ISR/FSRTomonobu Tomura

(University of Tsukuba)

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Introduction ISR/FSR is becoming one of the dominant

sources of systematic uncertainties for in-situ JES calibration method.

CDF and DZero have been using different approach to estimate ISR/FSR uncertainty. Better to use consistent way for the top mass

combination.

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Previous CDF Scheme Use DY data (no FSR) for the extraction

of reasonable range of Pythia parameters to describe ISR uncertainty. FSR syst. is based on the ISR study.

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Previous DZero Scheme

Use different mixture of Alpgen+Pythia ttbar+0p, 1p, 2p samples to describe the uncertainty. Systematic uncertainty: difference between

tt0p and weighted sum of 0p, 1p, 2p

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Merits and Demerits CDF

Advantage Data driven. Separate treatment

for ISR and FSR.

Disadvantage Extrapolation to a

higher Q2 region based on the linear dependence on log(Q2).

Not all the diagrams.

DZero Advantage

Many diagrams. No extrapolation.

Disadvantage No variation in soft radia

tion (below the threshold).

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Comparison of Samples We compared some kinematical

distributions of samples for the systematics. Pythia Nominal ttbar Alpgen+Pythia

ttbar+0p(3.60 pb) and ttbar+≥1p(1.13 pb) Pythia ISR More/Less Pythia FSR More/Less

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Jet Multiplicity

Left plot: Njets distributions for various MC Right plot: difference from the Pythia nominal

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Leading Jet (ET & )

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5th Jet (ET & )

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R between 5th jet and b/q

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Acceptance (≥4jets, ≥1b-tag)

Sample Acceptance (%)

Pythia Nominal 2.88

Alpgen ttbar+0p/1p 3.03

Pythia ISR More 2.88

Pythia ISR Less 2.82

Pythia FSR More 2.78

Pythia FSR Less 2.87

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N(Nj>=5)/N(Nj>=4)

Sample Fraction

Pythia Nominal 0.251

Alpgen ttbar+0p/1p

0.268

Pythia ISR More 0.275

Pythia ISR Less 0.247

Pythia FSR More 0.256

Pythia FSR Less 0.258

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Plan / Proposal Use Alpgen+Pythia as the nominal sample. ISR/FSR Uncertainty “Signal Modeling Uncertainty”

Vary Alpgen parameters to cover the uncertainties in the prediction.

Parameters to vary alpha_s, factorization scale, etc. Not 0p/1p sample fraction because it is determined by matching.

How much to vary Control sample? Theoretical input?

Or, reweighting the sample to construct + and – systematic samples.

Alpgen Validation / Parameter Study are urgent issues.