling measurement
need more/hard works
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Summary

• Studies on channels of ttH,H?bb, ttH,H?WW () and
  WH,H?WW () with ATLAS real detector geometry
  full simulation
• Coupling measurement
• ttH,H?bb (gt, Higgs120GeV), potential discovery
  channel
• ttH,H?WW () (gt, Higgs 160GeV)
• WH,H?WW () (gW, Higgs 170GeV)
• Number counting experiments gt BKG control is
  crucial
• Mass peak reconstruction
• possible in ttH,H?bb
• Hopeless in ttH,H?WW(), not yet tried in
  WH,H?WW()
• Background uncertainties
• JES(Btaging in ttH,H?bb) dominate the
  uncertainties of BKGs
• ttbar or ttjj normalization important
• Data driven method needed
• More advanced methods during 30fb-1 data
  gathering
• Statistical part of systematics reduced with
  30fb-1 data
• Still rooms for improvement of these analysis
• Good feasibilities, more efforts needed

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Bak up Main Issues for ttH,H?bb Channel

• After analysis Higgs purity is 30
• large tails and width in the bb invariant mass
• No visible peak on top of physical background,
  side band extraction very difficult
• The main problem is the b-jets exchange between
  top quarks and the Higgs
• b-jet from Higgs used for the hadronic top 36
• b-jet from Higgs used for the leptonic top 30
• Higgs boson reconstructed with only one correct
  b-jet 55 (The other wrong jet is mainly coming
  from the top quarks)
• Big uncertainty for the ttjets cross section
• Extracting the background shape and normalization
  in data is crucial for this channel
• Using loose and tight b-tagging cuts looks
  promising
• Background shape independent from b-tagging cuts

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Bak up Main Issues for ttH/WH,H?WW() Channel

• Higgs reconstruction difficult
• At least two neutrinos, one from Higgs W decay,
  one from top W decay
• With a virtual W, which could decay leptonic or
  hadronic
• Large combinatory in complex final states
• Lepton isolation is crucial in background
  suppression
• Several isolation methods are tried
• Could improve by Multivariable methods, with
  better understanding of real data
• Big uncertainty for the backgrounds cross section
  normalization
• Theoretical uncertainties could improve with
  understanding of real data
• Statistical part of systematics could be reduced
  with full 30fb-1 real data