CDF TOP ISSUES

This talk is about present efforts in CDF to understand a recent version of the Monte Carlo generator (PYTHIA V6.4), which includes the latest and fanciest parton shower model and color reconnection. All CDF top measurements have been done using PYTHIA V6.2.

How much are the measurements affected by the new version? This version is used by ATLAS top analyses and it is important to see if it fits the Tevatron data.

Outtline:

Motivation

How different is the new shower model?

How different are the jets?

What is the effect on the top mass measurement?

Status of the validation

CDF is studying top quark production and decay properties. The LBL group is working on a precision measurement of the top quark mass .

Motivation: Top Mass

2

Quantum loop corrections to many EWK observables are sensitive to the top mass

Top Mass is highly correlated to MW and MH in Standard Model EWK theory

~ Mt2 ~ log(MH)

EWK fits using 15 SM precision measurements give very large error on MT and MH. Direct Mtop measurement reduces uncertainty.

MW and Mtop measurements

Mtop = 172.2± 1.0 (stat.) ± 0.9 (JES) ± 1.0 (syst) GeV/c2 = 172.2 ± 1.7 GeV/c

LBL's is the best Tevatron mass measurement (uses 2.7 fb-1).

It contributes 46.1% towards the mass average.

(Paul Lujan, Jeremy Lys, Igor Volobouev, Jason Nielsen + LG)

MW= 80399 ± 25 MeV

EWK Fit: Winter 2008

Summer 2008 best Fit

MH =84+34-26 GeV/c2

and

MH< 154 GeV/c2 at 95% CL

Direct limit:

MH > 114 GeV at 95% CL

adding the direct limit

MH< 185 GeV/c2 at 95% CL

Need to reduce the uncertainties.

For Mt = +1.2(-1.2) GeV MH =+9 (-8)GeV

For MW= +25(-25) MeV MH= -13(+17)GeV

Summer Conferences EWK Fit, gives MH < 185 GeV/c2

Top Mass measurement

What ingredients in the measurement depend on the MC used? Can we include the effects from the new MC in the systematics?

Jet energy corrections and systematics

Parton shower uncertainties (pQCD)

ISR and FSR uncertainties (pQCD)

Hadronization uncertainties (non-perturbative)

t t → W+ b W - b

→ j1 j2 b l ν b

Jet energy scale uncertainties are the major contributors to the top mass systematics. To study this we use for

jets the variable

 JES = number of s.d. away

from the central value





Mtop Measurement (schematic)

Mtop = 172.2 ± 1.0 (stat.) ± 0.9 (JES) ± 1.0 (sys) GeV/c2 = 172.2 ± 1.7 GeV/c2

To evaluate ΔJES we use a 2D likelihood with Mt and ΔJES.

We “constrain” the W mass to the

measured value, using the 422 selected events (85 34 backg),

thus obtaining the ΔJES from data.

Also find DJES = (0.09  0.29)σ (statistics limited)

Mass and ΔJES Calibrations

Top Mass systematics

Meetings with MC experts to understand overlapping sys (when I was at CERN in April, I organized a couple of meetings there).

Meeting with D0, so that we do similar things to help with the mass

combination.

P. Skands (PYTHIA), D. Wicke have been evaluating the Color

Reconnection contribution (not included in our analysis) and written papers saying that it is an additional 1 GeV to be added to the 0.88 GeV (systematics coming from MC).

Tevatron has delivered 5.8 fb-1

CDF has recorded on tape 4.8 fb-1

Statistical error will get smaller, both

(stat) and (JES) uncertainties.

Measurement soon will be dominated by systematic uncertainties.

MC dependent systematics are in red

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

New PYTHIA

PYTHIA V6.4 versus V6.2

Our top mass measurements have been done using V6.2 (2003)

Color reconnection effects are included in PYTHIA V6.4.

Changes in V6.4

Parton shower uses pT ordering rather then Q2 ordering

ISR and FSR also uses a pT ordering algorithm

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

Model interleaves MPI process with ISR evolution off the hard process

New model for beam renmants, including baryon junctions

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.)

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

CDF Studies of CR Effects

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

generator (V6.4) with existing data. (see Perugia, Oct. 2008). Tuning of UE and MPI parameters not yet redone after the

inclusion of LEP data.

P. Skands is helping us generate events with the new tunes. We have already tried previous versions, also run the new

tunes done for Perugia. More work on their part is expected.

More comparison with Tevatron data is needed.

We are looking at other samples to see if the new model fits the data: dijet, gam+jet . Compare jet shapes with data.

In the mean time, we are pursuing studies relevant to the

top mass measurement.

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 needs work

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 new parton shower have:

wider 2 distributions

wider R “

Samples with new parton shower have:

larger

Samples with new parton shower have:

larger

Jets with new parton shower 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(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 with R = 0.4

What did we learn about jets?

The jets are wider in S0, i.e. less energy in a cone of 0.4 radius. The b-jets are shifted by 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)

New parton shower gives jet with less energy in cone of R=0.4

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” tune)

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

ACR (old shower+CR) shows very little effect from CR =-0.33 0. 27 GeV

NOCR: Event matching finds large MW, ME fit compensated for this with a large value of JES, resulting in Mtop = -1.28 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  0.41 GeV

Summary

We find the following CR values from the “pro” tune files: -0.33  0.27 GeV from ACR -0.45  0.41 from S0-NOCR, both consistent with zero, <0.45 GeV

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

Tune S0-pro 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 GeV b-jets : (mt) = 0.38 GeV that is 0.87 GeV, most of the MC related systematics.

