MC Studies of Color Reconnection

PYTHIA V6.416 includes color reconnection effects

We now have many top samples to work with at M=170 and 175 GeV

(thanks Sasha and Nathan).

Pythia V6.4 has new features available:

New parton shower model

New underlying event model

Inclusion of color reconnection with Skand's et al model

Can we use them to extract the CR systematics ?

How do these samples differ from V6.216 ?

Is there a difference between the new (175) and old (170) samples?

Are the jet shapes different?

Are the jets different?

So far the model has been tuned on Minbias events

We need to look at other samples to see if the new model fits the data: dijet, gam+jet

In the mean time many of us are applying the top mass machinary to evaluate mass shifts obtained with the new samples.

We are also looking at jets in top sample:

usual cuts : PT>20 GeV , jet corrected at L5

P lot: fraction of PT(jet)/PT(parton) in cone of DR=0.4

DE (parton-jet at level 5)

Compare old and new PYTHIA for jets from W-jets and b- jets.

Compare V6.216 (tune A) and V6.416 (S0, tune ACR, NOCR)

Procedure

Put a cut on the run number so that lumi profile is very similar

Given a MC sample, for each event we match the partons from top decays to the observed jets (Ntight = 4)

We then know which jet is light quark jet and which ones are b-jets

We correct the jets at L5

Compare pt5/pt0 and/or dE

in different samples

We calculate M(W) and M(top)

using the matched jets

PT(ttbar) and top PT for 4 tunes

ACR case has PT(ttbar) similar to V6.216

NOCR-V6.2

M=175 GeV

V6.2 (tune A)

V6.4 ACR

V6.4 NOCR

v6.4 S0

PT(jet)/PT(parton) for q and b jets

Jets look wider for q-jets, shifted for b-jets

M=175GeV

V6.2 (tune A)

V6.4 ACR

V6.4 NOCR

v6.4 S0

Reconstructed W and top mass at L7

MW shifted by 0.5 GeV, and M(top) shifted for S0

ACR-V6.2

S0-V6.2

NOCR-V6.2

M=175 GeV

V6.2 (tune A)

V6.4 ACR

V6.4 NOCR

v6.4 S0

Compare two S0 samples at 170 GeV

S0 samples comparison shows differences in jet shapes

S0-V6.2

NOCR-V6.2

W and top masses comparison

S0 case run by Nathe and Sasha show definite shifts in top and W mass

ACR-V6.2

S0-V6.2

NOCR-V6.2

Can this explain Andrea's puzzle?

Comparing two S0 samples (old and new) show jets and masses to be different. Likely source: different parameter settings.

Parameter settings

When comparing with default PYTHIA, i.e., trying to evaluate a CR systematics, we have to be sure that old and new PYTHIA are the same. Tune A is used as a checking point.

Results are confusing at this point. Will see this next page

Sasha and Nathan have been looking at the parameter settings as well as talking to Peter Skands

What do we learn?

S0 results for 170 and 175 GeV disagree (as seen earlier), but uncertainties are too big to say anything.

Tune A at 175 GeV looked very different, again not from here

Same conclusion for most other cases

Color reconnection

Backup slides

Backup slides

PYTHIA V6.416 versus V6.216

Our top mass measurements have been done using V6.216 (2003)

Changes in V6.416

Parton shower uses pt ordering rather then angular ordering

Multiparton (MPI) interactions have been added

ISR and FSR also have PT ordering algorithm

New model for beam renmants, including baryon junctions

Model interleaves MPI process with ISR evolution off the hard process

Color reconnection added with an “annealing model” by P. Skands

P. Skands and D. Wicke hep-ph/0703081v1 (March 2007)

D. Wicke and P. Skands hep-ph/0807.3248 v1 (July 2008)

D. Wicke and P. Skands TOP08

New PYTHIA model (P. Skands etc.)

Color reconnection in PYTHIA

Color reconnection in PYTHIA V6.4

Effects on top mass

Wicke + Skands analysis (toy MC for top mass) conclude:

(mt) ~ 0.7 from new shower effects

~ 0.5 from color reconnection effects

We (MTM3 as an example) have the following systematics:

generator: (mt) = 0.51 +- 0.37 GeV

ISR/FSR: (mt) = 0.29 +- 0.26 GeV

Using V6.416 (S0) to estimate systematics should cover most of these systematics, in addition to color reconnection.

We have to avoid double counting, so we should try to separate the two effects . Not clear how to do this.

2002-04-21T14:25:46