Janice Drohan, Simon Dean, Nikos Konstantinidis

1
Search for Low Mass SM Higgs in the Channel
ttH(H?bb)

• Janice Drohan, Simon Dean, Nikos Konstantinidis
• UCL Christmas Meeting
• 12/12/2005

2
Topics

• Overview
• Evolution of ttH(H?bb)
• TDR analysis
• Recently redone
• Changes at UCL
• Hadronic W rescaling

3
Overview of Channel

• One of three channels important for Higgs search
  in low mass region (110-150 GeV)
• Also H??? and qqH?qqtt
• LEP search limit
• 114 GeV lt mH (SM) lt 237 GeV (95 C.L.)
• H?bb is the dominant decay channel for mHlt135 GeV
• Direct production difficult to extract from the
  huge QCD 2-jet background
• Associated production.

4
Event Topology
4 b-jets 2 light jets 1 isolated
lepton Missing momentum
5
Backgrounds

• Irreducible Backgrounds
• ttZ much lower cross-section than signal
• ttbb
• Reducible Backgrounds
• Misidentified b-jets
• ttjj, Wjjjjjj, Wbbjj etc
• Latter two suppressed by reconstruction of top
  decays
• ttjj is the main background

6
TDR Analysis

• Detector and Physics Performance Technical Design
  Report (1999)
• Preselection
• 1 isolated lepton, pTegt20GeV, pTµgt6GeV, ?lt2.5
• At least 6 jets, pTgt15GeV, ?lt5
• Exactly 4 tagged as b-jets
• W-gtl?
• constrain invariant mass of lv to W mass, solve
  for pzv
• W-gtjj
• Pairs of non b-jets with mjjmW25GeV
• Reconstruction of 2 top quarks
• Minimise
• Reconstructed top masses must be within 20GeV of
  mt

7
TDR Analysis

• Analysis done with Pythia, CTEQ2L
• Results for 30fb-1

8
Updated Analysis

• Brought up-to-date in 2003/4
• Updates included
• CTEQ5L
• AcerMC to model ttbb and ttZ backgrounds (matrix
  elements)
• Lepton pre-selection updated for new trigger
  menus 25GeV electron, 20GeV muon, 20GeV jets
• Results

9
Studies Made At UCL

• Hadronic W decaying to three jets
• Ejet/Eb-quark distribution is much worse when the
  b-quark decays semi-leptonically (Br(b?lvc) 20)
• Small correlation observed between jet and
  neutrino energies
• pT dependant correction to jet energy and
  equivalent correction to missing ET

• 29 of events with correct jets assigned - low
• ?2 replaced with ?2
• Likelihood technique

10
Hadronic W Rescaling

• Simple rescaling sets mjj mW(PDG) by scaling jj
  vector
• Gaussian polynomial fit to EWreco/EWMC
• ? improves but RMS wider
• Partially effective

11
Hadronic W Rescaling

• Allow each light jet to scale independently
• ?2 minimisation to improve hadronic W
  reconstruction
• Uses Minuit

12
Results

• Improved ? and RMS
• Improved mass distribution

13
Rescaling Parameters

• Both always raised or lowered
• Follows from form of ?2 and mjj 2E1E2(1-cos?)

14
That's It

• Merry Christmas UCL HEP group!