Update on PYTHIA V6.4 Studies

Strong color correlations between the hard process and the underlying event are implied by tune A and similar tunes. These effects may be interpreted as sign for color reconnection.

The issue has been studied at LEP for the W mass measurement

CR effects on the MW

measurement at LEP

contribute to systematics

CR(sys) = 8 MeV

out of 22 MeV (total sys)‏

(LEPEWWG hep-ex/061203)‏

Preliminary MC studies have

indicated possible contributions

to the top mass

systematics of order

CR(sys) 0.5 GeV

D. Wicke and P. Skands arXiv:0807.3248V1

LEP

Tevatron

This is an update over my talk of July 1

A number of plots were not understood.

Will revisit them today

On going studies: try to evaluate the color reconnection contributions to our top mass systematics.

PYTHIA V6.4 has a lot of new features:

new parton shower algorithm

different way of treating ISR

different underlying event

inclusion of Color reconnection effects

Presently studying the difference in jet shapes in V6.2 and V6.4

Jet Shapes Variables in b-jets

V6.2 tune AV comparison of N(charged) in cone and φ and η Moments

The φ moments agree well with the data.

The η moments do not agree at all as already observed by Andrea and Hyunsu. The Moment dependence on PT(jet) is in clear disagreement with the data

For the moments distributions, we have not normalized to 50 GeV,

as the PT distributions for data and MC agree quite well and we are using only one mass point (175 GeV).

PYTHIA 6.2 agrees quite well with data for the

N(charged) variable.

Comparison among different tunes

Plots shown are for the default PYTHIA V6.2. The agreement is quantified by the KS value. A summary of the values for many samples is shown below.

V6.2-AV Aprolhl ACRlhl S0pro NOCRPg0 S0Pg0

b-jet PT 0.89 0.86 0.89 0.65 0.86 0.90

ET1 (b-tag) 0.85 0.71 0.74 0.93 0.85 0.88

N(charged) 0.57 0.13 0.84 0.20 0.18 0.30

η moments 0.00 0.00 0.01 0.00 0.00 0.00

φ moments 0.21 0.10 0.47 0.37 0.24 0.26

It is not clear on how to choose between the different tunes as none of the ones considered fair well on all of these variables. We need to do additional work to understand the situation.

Additional studies are being done and are now URGENTLY needed.

quark jet shapes in di-jets events

b-jet studies in di-jet events

MC Matching studies: more ISR?

Comparison of Number of tight jets in the Perugia0 and PYTHIA V6.2.

Also comparison of number of tagged b jets in the N(tight)=4 sample

These findings point to more ISR in the S0-Perugia0 samples. Looking into this.

Matching events:

V6.2 (tune A) 68%

V6.4 ACR 68%

V6.4 NCR-Pg0 59%

V6.4 S0-Pg0 59%

Perugia0 :

Less matching

More N(tight)

Less b-jets

in 4 jet sample

Moments for quark and gluon jets

Jet shapes studies have been around in CDF for a while

From Brigliadori, Castro, Margaroli's work we have the Expected moments for light quarks and gluons.

We have been looking at these variables and the distributions of N(charged) in jets, in an effort to understand the new tunes.

No-matching jets properties

match

match

no-match

Compare eta moments and jet eta for matching and no-matching jets. Relative shapes depend on the matching cuts, of course.

Wider distributions for eta moments and jet eta, point to the non

matching jets to be gluon jets

 and  Moments in no-match jets

tagged

no-match

For he light quark jets the no-match distributions look more like gluon jets.

The b-jets moments are different than the no-match moments at low PT. The phi moments look the same.

Maybe there is some bias in the eta moments for tagged jets at low PT

no-match

Moments-vs- PT(jet)

The Perugia0 tunes have more no-match jets, hence more gluons in the light quark sample (includes c quarks). It is harder to see a difference between the b jets and the gluon jets (right plots)

match

no match

no match

tagged

Note: the July 1 plots were wrong

Eta moments no-tag

b-jets, no tag requirements:

the matching and non matching jets show the same behavior.

This tells that distinguishing between b-jets and gluon jets from the moments is very difficult.

New variables should be investigated, such as N(charged)

Tagged b-jets eta moments

This shows that the eta of the tagged jets is what we expect. The eta moments, as seen in the previous plots, for the no match-jets are larger.

The tagged no match-jets jets have a very similar distribution.

Studies of non-matching jets properties

Previous studies have shown that the new tunes (S0-Perugia0) have more events for which the jets we see do not match the partons.

Also we see a difference in the N(charged particle in jets)

Need to understand better these differences

S0-Perugia0:

More non matching events

More N(tight) jets

Non matching jets have more charged particles

V6.2 (tune A) 68%

V6.4 ACR 68%

V6.4 NCR-Pg0 59%

V6.4 S0-Pg0 59%

Perugia0 :

Less matching

More N(tight)

Less b-jets

in 4 jet sample

Match-nomatch Ncharged

W-jets: the no-match have more nch than the match

b-jets : the no match have less then the match

V 6.2 V6.2 ACR(pro) S0Pg0

N(wjet) = 7.48, no-match = 7.99, 8.18, 8.30

N(bjet) = 9.91 7.76 7.89 8.06

the no-match have about nch=8 tracks

Studies on N(charged) in jets

We have seen earlier that the no-match-jets have more charged particles.

For the light quarks it is easy to explain, as we have concluded that the jet shapes show that the no-match jets have a large component of gluons.

What about the b-jets? Jet shapes cannot distinguish between b-jets and gluons. Are there other variables to look at/

Does N(charged) help?

This plot shows the no-match jets that are tagged. Very few events that are consistent with the mismatch rate.

