Comments on jets in Top Monte Carlo

Lina Galtieri for the MTM top mass analysis group:

work done with Si Xie, J. Lys, P. Lujan, P. Fernandez

Top Mass meeting

June 22, 2005

Jets in HERWIG and PYTHIA

b jets deviation from tree level partons (ΔR)

b jets and light jets mass

W mass at parton and hadron level

HERWIG MONTE CARLO

W1 and b1 have been adjusted from W0 and b0 to take into account parton shower effects

PYTHIA MC Top event

Here partons 44-47 will be added to compare with the b1 of HERWIG (ID=94 b parton).

Partons 14-15 will be modified (as in HERWIG) in the next step.

B jet direction changes in MC

∆R(Tree level – ID=94)

∆R(Tree level - Simulation)

∆R(Id=94-Simulation)

PYTHIA

width ~0.07

width ~0.1

width ~0.04

HERWIG

width ~0.04

width ~0.04

width ~0.004

width here refers to the width at half maximum. Tree level to simulation is different in PYTHIA and HERWIG: different physics effects?

The b jet directions at hadron level

Jets in HERWIG get large angle deviations (~0.1) when they acquire a mass

Effect seen at the parton level as well as the hadron level

Detector effects (resolution) are a small part of the ΔR we observe.

PYTHIA angle deviation is small(~0.02), detector effects same as HERWIG

∆R at hadron level : b0-had level jet and simulation

HERWIG

PYTHIA

resolution ~ 0.04

resolution ~ 0.04

width ~0.1

particles to jets

width ~ 0.02

Physics effects

Detector effects

parton to particles

parton to particles

particles to jets

cone of 0.4 used

b masses in HERWIG and PYTHIA

HERWIG

PYTHIA

b parton-jet (ID=94)

b JETCLU mass include

detector resolution

Jet mass for simulated jets is about the same, not so for ID=94 parton

PYTHIA b Mass varying PARJ(82)

PARJ(82)=1.0

PARJ(82)=2.0

PARJ(82)=3.0

PARJ(82)=4.0

mass for

PARJ(82)=3-4

looks like the HERWIG b mass.

Sjostrand and Mrenna point out that we can only make comparisons between the two MC at hadron level, i.e. after the parton shower and hadronization are complete (see next page)

PYTHIA default

PARJ(82)=1.0

tuned on LEP data

Jet masses at hadron level

HERWIG

PYTHIA

PYTHIA

Use cone algorithm to find jets at particle level, compute the mass for jets with cone=0.4

b jet mass ∆m=0.5GeV

q jet mass ∆m=0.01GeV

HERWIG

PYTHIA

b jet mass in HERWIG is larger by 0.5 GeV

HERWIG

PYTHIA

Mass difference Δm=0.5 GeV at hadron level

parton level

parton level

hadron level

hadron level

W mass studies

W mass at hadron level

HERWIG

PYTHIA

W mass at hadron level is larger for HERWIG than for PYTHIA

Not from underlying event (see Rick Field)

Parton shower and fragmentation?, production?

jet cone =0.4

jet cone =0.7

Δm=0.5 GeV

Δm=0.7 GeV

Summary

Hadron Jet direction in HERWIG is away from the tree level parton by ∆R~0.1. In PYTHIA it deviates only by ∆R~0.02. Is this due to different physics?

Jet resolution (i.e. from Simulation+reconstruction) ~ ∆R=0.04

Jet masses at the parton level cannot be compared in HERWIG and PYTHIA, as the parton shower and frag. are done differently and only at the end they can be compared

b jet masses at particle level differ by ~0.5 GeV.

W mass, reconstructed at hadron level, is 500 MeV larger in HERWIG than in PYTHIA: needs to be understood (in simpler final state, perhaps)

We have compared jets and W mass in HERWIG and PYTHIA MC

2002-04-21T14:25:46