The Top Quark Robert Roser FNAL CTEQ Summer School -- 2002 An experimentalists point of view… CDF.

The Top Quark

Robert Roser

FNAL

CTEQ Summer School -- 2002

An experimentalists point of view…

CDF

2An Experimentalists View of the Top Quark, R. Roser

This is only a cartoon….


Setting the Stage….

1977 discovery of bottom quark Standard Model calls for partner

- top quark The search was on!!!

The Talk will…1. Review what the standard model

tells us we should expect

2. Describe how we study them

3. Discuss have we determined experimentally thus far…

4. Take a peak into Run II DØ


Why Study Top?


Pair Production: pp tt

CDF + D0 in Run I each:

over 5 x 1012 collisions ~500 top events were produced in each

detector

Top Production at the Tevatron

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-1pb 4 105 dt L


Top Lifetime and Decay (SM) Top lifetime

top ~ 1/ M3

top~10 -24 secqcd ~ -1 ~10 -23 sec

BR(t Wb)

Branching ratios for tt decay modes

the top quark does not hadronize. It decays as a free quark!

e-e (1/81)

mu-mu (1/81)

tau-tau (1/81)

e -mu (2/81)

e -tau (2/81)

mu-tau (2/81)

e+jets (12/81)

mu+jets(12/81)

tau+jets(12/81)

jets (36/81)

Lepton+jets

Dileptons

All Jets


Decay Channels

l+

l-

b

b

b

l-

b q q

b

b

q

q

q

q

Dilepton Both W’s decay leptonically final state: llbb

Lepton + Jets One W’s decays leptonically final state: lqq bb

All-Hadronic Both W’s decay hadronically final state: qqqq bb


Finding Top is Hard

Proton anti-proton bunches cross 300,000 times a second

Assume top mass = 175 GeV Standard Model predicts a top

anti-top pair created once every 10 billion collisions

From 1992-96 CDF saw >5 trillion collisions so expect ~500 top quark pairs to be produced

The problem is identifying them!

Harder even than for a Chicago baseball team to win a pennant!


The life of an experimentalist Our “camera” is not fast enough to take a picture of a top

quark! We have to infer!

What do we know? Conservation of Energy Conservation of Momentum E=mc2

What do we want to identify? Electrons Muons Quarks Neutrinos b quarks


Colliding Beam Detectors Detector design is always a compromise between $$$, available

space, technological risk, readout and construction time Goal is to completely surround collision with detectors Arrange different types of detectors in layers

Measure particle’s position, momentum and charge first Type and kinetic energy second

Starting from center moving outwards: Tracking detectors within a magnetic field Electromagnetic calorimeter Hadronic calorimeter Muon chambers

CDF II Detector cross section


Particle Signatures Electrons - deposit all their energy in electromagnetic

calorimeter which can be matched to a track Photons - no track

Muons: Match signal in muon chambers to track


Particle Signatures (2) Quarks - fragment into many particles to form a jet

Leave energy in both calorimeters

Neutrinos - pass through all material Measured indirectly by imbalance of

transverse energy in calorimeters


b , b c l l

b vertex

Charged Particles

Secondary Vertex

Primary Vertex

Impact Parameter

Lxy

Finding b’s

b-quark lifetime c ~ 450mm b hadrons travel Lxy ~ 3

mm before decay

Secondary VerteX Tagging

((SVXSVX)) ~ ~ 50%50%

Identify semileptonic B semileptonic B decaydecay

Soft Lepton Tagging

((SSoftoftLLeptoneptonTTaggingagging)) ~ ~ 20%20%


Strategy for Discovering the Top Quark Look for collisions with a final state that matches a t-

tbar final state. If we find an event, is it a top quark?

