Color Reconnection Studies

PYTHIA V6.416 includes color reconnection effects

We now have some top samples to work with (thanks Sasha)

How do these samples differ from V6.216 ?

Are the jet shapes different?

How can we evaluate the Color Reconnection effects on the top mass measurement?

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.)

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 at L5, jet corrected at L5

Plots: 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 (tune S0)

plan to look at more variables +add more cuts

plan to look at corresponding jets with cone radius of 0.7

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

Distributions

look a bit wider for

V6.4 tune S0

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

Distributions

look a bit wider

for V6.4, tune S0

What do we learn?

Not much at this point, I have to improve the cuts

(include only jets that are isolated from other jets)

We seem to overcorrect the jets below 40 GeV

Plots just off the press, need to check my code

Looking at jets in top events is not conclusive (4 jets in each event, very few events)

We should look at di-jets or gam+jets, which can be compared with lots of data

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.

Color reconnection

Backup slides

Backup slides

Color reconnection in PYTHIA

Color reconnection in PYTHIA V6.4

2002-04-21T14:25:46