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First look of ME study on ttH(  bb) channel

First look of ME study on ttH(  bb) channel. Univ. of Tokyo Y.Kataoka. Feb.21.2003 @CERN. Previous Analysis (1). We introduced modified c 2 method to solve the Combinatorial BG . In this method, we used c 2 values based on mass information,

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First look of ME study on ttH(  bb) channel

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  1. First look of ME studyon ttH(bb) channel Univ. of Tokyo Y.Kataoka Feb.21.2003 @CERN

  2. Previous Analysis (1) • We introduced modified c2 method • to solve the Combinatorial BG. • In this method, we used c2 values based on mass information, • likelihood output of angle information. 30fb-1 • Reconstructed • Higgs mass plot • (MH=120GeV) ttbb BG is dominant. Mbb

  3. Previous Analysis (2) • Number of events (30fb-1) Combinatorial BG Is still large (true comb.) • Details are in the previous talk (Dec.12,2002).

  4. Matrix Element Study • Modified c2 method is not effective to solve Combinatorial BG. • We start a new study based on “Matrix Element”. This ultimate method may solve the problems. • Is it possible to improve the current result ?

  5. Event Selection ( The same as the previous study ) • Number of isolated lepton = 1 for electron, pT>25GeV, |h|<2.5 for muon, pT>20GeV, |h|<2.5 • Number of light flavor jet >= 2 pT>20GeV, |h|<2.5 pT>40GeV for at least one jet • Number of b-jet = 4 pT >30GeV • Decay mode • H b b • t l nb • tj jb

  6. Matrix Element After the event selection, • we calculate an event weight: “-log(ME)”, • and use this weight to solve the Combinatorial BG. We can choose the most probable combination. Matrix element is calculated by Grace (2.2.0). Input: 4-vector of 8 final fermions (Hbb, tj jb, tl nb ) 80 diagrams and their interference

  7. Detector Resolutions • In order to take detector resolutions into accounts, we set Higgs, top, W decay widths to 20GeV (temporary). • In the parton level, we check the outputs of grace. • Blue line is distribution • with natural decay width. • Red line is distribution • with smearing. If we have good energy resolution, we have very good performance. -log(ME) Small |ME|2 Large |ME|2 Wrong comb. Correct comb.

  8. Parton Level Study First, we use the parton level 4-momenta of the ttH signal sample (MH=120GeV). 1. all partons are correctly assigned (red line) 2. b and b-bar exchanged (blue line) • Wide widths are included • for W, t , H -log(ME) ME method has the power of discrimination between correct and wrong assignments.

  9. Detector Level Study Next, we use the observed 4-vectors (Atlfast output). 1. jets and leptons are correctly assigned (red line) 2. b and b-bar exchanged (blue line) -log(ME) The discrimination power is decreased.

  10. Parton level study shows: ME method is powerful to discriminate Combinatorial BG. • Detector level study shows: smearingin the detector level decreases the discrimination power. More studies are necessary.

  11. Conclusions • We apply the ME method to solve the Combinatorial BG. • We find the ME method has discriminating power. • More studies on detector smearing are necessary to apply this method. • We also hope to apply this method to BG rejection (ttbb,tt).

  12. Future Plans • We also start to study the ME method based on 7-body final particles. • 8-body • H b b • t l n b • tj jb • 7-body • H • t l nb • tj jb • We expect this 7-body ME has more power on Combinatorial BG than the current 8-body ME. • We will report new results in the very near future.

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