Maybe yes…… maybe no Sometimes events which do not contain a top quark

will mimic the final state of a t-tbar event. (background)

Estimate how many background events you expect given the size of your data sample and analysis cuts

Count how many data events we observe and compare that to the number of background expected


Lepton + Jets Selection Signature:

one central and isolated high PT e or

missing ET from the

3 or more jets

Dominant Backgrounds: Non W background (fake leptons) pp W + Jets

Signal region is: S/B

Reduce the background fraction by “b-tagging”

top events always contain b jets, W+jets events usually do not

b

l-

b q q

W+ 3 jets Secondary VerteX Tagging

((SVXSVX)) ~ ~ 50%50% tag b-quarks using displaced vertex

Identify semileptonic B decay semileptonic B decay

((SSoftoftLLeptoneptonTTaggingagging)) ~ ~ 20%20% tag b-quarks using semileptonic

decays

GeV) 20(PT

GeV) 20E( T GeV) 15(E jet

T


Backgrounds

What types of processes can mimic these top events?

1. P P W- e- (no jets)

2. P P W- g d u u e- W+ 4 jets(No b’s)

3. P P W- g d b b e- W+ 4 jets(Looks like t t decay)


Silicon Vertex Detectors Work (in a hadron

collider)!


Lepton+jet cross sections

Sample Observed Background tt (pb)

b l X(SLT)

40 22.6±2.8 9.2 +4.3-3.6

Displaced Vertex (SVX)

34 9.2±1.5 5.1 +1.7-1.4

Sample Observed Background tt (pb)

b l X(SLT)

40 22.6±2.8 9.2 +4.3-3.6

Displaced Vertex (SVX)

34 9.2±1.5 5.1 +1.7-1.4

Control

Region

Region

Signal

Background


tt Production Cross Section

CDF

Measurement of the cross section is primarily a “counting experiment”

1.71.4-6.5 )t(t 1.7.95 )t(t pb

pb

DØ

LA

NNtt bkgobs

)(


Check #1 – Are we tagging b’s?

Top events have 2 b quarks. Check that jets tagged as b’s

look like b’s. C distribution for jets with a

secondary vertex (SVX b-tag) in the W +3 jet sample.

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l-

b q q

b

W

C of b-Tagged Jets


Check #2 – Are we seeing W’s?

Use loose b-tagging (Jet Probability) algorithm to find additional double tagged events

It’s natural to try to reconstruct the invariant MJJ mass in case of double tagged events ( (no W mass region cut))

M GeV/cW 2 77 2 4 6. . ( )stat syst

l-

b q q

b

W

+jets W Mass in the top sample


Properties of the Top Quarkor

Are you convinced yet?Is this object consistent with the Standard Model? What is the mass of the top quark? How often is it produced? How does it decay? (branching ratios)

How often is the final state dileptons How often is the final state lepton+jets How often is the quark in the top quark decay a b? Are there any rare decay modes? ( t Z c)

Exotic Physics IS there something not predicted by the Standard Model Search for a new particle X t t


Top Quark Mass

One of the first and most important measurements is the mass.

One of the principle goals in Run 1 was: to determine the top mass in each of the decay channels Check to see that the excess observed in each decay channel

is coming from a single object Different decay channels have different systematic

uncertainties in the measurement Seek precision!

CDF and D0 have invested a tremendous amount of effort in this measurement. As a result the uncertainty has improved from ~10% to ~4%


Top Quark Mass

How do we measure the mass?

It is the same for top events with the following complication

1. t1 Wb jet – jet – jet

2. t2 Wb lep – – jet

Each event has 2 top quarks Two chances to measure its mass in each event We don’t know which decay products belong to which

object


W+

W-

t t

b-jet

b-jet

jet

jet

X

+jets Event Reconstruction

Use 2C constrained fitting technique with the following constraints M = MW

Mjj = MW

Mt1 = Mt2

24 combinations: 12 correspond to the jet parton

match every combination has 2

solutions for neutrino PZ

Take lowest 2 combination with 2 < 10

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Event Reconstruction on Top Events Distribution of reconstructed

MC top events with at least 1 b-tag

30% correct jet assignment (solid)

20% correct jets but wrong

combination (cross-hatch)

50% mismatch between parton and its jet (hashed)

Most events have extra jets from gluon radiation

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Note: While the proper parton assignments affects the width of the mass distribution, it does not shift the mean.


