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Effect of extra jets on top mass measurement

Effect of extra jets on top mass measurement. Regina Demina, Elizabeth Groves University of Rochester 12/04/04. Monte Carlo sample. Monte Carlo – full simulation produced by Michael Begel – ALPGEN+PYTHIA tt0j, tt1j, tt2j One W forced to decay leptonically, the other hadronically.

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Effect of extra jets on top mass measurement

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  1. Effect of extra jets on top mass measurement Regina Demina, Elizabeth Groves University of Rochester 12/04/04

  2. Monte Carlo sample • Monte Carlo – full simulation produced by Michael Begel – ALPGEN+PYTHIA • tt0j, tt1j, tt2j • One W forced to decay leptonically, the other hadronically Regina Demina, Top mass workshop

  3. The question • Consider only four leading jets, no matter how many jets are there in the even. • How often one of these jets is NOT a top decay product, e.g. a gluon radiation jet? • Procedure to match top decay products • Identify partons from top decay: • b and bbar quarks with top being the parent • q or qbar from W decay, with W from top decay • Match jets to partons by DR<0.5 Regina Demina, Top mass workshop

  4. Fraction of misidentified jets Regina Demina, Top mass workshop

  5. Mt in tt + jets Kevin Black – effect of extra jet on mass measurement in template method. ttbar + 0j ttbar + 1 j ttbar +2 j 4 jets, 1 not matched 4 jets, all matched Ensemble tests ~2.5 GeV topological ~2 GeV tagged Regina Demina, Top mass workshop

  6. MW in ttjets • Consider 4 most energetic jets, not matched to b-quark tt1j Nj=5 Both jets matched to W-decay products 80GeV At least one is not matched to W-decay product 80GeV Regina Demina, Top mass workshop

  7. 4 and only 4 jets Mass of two jets Not matched to b-quark At least one is not matched to W-decay product MW in tt+jets tt0j <MW>=86 GeV tt1j <MW>=95 GeV tt2j <MW>=97 GeV 200 GeV Regina Demina, Top mass workshop

  8. 5 and only 5 jets Consider only 4 most energetic jets Mass of two jets Not matched to b-quark At least one is not matched to W-decay product MW in tt+jets tt0j <MW>=92 GeV tt1j <MW>=104 GeV tt2j <MW>=108 GeV 200 GeV Regina Demina, Top mass workshop

  9. Christian Schmitt Effect on HitFit Chi2 from tt0j Chi2 from tt1j Regina Demina, Top mass workshop

  10. Each process can produce a spectrum of jet multiplicities How do we combine tt0j,tt1j,tt2j? Njets tt1j tt0j <Nj>=3.7 <Nj>=3.3 tt2j <Nj>=4.1 Regina Demina, Top mass workshop

  11. Approach #1: weighted sum • Add processes straight up, weighted by cross section*selection efficiency: • Add contributions of tt0j, tt1j, tt2j • Double counting because tt0j includes radiation by Pythia Regina Demina, Top mass workshop

  12. Approach #2: trust Pythia • If Pythia includes radiation why not just trust Pythia? tt1j <Et>=44GeV tt0j, extra jet produced by Pythia <Et>=35GeV Pythia “radiation on-off” approach underestimates systematics! tt2j <Et>=46GeV 400 GeV Regina Demina, Top mass workshop

  13. Approach #3: exclusive production • tt0j for W+4jet bin, tt1j for W+5j, tt2j for W+6j Regina Demina, Top mass workshop

  14. Verification with data • Can constraint the fraction of ttj by measuring the fraction of N(Nj>=5)/N(Nj>=4) Regina Demina, Top mass workshop

  15. How to assess systematics on top mass • Use Njets distribution of single and double tagged data to fit for the fraction of tt+nj • Open question: should we let the absolute cross section float? • Vary the mixture within uncertainty and extract top mass • With higher statistics additional information can be used to constraint the fraction of ttnj: • MW, Pt(top), Pt(ttbar), Dphi (t-tbar) – Jiri’s talk Regina Demina, Top mass workshop

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