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
0
0.2
0.4
0.6
0.8
1
1.2
3 4 5 6 7Njets
Fra
ctio
tt0j
tt1j
tt2j
combined
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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0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
3 4 5 6 7Njets
Fra
ction
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)
-
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
1
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
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