Mass Fitting Method Reconstructed top masses

from data are compared to parameterized templates of top and background MC

Use a continuous likelihood method to extract top mass and statistical uncertainty

Mtop is the minimum of the

log-likelihood distribution

top corresponds to a

change of 0.5 units in the log-likelihood

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An example of parameterized templates


+jets Top Mass

Subsample Events S/B Mtop

±Mstat

GeV/c2

SVX doubleSVX singleSLTNo tag

5151442

25/17.5/12.5/11/1

170.1 ± 9.3178.0 ± 7.9142.0 -14

+33

180.8 ± 9.0

Combined 76 175.8 ±4.8

To get additional precision, Split data into 4 exclusive sub-samples with different S/B ratios

SVX double tagged SVX single tagged SLT without SVX tags eventsnon-tagged events (ET(jet4)>15 GeV)


+jets Top Mass Multiply the likelihoods and

combine the result from the 4 samples together

Systematic uncertainties are:

2 top c/GeV )(3.5)(8.40.176M syststat

Source Value (GeV/c2)Jet Energy Scale 4.4Initial State Rad. 1.3Final State Rad. 2.2Bckg Spectrum 1.3b-tag bias 0.4PDF 0.3MC Generator 0.1Total 5.3


Top Quark Mass A variety of methods are used

to measure the mass of the top quark. Lepton+Jets channel uses a

constrained fitting technique. Because of the two neutrinos in

the final state, the dilepton channel is underconstrained. A weighting procedure is used to

find “most probable” mass.

2GeV/c 1.53.174 topM

Phy Rev Lett. 79, 1992 (1997)Phy Rev Lett. 80, 2767 (1998)Phy Rev Lett 82, 271 (1999)

Phy Rev Lett 80, 2063 (1998)Phy Rev D 58, 052001 (1998)


Higgs Boson Constraint from Mw and Mtop

A tough way to make a living….


Mt-tbar in lepton+jets Studies of the t-tbar invariant mass

spectrum provide a general search for heavy objects decaying to top pairs

Some Technicolor theories predict existence of heavy objects that decay to t t pairs. Top gluons and a Z’ in topcolor

assisted Tecnnicolor Mttbar is the same as Mtop with the

following exceptions: 2 cut is loosened to 50. 3C fit (Constrain Mtop=175) To guard against wrong

combinations, remove the constraint and calculate the 3 body mass. It must lie between 150 200 GeV/c2.

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No evidence for a deviation from expectation. KS Prob. ~20%


Search for Resonances

Assume data contains only Z’tt, s.m. tt, and QCD W+jets.

-

Perform binned maximum likelihood fit using Mtt , W+jets, and Z’tt templates.

95% CL limits obtained by mapping the likelihood as a function of Mx

Systematics include: jet ET, MTOP, gluon radiation, b-tag, bkg shape, acceptance, and luminosity.

A leptophobic Topcolor Z’ with natural width 0.012M (0.04M) is excluded by the data for masses lower than 480 (780) GeV/c2.

t-tbar invariant mass spectrum can be used to set limits on X tt, for example, on the narrow resonance Z’ in topcolor models


Top Pt Distribution

Extended technicolor predicts deviations in top quark production variables such as PT.

Lepton+jets MTOP event selection

Require Njet154 or NTAG 1

Perform Top Mass kinematic Fit w/MTOP constrained to 175 GeV/c2

and 2 <10. Observe 61 events w/expected

background of 24 5.8 Due to the correlations of the 2

top quarks in the event, only the hadronic side of top PT is measured.

One can also look at the top quark transverse momentum distribution for indications of new ttbar production mechanisms.


Top Pt Spectrum Unfold resolution smearing and correct for

acceptance as a function of PT.

4 bins of “true top PT” are used: Determine response functions for signal and

background from MC + detector sim.

Perform unbinned maximum Likelihood Fit to response functions.

Systematic uncertainties : MTOP, jet ET scale, gluon radiation, background shape, and acceptance.

