Measurement of the Top Quark Cross section in the tau+jets Channel
Effect of extra jets on top mass measurement
description
Transcript of Effect of extra jets on top mass measurement
Effect of extra jets on top mass Effect of extra jets on top mass measurement measurement
Regina Demina, Elizabeth Groves
University of Rochester
12/04/04
12/04/04 Regina Demina, Top mass workshop 2
Monte Carlo sampleMonte Carlo sample• Monte Carlo – full simulation produced by Michael Begel –
ALPGEN+PYTHIA– tt0j, tt1j, tt2j
• One W forced to decay leptonically, the other hadronically
12/04/04 Regina Demina, Top mass workshop 3
The questionThe 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
12/04/04 Regina Demina, Top mass workshop 4
Fraction of misidentified jetsFraction of misidentified jets
Jet quality cuts
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tt0j
tt1j
tt2j
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matched
12/04/04 Regina Demina, Top mass workshop 5
Mt in tt + jetsMt in tt + jets
ttbar + 0jttbar + 1 jttbar +2 j
Kevin Black – effect of extra jet on mass measurement in template method.
4 jets, 1 not matched4 jets, all matched
Ensemble tests~2.5 GeV topological~2 GeV tagged
12/04/04 Regina Demina, Top mass workshop 6
MW in ttjetsMW 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 products80GeV
80GeV
tt1jNj=5
12/04/04 Regina Demina, Top mass workshop 7
MW in tt+jetsMW 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
<MW>=86 GeVtt0j
tt1j
tt2j
<MW>=95 GeV
<MW>=97 GeV
200 GeV
12/04/04 Regina Demina, Top mass workshop 8
MW in tt+jetsMW 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
<MW>=92 GeVtt0j
tt1j
tt2j
<MW>=104 GeV
<MW>=108 GeV
200 GeV
12/04/04 Regina Demina, Top mass workshop 9
Effect on HitFitEffect on HitFit
• Christian Schmitt
Chi2 from tt0jChi2 from tt1j
12/04/04 Regina Demina, Top mass workshop 10
NjetsNjets
• Each process can produce a spectrum of jet multiplicities
• How do we combine tt0j,tt1j,tt2j?
tt0j tt1j
tt2j
<Nj>=3.3 <Nj>=3.7
<Nj>=4.1
12/04/04 Regina Demina, Top mass workshop 11
Approach #1: weighted sumApproach #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
tt0j tt1j tt2j
eff, % 13.9% 19.5% 23.3%
sigma, pb 6 2.5 0.56
sigma*eff 0.834 0.4875 0.13048
relative weight 57% 34% 9%
12/04/04 Regina Demina, Top mass workshop 12
Approach #2: trust PythiaApproach #2: trust Pythia• If Pythia includes radiation why not just trust Pythia?
tt0j, extra jet produced by Pythia <Et>=35GeV
tt1j <Et>=44GeV
tt2j <Et>=46GeV
400 GeV
Pythia “radiation on-off” approach underestimates systematics!
12/04/04 Regina Demina, Top mass workshop 13
Approach #3: exclusive Approach #3: exclusive productionproduction
• tt0j for W+4jet bin, tt1j for W+5j, tt2j for W+6j
Relative conribution of jet multiplicities
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Fra
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tt0j onlyweighted sumexclusive matching
12/04/04 Regina Demina, Top mass workshop 14
Verification with dataVerification with data• Can constraint the fraction of ttj by measuring the
fraction of N(Nj>=5)/N(Nj>=4)
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method
N(J>=5)/N(J>=4)
Pythia
weighted sum
exclusive matching
double tagged data
12/04/04 Regina Demina, Top mass workshop 15
How to assess systematics on top How to assess systematics on top massmass
• 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