Color Reconnection Systematics

Thanks to all that are contributing to this effort:

Sasha Golossanov, Nathan Goldschmidt

A. Castro, P. Lujan, P. Mehtala, Jian Tang, Marco Trovato, et al.

Top mass conveners, top conveners

Peter Skands for his availability and continuous help.

This systematic uncertainty has not been included as yet, although we heard talks about this for many years now.

We are now trying to understand how to include it, with the help of our PYTHIA friends.

In recent months in CDF many samples have been generated and some effort is being made to study this effect.

We have a better understanding of the issues, but no concrete proposal on how to proceed yet.

This systematic uncertainty has not been uncluded as yet, although we heard talks about this for many years now.

We are now trying to understand how to include it, with the help of our PYTHIA friends.

In recent months m any in CDF samples have been generated and some effort is being made

PYTHIA V6.416 versus V6.216

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

Color reconnection effects are included in PYTHIA V6.416

Changes in V6.416

Parton shower uses pt ordering rather then mass ordering

Multiparton (MPI) interactions are now part of the parton shower

ISR and FSR also uses 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 M. Sandhoff and 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)

Effects on top mass

The complete set of changes is included in tune S0 of PYTHIA V6.416

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

(mt) ~ 0.7 from new shower effects

~ 0.5 from color reconnection effects

Tune S0 includes systematics that we are already taking into account

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.

MC studies of CR Effects

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 . Compare jet shapes with data.

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:s

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)

Procedures

A. 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 (no out of cone cor.). To check if jet shapes have changed we do the following:

Compare Pt5/Pt0 and/or dE in cone of R=0.4

We calculate M(W) and M(top) using the matched jets

B. We apply to each sample the top mass measurement

analysis to obtain a mass and an uncertainty. Compare

the values to estimate a systematics.

PT(ttbar) and top PT for 4 tunes

ACR case has PT(ttbar) similar to V6.216. Top PT not affected.

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

M(W) shifted by ~0.5 GeV, also 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

Comparison of A and B methods

We compare the results obtained with the two methods.

For the MC event matching method, we are only looking at the signal. The ME mass measurement method is applied only to the signal events for this comparison. The numbers in red show deviation of more then 2 sigmas.

The ACR case seems to deviate the least from the nominal V6.2 case. This is expected as only the CR part has been changed.

Top mass reconstruction studies

ACR should be directly comparable to tune A, the difference being due to Color Reconnection.

NOCR and S0 are expected to have larger M

More studies

Using the 170 GeV samples in the di-lepton channel

Consistent with previous results

Summary and Conclusions

First studies indicate that jet shapes in PYTHIA V6.416 are different from those in V6.216, as expected.

Planning to compare the new PYTHIA with di-jet and gam+jet data, to see if it fits the data.

Using ACR to evaluate the systematics could be the simpler thing to do, but we have to look at di-jet and gam- jet data to see if Acr fits the jet shapes

Color Reconnection Studies

Backup slides

Color reconnection in PYTHIA

Color reconnection in PYTHIA V6.4

2002-04-21T14:25:46