Search for the Standard Model Higgs Boson at D

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Search for the Standard Model Higgs Boson at DØ

• Michele Petteni
• Imperial College London
• On behalf of the DØ Collaboration

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Talk Outline

• What do we expect from the Standard Model?
• Higgs production and decay at the Tevatron
• Expectations for the Tevatron
• Brief description of DØ and the Tevatron
• Analysis channels
• Outlook

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The Higgs Boson

• Higgs boson needed for Electroweak symmetry
  breaking
• Allows for difference in W,Z and photon mass
• Generates masses for fermions
• Neutral, scalar particle

• Theoretical limits
• WL?WL scattering, mHlt 1 TeV
• Other limits depend on the scale of new physics

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Experimental Constraints

• Higgs couples to mass
• W and Top mass measurements constrain mass of
  Higgs via effect of radiative corrections.
• Global Electroweak fits give indication of Higgs
  mass
• Recently updated for new top mass measurement
  from Tevatron.
• Direct Searches
• This has not changed from LEP limit, 114.4 GeV

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Higgs Production and Decay
Excluded by LEP

• Two search strategies
• Below 135 GeV look for associated production with
  2 bjets from the Higgs decay.
• Above this limit exploit gluon fusion and
  reconstruct the two gauge bosons.
• Possibility of third strategy in the intermediate
  mass range?
• More on this later

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Why the Tevatron?

• Sensitivity studies performed
• Higgs Working Group Collaboration,hep-ph/0010338
• Tevatron Higgs Sensitivity Study Group,
  FERMILAB-PUB-03/320-E
• Studies have some inherent assumptions
• For low mass Higgs
• Excellent b-tagging to identify jets from the
  Higgs
• Lepton-id to identify the Z/W
• Jet resolution is essential to disentangle the
  mass peak
• For high mass Higgs
• Hinges around lepton identification

8 fb-1 for Run IIb

• Sensitivity studies highlight important facts
• Not one golden channel
• Combined results from both Tevatron experiments
• Evidence for light mass Higgs is feasible at the
  Tevatron

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The DØ Detector
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Operations Report
Peak luminosity gt 1.2x1032
Data used in analysis

• Luminosity progressing steadily and in a regular
  manner
• 1 fb-1 delivered
• DØ operating well and is stable
• Analysis presented taken with data up to latest
  shutdown

Stable at 88 efficiency
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Experimental Results

• Low mass Higgs
• Z/? (? ee-) n jets x-section
• WH ? e ? b b
• ZH ? ? ? b b
• Intermediate mass Higgs
• WH ? WWW
• High Mass Higgs
• H ? WW

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Z/? (? ee-) n jets

• Z/W n jets main background to light mass higgs
  searches
• Main cuts
• Two leading electrons pT gt 25 GeV, consistent
  with Z mass
• Jet pT gt 20 GeV, ? lt 2.5
• Results normalised to inclusive cross-section
• Jet multiplicity figure compared with NLO MC and
  to matrix-element - parton shower treatment
  (ME-PS)
• Jet pT spectrum compared to Alpgen Pythia
  normalised to data

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Cross-section Ratios

• Next step measure Zn bjet cross-sections.
• Higgs limit

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WH Searches W(?e?)jets

• Results are an update on the analysis published
  in PRL 94, 091802(2005).
• Includes data shown in the PRL paper (174 pb-1)
• WH process has a higher x-section than ZH and
  benefits as only one lepton to identify
• Main backgrounds are Wjj (before b-tagging) and
  subsequently tt, single top, and WZ and the ever
  present QCD

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WH Searches W(?e?)jets (2)

• Main cuts
• Isolated electron, pT gt 20 GeV, ? lt 1.1
• Missing ET gt 25
• Two jets, pT gt 20 GeV, ? lt 2.5
• After basic selections 3844 events, expect 3256
  W/Zjets events
• Good agreement with MC

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WH Searches W(?e?)jets (3)

• After b-tagging see 13 events expect 10.2
• B-tagging efficiency major source of uncertainty
• Set limit using a mass window for 4 Higgs mass
  values