It is intesting to see that the tune S0-Perugia0 has more charge particles than V6.2 and ACR-pro

What we have learned

The S0-Perugia 0 events differ from the PYTHIA V6.2 events in a number of ways:

Events have more tight jets

About 58% of the events have non-matching jets (compared with 68%)

Events have less b-tagged jets in the N(tight)=4 sample

Jet Properties:

The matching jets (light quarks and b quarks) have different energy in the cone of 0.4 then PYTHIA V6.2. This is at the origin of a large top mass shift

The jet shapes of the non-matching jets in light quarks have a very large contamination of gluon jets

For b-jets the moments cannot distinguish between b-jets and gluon jets

The no-matching jets have more N(charged) particles in the jet

Looking at this variable to see if it can help separating gluon jets from b-jets

Top Mass Measurement and CR

Backup slides

Comparison: E in cone of 0.4

S0Pg0 (S0Perugia0) compared with our default (V6.2 tune A)

S0Pg0 -Nominal

 E (cone) GeV

W-jets +0.30 ± 0.15

b-jets -0.40 ± 0.15

The S0-Perugia0 tune has different behavior for the b-jets

S0-Perugia0 W and Top mass shifts

M=175 GeV Mw

V6.4 NCR-Pg0 +1.07

V6.4 S0-Pg0 +1.00

M=175 GeV Mtop

V6.4 NCR-Pg0 +0.33

V6.4 S0-Pg0 +0.33

The S0- Perugia0 and corresponding NOCR have a small top mass shift

Summary of Matching and ME fits

We find the following CR values from the “pro” tune files: CR--> -0.41  0.37 GeV from ACR (-0.4  0.3 GeV, Winter Conf)

CR --> -0.34  0.38 from S0-NOCR, CR --> +0.15  0.45 from the Perugia0 tunes

The S0-pro (S0Pg0) tune gives Mtop = -1.8 GeV (-1.5 GeV) this is directly related to different jet and events shapes, due to a different p-shower

Tune S0 tunes include systematics that we are already taking into account ,i.e. generator : (mt) = 0.51 +- 0.37 GeV ISR/FSR : (mt) = 0.29 +- 0.26 GeV OOC : (mt) = 0.52 GeV b-jets : (mt) = 0.38 GeV that is 0.88 GeV, most of the MC related systematics.

More comparison of the S0 tune with Tevatron data needs to be done URGENTLY. We need to disentangle the various contributions

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.

Jet Shapes Studies

Comparison of jet variables MC- Data

Continuing studies on color reconnection systematics require the understanding of jets from PYTHIA V6.4

We have looked at jet variables and compare them to jets in top data. Only b-tagged jets are considered in this comparison.

There are 698 jets tagged by our secondary vertex algorithm.

N(events)= 578 with the topology lepton+4 jets (PT>20 GeV) .

N(background)= 134  34 events.

Variables:

Number of charged particles

Eta moments

Phi moments

We have many histograms. I only show a few of them.

Procedures

A. Given a MC sample, for each event we match the partons from top decays to the observed jets (Ntight = 4). This is match of the 4 partons to the 4 jets in the event.

We then know which jet are light quark jets and which ones are

b-jets.

To check the changes between MC's we compare a number of variables for the different tunings, for example:

Compare E(parton) and E(jet) in cone of R=0.4

Compare 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.

For methods A and B, we compare results obtained for V6.2(tune A) old MC (used for CDF measurements ) V6.4 (tune ACR) only CR added to old shower V6.4 (tune NOCR, S0) new shower, wo/w CR

Use the l+jets sample: events with 1 lepton + 4 jets (Et>20 GeV)

Summary of studies on Mtop

Comparison of V6.2 (nominal) to V6.4 (the “pro” files)

Using both methods, i.e., reconstructing top mass with event matching and with our ME method.

ACR (old shower+CR) shows little effect from CR = -0.41  0.37 GeV

NOCR: Event matching finds large MW, ME fit compensates for this with a large value of JES, resulting in Mtop = -1.5 GeV . For JES= 0 we get Mtop = -0.7  0.2 GeV

Color Reconnection Systematic

S0-pro : Mtop = -1.8 GeV, expected because of -1.3 GeV b-jet shift.

S0-Perugia0 : the light quark jets are more shifted then the b-jets. This shifts the W mass considerably (~ 1GeV). The top mass goes up for this reason. The ME fit gets a large DJES to reconstruct the W mass properly, this moves the jets down resulting in a large DMtop

NOSR-Pg0: same as above .

Bottom line: what is the CR systematics? From ACR (pro)-A(pro) CR = -0.41  0.37 From S0(pro) and NOCR CR = -0.34 0.38 From the Perugia0 tunes CR = +0.15 0.45

More statistics will help. At this point it seems that CR ~0.5 GeV

Color Reconnection Systematics

Strong color correlations between the hard process and the underlying event are implied by tune A and similar tunes. These effects may be interpreted as sign for color reconnection.

The issue has been studied at LEP for the W mass measurement

CR effects on the MW

measurement at LEP

contribute to systematics

CR(sys) = 8 MeV

out of 22 MeV (total sys)

(LEPEWWG hep-ex/061203)

Preliminary MC studies have

indicated possible contributions

to the top mass

systematics of order

CR(sys) 0.5 GeV

D. Wicke and P. Skands arXiv:0807.3248V1

LEP

Tevatron

Jet Shapes:moments

The eta and phi moments are sensitive to the width of the parton shower. We use calorimeter (both electromagnetic and hadronic components) information to evaluate the moments

We sum over all towers:

And similar expression for the phi moments,

Used in CDF to distinguish quark jets from gluon jets in ttbar production in the 6 jets topology

2002-04-21T14:25:46