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

L+jets:Sample Composition

In 1.9 fb-1 find 371 events

Estimated background:

70  17 events

But: are these events

only top+SM background?

Event Selection

Isolated lepton, PT > 20 GeV

MET > 20 GeV (neutrino)

N (jets): only 4 jets with ET >20 GeV

≥1 b-tag by the SVX algorithm

Background :

Mistag in W+light quarks

non-W QCD

Physics background: Wbb, Wcc

Single top, WW, WZ etc.

~85%

~15%

Top Mass Measurement ME(1)

measured quantities

Incoming partons

parton level quantities

normalization

acceptance

Transfer functions

For each event we evaluate a likelihood as a function of the top mass and ΔJES (related to the jets momenta measurements)

All possible jet permutations are included with weights = wi.

We integrate over phase space (d Φ) and Matrix Element (M) for t t production and decay.

T

24 Permutations

Top Mass: Transfer Functions (2)

The transfer functions for a given parton x, give the probability that we observe y. Detector effects, resolutions etc. are included

Both angular and PT transfer functions are used

Multiplied by efficiency for proper normalization

Transfer functions depend on jet mass as well as on PT (in  bins). Also they are evaluated for 25 values of JES.

PT ratio= PT(jet)/PT(q)

PT(q)=40 GeV, mjet=30 GeV

Top Mass : integration (3)

From 32 parameters in z1 +z2 = q q' b1 + lep  b2, assumptions on incoming partons, lepton masses, charged lepton P and energy-momentum conservation leave a 19-dimensional integration, performed by Quasi-Monte Carlo method.

Integration variables: M12 and M22 , the hadronic and leptonic top mass squared m12 and m22 , the hadronic and leptonic W mass squared  = log(q/q') , log of ratio of momenta of the two q from W PT(t t), priors from MC  (parton-jet) ,  (parton-jet) for each jet. Mass of each p-jet. All jet priors from MC

Tools: Jet Reconstruction (4)

Source of the largest uncertainty on the top mass measurement

Use calorimeter information only

Jet calibration done in many steps

3% systematics at high pT

Use cone algorithm

In Situ JES calibration (5)

Likelihood parameters are mt and JES

We shift each jet by the factor

JES = 1 + JES x JES(pT,)

where JES(pT,) is the systematic uncertainty on the jet pT

JES is determined using the decay

W  j1 j2

and using the measured value for the W mass

Precision on JES is determined by the statistics we have, thus a systematics uncertainty is now a statistical one

Top Mass: MC Calibration(6)

We use 12 mass point between 160 and 185 GeV/c2 to calibrate the method

Mmeas =( 0.953 0.009)  minput

m(172)=1.5 GeV/c2

Top Mass Results (7)

KS=0.85

Expected uncertainty distribution from MC. The arrow shows the uncertainty for the data sample (422 events). 49% of the pseudoexperimets are below the arrow.

The peak of the likelihood for each MC events compared with the distribution for the 494 events. We cut the likelihood at a value of 10 to reduce background and badly reconstructed events

Cross section for ttbar production

Background to many searches: Higgs 30%,

SUSY trilepton xx%

It is important to reduce systematics.

CDF average summer '08

7.0 ± 0.3 (stat.) ± 0.4 (syst)

~0.2 of the syst error comes from MC related uncertainties,

Contribution from the l+jets+SVX b-tag topology is 44%.

Any other measurements are affected?

MPI Models in Pythia 6.4

Old Model: Pythia 6.2 and Pythia 6.4

“Hard Interaction” + virtuality-ordered ISR + FSR

pT-ordered MPI: no ISR/FSR

Momentum and color explicitly conserved

Color connections: PARP(85:86)  1 in Rick Field’s Tunes

No explicit color reconnections

New Model: Pythia 6.4 and Pythia 8

“Hard Interaction” + pT-ordered ISR + FSR

pT-ordered MPI + pT-ordered ISR + FSR

ISR and FSR have dipole kinematics

“Interleaved” with evolution of hard interaction in one common sequence

Momentum, color, and flavor explicitly conserverd

Color connections: random or ordered

Toy Model of Color reconnections: “color annealing”

MPI create kinks on existing strings, rather than new strings

Hard System + MPI allowed to undergo color reconnections

Color Reconnection

Searched for at LEP

Major source of W mass uncertainty

Most aggressive scenarios excluded

But effect still largely uncertain Preconnect ~ 10%

Prompted by CDF data and Rick Field’s studies to reconsider.	What do we know?

Non-trivial initial QCD vacuum

A lot more colour flowing around, not least in the UE

String-string interactions? String coalescence?

Collective hadronization effects?

More prominent in hadron-hadron collisions?

What (else) is RHIC, Tevatron telling us?

Implications for precision measurements:Top mass? LHC?

OPAL, Phys.Lett.B453(1999)153 & OPAL, hep-ex0508062

Sjöstrand, Khoze, Phys.Rev.Lett.72(1994)28 & Z. Phys.C62(1994)281 + more …

Existing models only for WW  a new toy model for all final states: colour annealing

Attempts to minimize total area of strings in space-time (similar to Uppsala GAL)

Improves description of minimum-bias collisions

PS, Wicke EPJC52(2007)133 ;

Preliminary finding Delta(mtop) ~ 0.5 GeV

Now being studied by Tevatron top mass groups

Slide from P. Skands' talk, Perugia MC workshop, October 2008

Color reconnection in PYTHIA V6.4

2002-04-21T14:25:46