Define ratios Ri which represent the fraction of events in given PT range

R1: 0 < PT < 75 GeV/c

R2: 75 < PT < 150 GeV/c

R3: 150 < PT < 225 GeV/c

R4: 225 < PT < 300 GeV/c

0.025) :(SM C.L. 95%at 16.0

0.84) :(SM 07.017.066.0

4

21

R

RR


Search for Rare Decays Good region to search for non-Standard Model physics Two FCNC t-decays, t Zq and t q (q=u or c), were

investigated Within SM, these decays are suppressed ~ 10-810-12

t Zq : search for events where at least one top decays via this mode, tt Wb Zq observe: 1 event (Z +4jets) background: 0.6from WW/ZZ+jets

t q : search for events where at least one top decays via this mode, tt Wb q

observe: 1 event ( l, ET and 2 jets)

background: 0.5 (W l) and 0.5 (W qq)

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Br(t q) at c.l. 32% 95%.Br(t q) at c.l. 32% 95%.

Br(t q) at c.l. 33% 95%Br(t q) at c.l. 33% 95% ,Z


Single Top Production

The weak interaction is used to produce the top quark (mainly through W* and W-gluon fusion at 2 TeV)

The cross section for these two processes are proportional to |Vtb|2 so one has a way of directly measuring this CKM matrix element without any assumptions about the number of generations

Main Problem: Although single top is produced at roughly ½ that of t-tbar at the Tevatron, the signal is in the W+2 jets sample. (compared to the W=4 jets for the lepton+jets decay of ttbar. Consequently, QCD background swamps the signal!

Single Top Production was not observed in Run 1


Single Top Production

Event Topologies W-gluon fusion: = 1.700.1 pb

(Stelzer 1998) One high-Et b jet (from top) One soft b jet (possibly missing) High Et light quark jet W decay products

W* Production: = 0.730.04 pb (Smith 1996) 2 hard b-jets W decay products

Backgrounds include: Wbb, Wcc, Wc, mistags, and tt production.


Single Top using HT Distribution

Given the low statistics of Run 1, we search for the combined processes.

Event Selection: Good W+1,2,3 jet events At least 1 jet SVX b-tagged 140 < Mlb< 210 GeV/c2

65 Events Observed Background from t-tbar

production and QCD background. Est. 62.4 +/- 11.5 Events

Signal (W* and Wg) Est. 4.3 Events

jets) all and ,E lepton,over (summed H TT TE


Results on Single Top Production with HT

Perform an unbinned Likelihood fit to the HT distribution to extract the 95% CL limit on single top production.

95% CL limit on single top production (Wg + W*):

pb 5.13

Still 6 times above SM expectation


Search for Wg Pseudo-rapidity () of the

non-tagged (light quark) jet tends to be positive for top and negative for antitop.

Assume + (-) charged lepton is from top (tbar).

Q * distribution is asymmetric for signal events.

Signal


Limits on Wg->top

After selection cuts : Observe 15 events Expected Total Bkg 12.9 2.1 Expected signal 1.2 0.3

Binned maximum likelihood fit using Q distribution.

Fitted Signal: 1.4 +4.2

- 3.4 w/ bkg constraint

0.0 +6.7 - 0.0 w/out

Systematic uncertainties of MC gen, gluon radiation, PDF, and acceptance taken into account.

Extract an upper limit: W-

gluon < 15.4 pb at 95% c.l.


W Helicity

Verify that top decays to a spin 1 particle! The 0 helicity (longitudinal) mode of the W couples to fermion mass, so top

quark decays provide the only means for its study. Fraction of longitudinal W’s(tree level, top rest frame):

F+= 0 in SM Information on W helicity can be obtained from lepton Pt spectrum

70.0

21

2

)0()1(

)0(

2

2

2

2

0

W

t

W

t

ww

w

Mm

Mm

hh

hF


W Helicity

Lepton pT distributions in tbl distinguish the two helicity states.

