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Extra jets in ttbar events

Extra jets in ttbar events. Regina Demina University of Rochester 6/27/07. Systematics due to extra jets. DØ’s approach – measure it on ttbar events In top mass analysis (in l+jets channel) we select events with four and only four jets

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Extra jets in ttbar events

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  1. Extra jets in ttbar events Regina Demina University of Rochester 6/27/07

  2. Systematics due to extra jets • DØ’s approach – measure it on ttbar events • In top mass analysis (in l+jets channel) we select events with four and only four jets • We use events with four or more jets to verify our Monte Carlo in how well does it model extra jets • We vary MC parameter to reproduce the data number for the fraction of events that have extra jets in them • It was easier to do in the “pre-matching” era • We varied the fraction of ttj events • Cannot do this anymore

  3. What to measure? • It is difficult to define a measurable quantity that is experiment independent • It is possible to compare data with specific detector MC • Can define • R=N(ttbar +0 jets)/N(ttbar + 1 or more jets) • Measure in ttbar l+jets data • Compare to MC • PYTHIA • ALPGEN tt+0lp, +1lp, +2lp

  4. R=N(j=4)/N(j>4) • The result from l+jets data R=1.71±0.25 Ntags

  5. ALPGEN MC • Divide tt+0lp(excl), tt+1lp(excl), tt+2lp(incl) into l+3particle jets, l+4particle jets, and l+5 or more particle jets Vary the fraction of l+5 particle jets to get the right R R(3pj +4pj) =1.33 R(5pj) =1.56 In data between 1.46 and 1.96

  6. Data to MC comparison Fraction of l+5pj should be varied from -50% of the nominal weight to using 100% of l+5pj.

  7. How to assess systematics on Mt • Compare top mass from 3,4,5 pj and vary within suggested uncertainty to check the effect on top mass • Next look at kinematics of the extra jet • Ej, etaj, DR(j-bjet), DR(j-light jet)

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