PYTHIA-vs-HERWIG for Top Mass

Are jets better measured by PYTHIA or HERWIG?

Jet Shapes from Mario's analysis

Jet correction studies:

Use -jet balance to check corrections

Relative correction

Jet response

Underlying event

Out of cone corrections

Summary: can we tell which jets are better?

Is the mass analysis sensitive to t-tbar spin correlations?

Jet Shapes studies (Mario's)

Mario's jet shapes: CDF-7240

(published in PRD, 2005)

PYTHIA jets fit the data better

For PT<65 GeV

HERWIG jets are narrower by

5% at PT=50 GeV

For PT>80 GeV

HERWIG jets are wider by

5% at PT=125 GeV

Most jets in top events are below 150 GeV

Midpoint algorithm used, R=0.7

Corrections made from PYTHIA tune A

Statistical and syst. uncertainties used.

Shows fraction of PT in annulus 0.3-0.7

Comments:

PYTHIA used for the corrections.

Discrepancy only in R=0.3-0.7

Results:

-jet balance: (rel. corr. only) NIM

HERWIG has 2% less PT in cone of R=0.4

PYTHIA has 2% more PT in cone of R=0.4

Plot shows test of corrections on -jet data and in MC samples.

Relative corrections calculated from di-jet samples: separately for data and MC (using PYTHIA)

Systematics calculated by varying the selection cuts

Top jets:light and b quarks

at parton level)

Jets in -jet events

(uncorrected)

Jets in -jets are softer than top jets

Mousumi Datta

Jet response (NIM absolute corrections)

HERWIG is better then PYTHIA on jet response

See CDF-7450, Bhatti et al.

PYTHIA used to calculate the

absolute corrections.

Systematics from many sources. The largest one from uncertainty of calorimeter response to charged particles.

GLASH used for calorimeter simulation.

-jet balance (partial corr.) NIM

Check of relative and and absolute corrections on -jet events

HERWIG-data ~ - 2.0%

PYTHIA -data ~ + 1.8%

No way to choose

For -jet balance studies see CDF-7452, Canelli et al.

Underlying event (NIM)

Rick Field's studies show PYTHIA tune A fits data better

For PT> 10 GeV/c HERWIG is lower than the data by ~ .5 GeV/c. This is the Sum(PT) in the transverse region (A=4/3*), for a radius of 0.4 we expect 0.5*3/4*.16=0.06 GeV discrepancy. This is ~0.1 GeV (20%) for the UE=0.4 GeV evaluated from calorimeter measurements (see next page)

Out of cone (NIM)

-jet balance results (PT jets ≤ 100 GeV). Left plot shows PT in annulus 0.4-1.3: includes both UE and Out-of-Cone PT

OOCC and UE (0.4 GeV for R=0.4) determined from PYTHIA dijet samples.

Systematics include difference between PYTHIA and HERWIG.

HERWIG (blue) is closer to the data than PYTHIA

-jet balance: all corrections NIM

All corrections applied (UE and OOCC determined from PYTHIA)

HERWIG-data ~ - 2.0%

PYTHIA-data ~ + 1.8%

No way to choose

Summary

Mario's plots show that PYTHIA is better than HERWIG between R= 0.3 and 0.7 .

Jet response show that HERWIG is slightly better than PYTHIA

Out of cone plots show that HERWIG is better than PYTHIA between R= 0.4 and 1.3.

-jet balance data show that HERWIG is below the data by ~2% PYTHIA is above the data by ~1.8% at every level of correction.

Both PYTHIA and HERWIG disagree with the data at the 2% level

(note, however, that jets in -jet are softer that top jets)

Are the ME analyses sensitive to spin correlations?

PYTHIA does not have spin correlations in the Matrix Element.

2002-04-21T14:25:46