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

-

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