Update on CR Systematics

There is a lot of activity from the PYTHIA team to tune the new

generator (V6.4) with existing data. (see Perugia, Oct. 2008).

Used LEP data :event shapes, fragmentation functions and flavor spectra.

The flavor and hadronization parameters are tuned for the new PT ordered shower. New UE and MPI model parameters not tuned. (Done for old Q 2 ordered parton shower).

Some comparison to Tevatron data was shown pT(Z) and single jet distributions. The tuning is going on. The parameters are not stable, it is a moving target!

Nathan (thank you!) has generated ttbar events with the Perugia tunes, called “pro”. We look at these today.

Compare V6.216 (tune A) and V6.416 (S0, tune ACR, NOCR)

The CDF top group has been generating samples with PYTHIA V6.4 to

evaluate the effects on the new parton shower, UE and CR models.

Look at jets in the top sample to compare them with the V6.2 MC

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.

Apply the top mass machinery to evaluate mass shifts obtained with the new samples.

The CDF top group has been generating samples with PYTHIA V6.4 to

evaluate the effects on the new parton shower, UE and CR models. Two types

of studies are being done:

Look at jets in the top sample to compare them with the V6.2 MC

applying the top mass machinary to evaluate mass shifts obtained with the new samples.

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

Samples with new parton shower have:

larger

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

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 cone R=0.4

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:

E(V6.12) -E(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



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, E and jet mass 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

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. generator: (mt) = 0.51  0.37 GeV ISR/FSR: (mt) = 0.29  0.26 GeV OOC : (mt) = 0.52 b-jets : (mt) = 0.38 that is, most MC related systematics (0.87 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.

Top Mass Measurement and CR

Backup slides

Monte Carlo Issues: PT(ttbar)

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)

Discussions with the PYTHIA authors were motivated by the disagreement of the pT(ttbar) distribution between PYTHIA and HERWIG

Solution: PYTHIA V6.4, tune S0, gives a correct

pT(ttbar) distribution. However, V6.4 includes color

reconnection (CR) effects, not present in V6.2.

CR effects at LEP, W mass

CR at the Tevatron

Systematics

on top mass

can be as large as 1 GeV

Some Tevatron plots with new tunes

From Hendrix Hoeth talk (Perugia 2008). New tunes (called professor from tools used) use LEP data: event shapes, fragmentation functions and flavor spectra. The flavor and hadronization parameters are tuned for the new PT ordered shower. New UE and MPI model not tuned. (Done for old Q 2 ordered parton shower).

Some comparison with Tevatron data has been shown at Perugia

Tune S0 (used by CDF), Tune Moraies (used by ATLAS).

Perugia Tunes

Not only more (charged particles), but each one is harder

Perugia tunes of new model, using Tevatron 630/1800/1960 GeV data

Average track pT as a function of multiplicity: sensitive probe of CR?

Used to fix CR strength parameter in tunes

Data from CDF, N. Moggi et al., 2008

From Peter Skands talk at Perugia

Mass systematics

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