Lepton pT discriminates Better measured than angular

correlations Unaffected by combinatorics or

reconstruction Higher Statistics by using 3 jet

events and dilepton events


W Helicity Backgrounds include W+jets, fake

leptons, HF production, Z, and WW.

Unbinned maximum likelihood fit to the MC predicted expectations for longitudinal (70% in the S.M.), left and background

F+1 determined by repeating fit with F0 constrained to the SM value of 0.70

F+1=0.11 0.15 (stat) 0.06 (sys)F0 = 0.91 0.37 (stat) 0.12 (sys)


Results of W Helicity Cont.

The measured fraction of longitudinally polarized W’s is not very compelling at the moment.

The table of systematics to the right indicate that this will be an interesting measurement in Run 2.

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A Sample of CDF Run I Results

c.l. %95at %33q)Br(t Z c.l. %95at %33q)Br(t Z

c.l. %95at %2.3q)Br(t c.l. %95at %2.3q)Br(t

Br(t eX or t X) 0 094 0 024. .Br(t eX or t X) 0 094 0 024. .

fusiongluon -for W

c.l. %95 at pb 13CDF topsingle

fusiongluon -for W

c.l. %95 at pb 13CDF topsingle

39.091.0Fo 39.091.0Fo

c.l. 95%at 75.0Vtb c.l. 95%at 75.0Vtb

pb 6.5 1.71.4 -

CDFtt

pb 6.5 1.71.4 -

CDFtt

GeV 6.5 176.0 topM GeV 6.5 176.0 topM

Z'

Z'

0.012M for

c.l. %95at GeV 480M

Z'

Z'

0.012M for

c.l. %95at GeV 480M

Z'

Z'

0.04M for

c.l. %95at GeV 780M

Z'

Z'

0.04M for

c.l. %95at GeV 780M

channel for W

c.l. %95 at pb 18*

CDF topsingle

channel for W

c.l. %95 at pb 18*

CDF topsingle


Fermilab Tevatron Collider

1992-96

Run I: 100 pb-1,1.8TeV

1996-2001

Major detector upgrades

2001-??? now

Run IIa: 2 fb-1, 1.96 TeV

Run IIb: 15 fb-1

Main Injector(new)

Tevatron

DØCDF

Chicago

p source

Booster

CDF

Wrigley Field


Run 1 Run 2

subprocess s

Numberof

Events

Run 2Run 1

Increased reach for discovery physicsat highest masses

Huge statistics for precision physicsat low mass scales

Formerly rare processesbecome high statisticsprocesses

Upgraded Tevatron and detector mean:

Extend the third orthogonal axis:the breadth of our capabilities


Tevatron Collider Upgrades Original Tevatron Design:

1030 cm-2 s-1

Run I (ended Feb 1996) Lum > 1031 cm-2 s-1

CDF/D0 integrated ~100 pb-1 ea.

Run II Upgrades: Main Injector (factor of ~5)

Initial Goal: 1032 cm-2 s-1

Recycler (factor of ~2) 2x 1011 antiprotons/hour 3x 1012 antiprotons/bunch Re-cool antiprotons from the

Tevatron

Later Electron cooling Crossing angle?

Bunches Initially 36x36 at 396 ns Ultimately(?) 141x121 at 132ns

s = 2 TeV (was 1.8 TeV)


Accelerator Status Goal: Reach luminosity of 9E31 by December.

Currently peak luminosity is 2E31 Problems with pbar efficiency into Tevatron and beam

stability interference between p’s and pbar’s? (very unexpected) Have requested 2 days/week for Tevatron studies

Two shutdowns scheduled, 2 week shutdown in progress and 6 week shutdown starting Oct.1

Integrated Luminosity Goal: 300 pb-1 by January ‘03

Bottom line… Disappointing thus far but they are starting to make real

progress Hope that cooling tanks will give 2-4 increase in luminosity Once the beam fits in the machine, more options are

available


Improved Detector Means...

