Search for neutral MSSM Higgs bosons decaying to ? pairs in pp collisions at 1.96 TeV (CDF)

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Search for neutral MSSM Higgs bosons decaying to
? pairs in pp collisions at 1.96 TeV (CDF)

• Jessica Leonard
• Oct. 23, 2006
• Physics 835

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MSSM Higgs

• How do particles have mass?
• SM Higgs mechanism
• Problems with SM Higgs may be solved in MSSM
• Five Higgs in MSSM 2 charged, 3 neutral
• This paper deals with neutrals (?)
• 1 CP-odd A
• 2 CP-even h (lighter), H (heavier)
• MSSM parameters
• mA
• tan ? vu/vd, where vu, vd are vevs of neutral
  Higgs that couple to up- and down-type fermions

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MSSM Higgs

• For large tan b, one CP-even Higgs has similar
  mass, couplings to A
• Dominant ? production mechanisms
• Gluon fusion
• bb fusion
• Dominant decay modes for A, mass-degenerate
  CP-even state
• ? -gt bb (90)
• ? -gt ?? (10)

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The search at CDF

• Tevatron CM energy 1.96 TeV
• Data collected at CDF II between 2002-04
• Integrated luminosity 310 pb-1
• ? -gt ?? for 90 lt mA lt 250 GeV/c2
• One ? -gt lepton neutrinos (?e, ??)
• Other ? -gt hadrons neutrino (?had)

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CDF detector
?-ln(tan(?/2))
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Event selection

• For the ? -gt ?? search, each event must contain a
  lepton candidate (e or ?) and hadrons consistent
  with a ? decay.
• Pre-select with a lepton plus track condition
• Lepton and track must both point into central
  cal.
• Must have azimuthal separation of gt 10o
• e/? determination
• Electrons only deposit energy in EM calorimeter
• Muons seen in muon chambers outside cal.
• Momentum of neutrinos missing transverse energy

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Event selection

• Hadron reconstruction
• Energy from ? decays are deposited in a small,
  contiguous region
• Four-momentum of this jet determined from tracks
  of charged particles and energies of ?0s within
  signal cone
• Particles in a ring outside the signal cone must
  have very low transverse energy (lt 1 GeV total).
  This isolation cuts out quark and gluon jets.

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Event selection

• Hadron reconstruction
• The jets must have 1 or 3 tracks (charged
  particles), and sum of charges ?1
• Invariant mass must be lt 1.8 GeV/c2 ( m?)
• Require (Ecl/Psigtrk)(0.95-f)gt0.1 to reject
  electrons
• Ecl value of energy deposit in the calorimeter
• f ratio of electromagnetic to hadronic energy
• Psigtrk scalar sum of track momenta in signal
  cone
• Require ETcl gt 15 GeV to reject muons

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Event selection

• Hadron reconstruction efficiency
• 38 at pThad 15 GeV/c
• 46 at pThad gt 25 GeV/c
• Probability of misidentifying quark or gluon jet
• 1.5 at ETjet 20 GeV
• 0.1 at ETjet 100 GeV

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Monte Carlo Simulation

• Event generation PYTHIA, with CTEQ5L PDFs
• Tau decays TAUOLA package
• Detector response GEANT-based model

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Backgrounds

• Dominant background Z/? -gt ??
• Second-highest fake ?s (misidentified quark
  or gluon jets) from processes like di-jet,
  multi-jet, Wjets, ?jets
• Third Z/? -gt ll, WW, WZ, ZZ, tt
• Background contributions determined by Monte Carlo

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Eliminating backgrounds

• Cut out low-energy multi-jet backgrounds
• Each e/? candidate must pass pT threshold of 10
  GeV/c
• Each ?had candidate must pass pT threshold of 15
  GeV/c, must also have opposite charge
• Require pTlpThadETmiss gt 50 GeV

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Eliminating backgrounds

• Cut out Wjets backgrounds
• Define
• p?vis (pl phad) ? ?
• p?ETmiss ETmiss ? ?
• Require p?ETmiss gt 0.6p?vis - 10 GeV/c
• Removes 85 Wjets events, keeps 95 of signal
• Cut out Z -gt ll decays with misidentified lepton
  reject events with lepton 1-prong ?had
  invariant mass within 10 GeV/c2 of Z mass

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Signal Acceptance

• For mA 90 GeV/c2, combined acceptance for
  ?e?had and ???had channels is 0.8
• For mA 250 GeV/c2, combined acceptance is 2.0

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Uncertainties

• Systematic uncertainties for particle ID
  efficiency ?had 3.5, e 1.3, ? 4.6
• Uncertainties in trigger efficiency ?e?had 2.1,
  ???had 1.4
• Jet -gt ? misidentification 20 (3 bg. overall)
• Uncertainty in signal acceptance
• Due to event-level cuts lt 2
• Due to PDFs 5.7

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Expected events from SM

• Number of expected standard model events passing
  all cuts
• 496?5(stat)?28(sys)?25(lumi)
• for a range of 438 to 554 events
• Breakdown
• 405 from Z/? -gt ??
• 75 from jet -gt ? misidentification
• 16 from all remaining background sources

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Data

• 487 observed events
• Reconstructed the mass (mvis) for each event
  invariant mass of visible tau decay products and
  ETmiss
• NOTE This is incorrect!
• Binned the data to compare with background and
  hypothetical Higgs data
• No signal evidence observed for mA 90 through
  250 GeV/c2

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Example plot mA 140 GeV/c2
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Exclusion limits
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Excluded regions

• Plotted excluded regions in parameter space for
  four situations
• mhmax vs. no-mixing
• ? gt 0 and ? lt 0

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Excluded regions
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Conclusions

• LEP excluded mA lt 93 GeV/c2, higher mA for small
  tan ?
• This search also excluded regions of higher tan ?
• Results similar to those of D0