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Top Mass in the all hadronic channel

Top Mass in the all hadronic channel. Run I analysis strategy Mass Reconstruction in RunI Status with RunII MC and data Plans. P. Azzi, T. Dorigo - Padova University & INFN Castro, A. Gresele – Bologna University & INFN I. Lazzizzera, S. Amerio – Trento University & INFN

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Top Mass in the all hadronic channel

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  1. Top Mass in the all hadronic channel • Run I analysis strategy • Mass Reconstruction in RunI • Status with RunII MC and data • Plans • P. Azzi, T. Dorigo - Padova University & INFN • Castro, A. Gresele – Bologna University & INFN • I. Lazzizzera, S. Amerio – Trento University & INFN • J. Konigsberg, G. Lungu, S. Sukhanov – Florida University

  2. Run I analysis strategy • The all hadronic channel: • BR is large (46%)  • Events fully reconstructable  • Large QCD background: S/B is very small  • RunI Analysis had to cope with low statistics and a low double b-tagging efficiency • Specific trigger was developed: S/B @trigger =1/500 • Two paths for improving S/B: • A)Tight Kinematic + >=1 btag: S/B ~ 1/4 • used for mass measurement • B) Loose Kinematic + >=2 btag: S/B ~1/4

  3. Run I Analysis Strategy (cont’d) Multijet Trigger Data Sample Large Jet Multiplicity Large Total Transverse Energy One B-tag jet present • Kinematic Cuts • Centrality • Aplanarity Two B-tagged jets Cross Section s=7.6+3.5-2.7 pb Mass Reconstruction M(t)=186±10±5.7 GeV/c2

  4. Run I Event Selection for Mass Reconstruction • Jet counting definition: • Et(raw)>15 GeV & |eta|<2 • Main background: QCD 22 processes with extra jets due to gluon radiation • Kinematic very different from tt • Use data before tag for the background shape • Signal contamination negligible • Cuts (corrected jet energies) • 6N(jet) 8 • Et 200 GeV • Et/ŝ0.75 (centrality) • Aplanarity+0.0025 x 3Et 0.54 <M(t)> 175 SET The Et cut has been lowered wrt to the value optimized for the cross section selection b/c of its bias on the background shape

  5. Mass Measurement in RunI • Fully reconstructable final state • Fit the six jets events to tt hypotesis (3 constraints): • M(jjb)1 = M(jjj)2 • M(jj)1=M(W) • M(jj)2=M(W) • Combinations: • No tag: 90 • 1 Tag: 30 • 2 Tag: 6 • Caveat: even in events with two tags we required only one of the b to be associated to the parton. In 30% of the cases the second tag is a charm from W decay. • In Run I we saw no significant improvement in top mass resolution from 1 to 2 tags One b Two b

  6. Mass Measurement in Run I (cont’d) Double tagged events (consistency check) M(t)=186±10±5.7 GeV/c2

  7. Run I Systematics Final weights in the combined mass result: 0.65 (l+jets), 0.19(dilepton), 0.15 (hadronic)

  8. The RunII trigger and datasets • Dedicated trigger: TOP_MULTI_JET (afterRun 143938) • Requires: N(jet)>=4 with Et>=15 and Et>125 GeV • Before Run 143938: L2 was only Et>130 GeV cut • Total luminosity available now is: Lint~107 pb-1 (with good run list and SVX requirement) • Available datasets (Andrea C.): • Events before Jan shutdown processed with 4.9.1hpt1 • Full statistics up to May27th processed with 4.11.1 • Signal MC: use official tt MC of the Top Group (inclusive)

  9. Work in progress and plans • “Top Hadronic Group” meets informally every two weeks • Basic philosophy for RunII analysis: • Full characterization of the multi-jet datasets (Njet>=4) in terms of kinematical properties and heavy flavor content • Currently we are validating data after the shutdown • Use of new tools (ME MC such as Alpgen) to have an extra handle on the background determination • In Run I no MC was available to properly describe high multiplicity events with heavy flavor • Optimize analysis selection for a S/B~1/1 (or better) in the signal region • Work in progress to evaluate a priori how well we can do in different luminosity scenarios

  10. First look using Run II MC • Machinery for mass reconstruction is already in place. • Work in progress to add top specific corrections • Run II Herwig MC • Jet corrected to Level7 • Compare the resolution for tt in the case of 0, 1 and 2 tags: • In RunI width was 24 GeV(22 GeV) with 0tag (1 tag) with full AA corrections.

  11. Comparison Run I vs Run II background shape Entries=1153 RUN I multi-jet data 0 tag after kinematical selection Full Run I jet corrections RUN II multi-jet data 0 tag after kinematical selection Top specific corrections missing!

  12. Run II signal and background shape • After a “Run I style” kinematical selection: • 0 tag (veto) : S/B= 1/100 , negligible top contribution • >= 1 tag: S/B=1/6 • >=2 tags: S/B=1/1

  13. Future Plans • Work in progress to reoptimize analysis to take full advantage of the CDF upgraded detector and increased statistics • Need to fully characterize (again): • Correlations of kinematical and b-tagging requirement • Issues on mass reconstruction from the 2 tags requirement • Background shape? • improvements in resolution? • Other ideas: bottom/charm difference? Interplay of different b-taggers? • Plan is to have a preliminary results for this Winter

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