Primary lepton (W->lnu) acceptance increases ~33% for electrons , ~15% for muons

B-tagging efficiency will increase by ~60% for single tags ( ~ 65%)

~200% for double tags ( ~ 20%)

Impact on Physics Data Samples: ~1000 SVX tagged W+3 jet events

(34 in Run 1) ~300 SVX double-tagged events

(8 in Run 1) ~150 Dilepton events (9 in Run 1)

Layer 00 increases the number of observed displaced tracks and hence b tagging and flavor tagging are improved.


What can be done in Run 2? Assumptions:

Statistical uncertainties scale as 1/N Backgrounds scale with luminosity and increase in acceptance to what was

determined in Run 1 Systematic uncertainties measured from data scale by 1/N Thus, we should

expect a factor of 4.7 for 2 fb-1 compared to the 90 nb-1 in 1b . Seems a bit optimistic -- we did NOT gain a factor of 2 from run 1a to 1b if the

above rule follows.

B-tagging Efficiency: Use top data samples (In Run I, MC was used)

Dilepton events Ratio of single to double tags in lepton + jets events

L00 and new vertexing algorithms Distinguish b’s from charm Use c distribution of tagged jets

Obtain Top Cross Section precision of 9% by Improve luminosity measurement

Use W l rate or # interactions/crossing to get 5% precision


SVX Cross Section Example Tagging efficiency can be measured in dilepton events.

(estimated at 1/2 of 1b or 4%) b and c content can be measured using c-tau distributions

in the data (estimated we reduce bckg uncertainty by 50%) Acceptance could be reduced by looking at Z+jets (from 9%

to 6%)

Assumption Fractional Uncert

Run 1b 16%

1/(lum) 7%

“Reasonable” 11%


CDF Data

CDF Run I L+Jets Top Mass

Source Value (GeV/c2)Jet Energy Scale 4.4Initial State Rad. 1.3Final State Rad. 2.2Bckg Spectrum 1.3b-tag bias 0.4PDF 0.3MC Generator 0.1Total 5.3

2GeV/c 5.60.176 Mtop

Measuring Mtop

Optimized Lepton + Jets Sample -

Largest systematics due to jet energy scale and gluon radiation

Run IIa Use only double-tagged events

250 events (5 events in Run I) 20% improvement on mass

resolution

Use large data samples to calibrate jet energy scales

Achieve 2-3 GeV precision


Mt Systematics in Run IIa Improve understanding of jets

Zee + 1 jet for jet balancing Wjj from b-tagged top events Zbb for b jet energy scale

Backgrounds Zee + 4 jets to understand W + 4 jets

Gluon Radiation Z + jets, W + jets and photon + jets

Biggest remaining problem Modeling of parton momenta observed jet


Top Physics Landscape with 2 fb-1

Measurement Precision

Top Mass 2-3 GeV/c2

(ttbar) 9%

(ll)/(l+j) 12%

B(t Wb) 2.8%

B(Wlongitudinal) 5.5%

Vtb 13%

B(t c) 2.8 X 10-3

B(t Zc) 1.3 X 10-2


What about a Light Higgs?

Precision measurements of top and W masses will severely constrain the mass of the Standard Model Higgs

Mw: CDF D ~ 35 MeV (2 fb-1/) ~ 20 MeV (10 fb-1/)

Mt: CDF or D 4 GeV (2 fb-1/) 2 GeV (10 fb-1/)


Summary The Run 1 Tevatron Collider program was very

successful Discovery of Top Quark A variety of “precision” measurements have been made The standard model has held up to scrutiny thus far…

The Tevatron, CDF and D0 have been upgraded Beginning to understand the detectors Getting data - Starting to make W and Z data samples

Now the fun starts! Only find top quarks at Fermilab until LHC Expect ~300 SVX double tagged events in Run IIa Precision top physics measurements

Mass to 2 GeV Cross section to 9%

The Top Quark Robert Roser FNAL CTEQ Summer School -- 2002 An experimentalists point of view… CDF.

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Transcript of The Top Quark Robert Roser FNAL CTEQ Summer School -- 2002 An experimentalists point of view… CDF.