MC Studies of Color Reconnection

PYTHIA V6.416 includes color reconnection effects

We now have many top samples to work with (thanks Sasha and Nathan)

How do these samples differ from V6.216 ?

Are the jet shapes different?

Will show plots of jet shapes on PYTHIA V6.216 default M=175 V6.416 S0 '' V6.416 ACR '' V6.416 NOCR '' V6.416 S0 M=170

Are the jets different?

So far the model has been tuned on Minbias events

We need to look at other samples

Start to look at jets in top sample:

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

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

Compare old and new PYTHIA for jets from W-jets and b quarks. Compare V6.216 (tune A) and V6.416 (S0, tune ACR, NOCR)

plan to look at more variables +add more cuts

plan to look at corresponding jets with cone radius of 0.7

Luminosity profile

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

PT(ttbar) for 4 tunes

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

S0-V6.2

ACR-V6.2

NOCR-V6.2

PT(top) at sim level

PT(top) is not affected much by the new parton shower or CR process

PT(jet)/PT(parton) for light jets

Distribution looks a bit wider for V6.4 tune S0

ACR-V6.2

S0-V6.2

NOCR-V6.2

S0 Comparison of 170 and 175 GeV

The jet shapes for light and b quarks are consistent

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

Distribution looks a bit wider and shifted for V6.4, tune S0

ACR-V6.2

S0-V6.2

NOCR-V6.2

Reconstructed W mass at L7

MW shifted by 0.5 GeV, RMS is larger (14. vs 15.5 GeV) for S0

ACR-V6.2

S0-V6.2

NOCR-V6.2

Reconstructed m(top)

In S0, Mtop is shifted by -0.8 GeV , RMS is 23.7-vs-21.7 GeV

ACR-V6.2

S0-V6.2

NOCR-V6.2

What do we learn?

The jets are wider in S0, so worse resolution.

We see this in both the W mass and the top mass

From other plots, not included here, I see that the jet widening happens mostly at low pT

The ACR case has smaller effects than S0

The NOCR shows less visible effects than S0

Probably the combined effects on ACR and NOCR explain the S0 behavior. We should quantify this statement

PYTHIA V6.4 is still being tuned by the authors

Color reconnection

Backup slides

Backup slides

PT(ttbar)

Courtesy of Mousumi Datta

Why change to V6.416 new tunes? PT(ttbar) now agrees with HERWIG. The PYTHIA authors believe this is what it should look like.

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