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ZH??? bb

• Due to the large branching ratio for Z decay this
  channel has about the same contribution as all
  the W channels combined
• However
• No easy handle to trigger the events
• trigger on missing ET from the jets and the
  presence of jets
• Highly sensitive to any mis-measurements
• Significant QCD background
• large cross-section
• difficult to estimate
• Main non-QCD backgrounds are Zbb, ZZ, ZW, Wbb and
  top (after b-tagging)

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Topology Cuts

• Require two acoplanar jets, missing ET and veto
  any isolated leptons
• In order to reduce the QCD/instrumental
  background, exploit correlation between missing
  energy from calorimeter cells, jets and tracks
• They should be aligned
• Form asymmetries and cut on this value (peaks at
  0 for signal)
• Cut on angles between jets and missing ET

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Estimation of Instrumental Background
sideband
sideband
Physics bkgd. from MC
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ZH??? bb Results

• Expect 2125, see 2140 before b-tagging.
• After b-tagging observe 9 expect 6.4
• Apply 50 GeV mass window
• Signal acceptance 0.3
• Zbb and Wbb major backgrounds
• Instrumental half of these (same as top)

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WH?WWW?l?l?qq

• Exploits like-sign leptons in final state
• Low backgrounds
• Clear signal
• Require 2 like sign leptons (veto 3rd)
• Missing ET gt 20 GeV
• Main backgrounds
• Physics, WZ?l?ll
• Charge flips from Z?ll, WW?l?l?,tt?ll, etc.
• Instrumental from real like sign leptons from
  semileptonic decays, ? conversions...

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WH?WWW?l?l?qq (2)
ee e? ??
Observed 1 3 2
WZ 0.430.03 0.330.03 0.160.03
Charge Flips 0.200.06 0.050.01 3.400.73
W/QCD 0.070.04 3.940.23 0.160.18
Total 0.700.08 4.320.23 3.720.75

• Different signals have different background
  contributions
• Observed limit covers nicely the intermediate
  Higgs mass region

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H?WW?l?l?

• Look in ee, ?? and e? channels
• Require
• Two opposite sign leptons
• Missing ET gt 20 GeV
• Cut out events with potentially large
  contribution to the missing energy from jet
  mis-measurement
• Veto low mass resonances and reduce Z contribution

Excellent agreement after basic cuts
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WH?WW?l?l? (2)

• In order to further suppress background use
  angular correlations
• Spin of Ws are correlated
• Leptons from Higgs are collinear, back-to-back
  for background
• Cut on opening angle between ll
• Mass dependent cuts

Selection acceptance varies from 16.7 to
3.9 Combine all channels using likelihood
functions Numbers of events depends on Higgs
mass, for mH 160 GeV expect 17.71.0 and see 20.
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Summary

• Preliminary results look good
• Good description of background processes and the
  understanding of environment has improved
  dramatically
• Analysis (always) need more work, striving to
  reach sensitivity outlined by sensitivity study
• Need advanced analysis techniques
• Tevatron has already delivered 1 fb-1 (0.8 fb-1
  on tape), by the end of the year we will have 1
  fb-1 to tape
• With this luminosity getting close to excluding
  at 95 a Higgs mass close to 115 GeV

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Backup Slide
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Comparison of WH published Results with
Sensitivity Prospective Study
Dijet mass window DØ Analysis (PRL 05) 174 pb-1 WH? e?bb 85,135 Prospective Study (03) normalized to 174 pb-1 and to WH? bbe? 100,136 Ratio Prospective DØ Analysis R0.72
Dijet mass resolution 14 /- 1 10 R0.71
Signal events (S) 0.049 0.145 R3.0
Background evts (B) 1. 07 1.76 R1.6
S/?B 0.045 0.11 R2.4
S/B 0.046 0.082 R1.8
We are currently missing a factor 2.4 in
sensitivity Prospective Study assumed Larger
ECAL coverage (30), improved em-id (40),
extended b-tag efficiency (50, 2 tags) and
30 less backgd (better dijet mass resolution) ?
Factor 2 in S/?B ? 2.4/ 2 1.2 difference
(only) in sensitivity Apply Advanced
techniques Missing Factors can be recovered