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

Regina Demina, Top mass workshop2 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

12/04/04Regina Demina, Top mass workshop3 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  R<0.5

12/04/04Regina Demina, Top mass workshop4 Fraction of misidentified jets

12/04/04Regina Demina, Top mass workshop5 Mt in tt + jets ttbar + 0j ttbar + 1 j ttbar +2 j Kevin Black – effect of extra jet on mass measurement in template method. 4 jets, 1 not matched 4 jets, all matched Ensemble tests ~2.5 GeV topological ~2 GeV tagged

12/04/04Regina Demina, Top mass workshop6 MW in ttjets Consider 4 most energetic jets, not matched to b-quark At least one is not matched to W-decay product Both jets matched to W-decay products 80GeV tt1j Nj=5

12/04/04Regina Demina, Top mass workshop7 MW in tt+jets 4 and only 4 jets Mass of two jets –Not matched to b-quark –At least one is not matched to W-decay product =86 GeV tt0j tt1j tt2j =95 GeV =97 GeV 200 GeV

12/04/04Regina Demina, Top mass workshop8 MW in tt+jets 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 =92 GeV tt0j tt1j tt2j =104 GeV =108 GeV 200 GeV

12/04/04Regina Demina, Top mass workshop9 Effect on HitFit Christian Schmitt Chi2 from tt0j Chi2 from tt1j

12/04/04Regina Demina, Top mass workshop10Njets Each process can produce a spectrum of jet multiplicities How do we combine tt0j,tt1j,tt2j? tt0j tt1j tt2j =3.3 =3.7 =4.1

12/04/04Regina Demina, Top mass workshop11 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 tt0jtt1jtt2j eff, %13.9%19.5%23.3% sigma, pb sigma*eff relative weight57%34%9%

12/04/04Regina Demina, Top mass workshop12 Approach #2: trust Pythia If Pythia includes radiation why not just trust Pythia? tt0j, extra jet produced by Pythia =35GeV tt1j =44GeV tt2j =46GeV 400 GeV Pythia “radiation on- off” approach underestimates systematics!

12/04/04Regina Demina, Top mass workshop13 Approach #3: exclusive production tt0j for W+4jet bin, tt1j for W+5j, tt2j for W+6j

12/04/04Regina Demina, Top mass workshop14 Verification with data Can constraint the fraction of ttj by measuring the fraction of N(Nj>=5)/N(Nj>=4)

12/04/04Regina Demina, Top mass workshop15 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),  phi (t-tbar) – Jiri’s talk