Color Reconnection systematics (1)

Our analyses use the PYTHIA V6.2 (tune A) Monte Carlo generator to calibrate the methods used for the top mass measurements.

The latest version (PYTHIA V6.4) includes new models for the

parton shower, the Multiple Parton Interactions (MPI), ISR and FSR, the underlying event and the Color reconnection.

Tuning of the new generator to data is in progress.

(see Perugia MC meeting , October 2008)

We have looked at two recent tunes:

Tune ACR(pro) , which includes only the new color reconnection model, the rest is unchanged.

Tune S0(pro), which includes all of the above changes.

Color Reconnection systematics (2)

E = E(parton) – E(jet)R=0.4

= E(Out of Cone)= EOOC

We have compared some jets properties after generation + detector simulation

PT = jet PT(R=0.4) after simulation + reconstruc.

Mj = mass of jet (R=0.4) after sim+ reconstruc.

E OOC= E(Out of Cone of 0.4) after matching + simulation + reconstuction

Comparing the old tune A with the ACR (pro) and S0(pro) we find that the S0(pro) tune differs the most from the Nominal tune (tune A). The b-jets

are the most affected with a shift of 1.3 GeV

Color Reconnection systematics (3)

The S0(pro) tune differs the most from our Nominal sample. It seems that the b-jets are at the origin of the discrepancy. The tuning done so far did not specifically look at heavy flavor jets. This should be done next.

In addition, since S0(pro) uses new modeling for ISR/FSR, parton shower, UE and CR, we have to investigate possible overlaps with the systematic uncertainties we are now using.

At this stage of our studies we evaluate the CR systematics using the

ACR(pro) tune, that includes only changes in the CR model.

Applying the top mass measurement method (MTM3) to this sample we obtain a shift in top mass of

M(top) = -0.33  0.31 GeV/c2

Work is in progress to compare the b-jets in top events with MC.

Top Mass Measurement and CR

Studies shown at the CDF-D0 meeting on 01/14/09

Plots added after talk of 01/14/09 (1)

Compare ET5 for TuneA and Tune S0. Luminosity profile is the same for the MC plots (up to P8). Data gets to P17, so different Lumi profile

V6.2 MC: to P8

data: to P17

Data: P0-12 and

P13-17

Number of z-vertices increases with the luminosity. The MC we have been using for this analysis corresponds to that of P8. Comparison of low-lumi data with high-lumi data is shown at righr.

Make averages here

Plots added after talk of 01/14/09 (2)

Compare data: Periods 0-12 and Periods 13-17

Conclusions: for ET of jet 5 and Njet distributions, we need to use the same luminosity profile, as the number of additional z vertices affects these distributions. Note: jets on the two left plots have no cut-off. To be taken with a grain of salt!

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 correction)

To check the changes between the 2 MC's we do the following:

Compare Pt5/Pt(parton) and 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.

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)

PT(ttbar) and PT(top) at parton level

PT (ttbar) for the new shower tunes is wider as advertised, ACR is still close to the old distribution. Not clear if the parameters we use are correct

PT(top) is not affected much by the new modeling.

M=175 GeV

V6.2 (tune A)

V6.4 ACR

V6.4 NOCR

V6.4 S0

Comparison of matching

The whole event is matched using R for each parton-jet pair. An overall 2 is calculated, best 2 < 200 are accepted as matched

M=175 GeV

V6.2 (tune A) 68%

V6.4 ACR 68%

V6.4 NOCR 60%

V6.4 S0 59%

Samples with the newer tunes (NOCR and S0) have:

wider 2 distributions

wider R “

Samples with new parton shower have:

larger

Jets in NOSR and S0 tunes are more displaced from the partons.

Comparison of Jet PT(at L5)

M=175 GeV

V6.2 (tune A)

V6.4 ACR

V6.4 NOCR

V6.4 S0

PT(jet)/PT(parton) for jets in top events

Distributions for V6.4 tune S0 look a bit wider (PT(jet)/PT(parton) smaller) and shifted for the b-jets

M=175 GeV

V6.2 (tune A)

V6.4 ACR

V6.4 NOCR

V6.4 S0

E= E(parton)-E(jet in R=0.4)=EOOC

M=175 GeV

V6.2 (tune A)

V6.4 ACR

V6.4 NOCR

V6.4 S0

For the S0 tune, there is less energy in the cone of R = 0.4

Plots show

difference between the top curves:

EOOC(V6.12

V6.4 new tunes)

What did we learn about jets?

The jets are wider in S0, i.e. less energy in a cone of 0.4 radius. We get on the average b-jets with a shift of -1.3 GeV.

The ACR case has smaller effects than S0

The NOCR shows less visible effects than S0 (-0.58 GeV b-jet shift)

Tunes with the new parton shower give jets with less energy in cone of R =0.4

Largest discrepancies in the “pro” files found in the variable EOOCt

Comparing Jet mass

M=175 GeV

V6.2 (tune A)

V6.4 ACR

V6.4 NOCR

V6.4 S0

Jet masses are different as well

Summary of jet findings

Shifts in PT, jet mass and E in a cone of DR=0.4

(values in red are shifted by >2s)

For tune S0 we observe large shifts in the energy and mass of jets.

The b-jets seem to be more affected than the light quark jets. The energy in a cone of DR=0.4 is smaller by -1.3 GeV.

Data-MC comparison V6.2 and 6.4

CDF Data (494 events in 2.7 fb-1), not enough to distinguish!

PYTHIA V6.2

PYTHIA V6.4 (S0)

Highest ET jet: there is a 2 GeV difference between the two MC samples

Reconstructed W and top masses

MW is somewhat shifted . Mtop shifted for both the NOCR and the S0 samples

M=175 GeV

V6.2 (tune A)

V6.4 ACR

V6.4 NOCR

V6.4 S0

Using event matching we find:

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 our ME method.

ACR (old shower+CR) shows little effect from CR = -0.33  0.31 GeV

NOCR: Event matching finds large MW, ME fit compensated for this with a large value of JES, resulting in Mtop = -1.3 GeV

S0 : Mtop = -1.7 GeV, expected because of -1.3 GeV b-jet shift. comparing NOCR and S0, we find CR (sys)= -0.45  0.46 GeV

Summary

We find the following CR values : -0.33  0.31 GeV from ACR -0.45  0.46 GeV from S0-NOCR, consistent with zero, <0.46 GeV

The S0 tune gives Mtop = -1.7 GeV this is directly related to different jet shapes, i.e., different p-shower

Tune S0 includes systematics that we are already taking into account ,i.e. , for the MTM3 analysis: generator: (mt) = 0.51 GeV ISR/FSR: (mt) = 0.29 GeV OOC : (mt) = 0.28 GeV b-jets : (mt) = 0.38 GeV that is, most MC related systematics (0.75 GeV)

More comparison of the S0 tune with Tevatron data need to be done before we use it. We also need to disentangle the various sys 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.

Mass systematics for the MTM3 analysis

Residual JES is mostly due to OOC systematics

Without the calibration systematics, the MC Dependent sys are

0.87 GeV

2002-04-21T14:25:46