Measurement of the tt Cross Section in the Dilepton Channel at Fermilab’s Run II

Measurement of the tt Cross Section in the Dilepton Channel at

Fermilab’s Run II

David Goldstein

David Goldstein, UCLA - CERN EP Seminar 27/10/03 2

Why was top expected?

History of searches

Why study top?

CDF II Detector (b tagging, RASNIKs)

Cross-section definiton/goals for summer 2003 dilepton analysis Physics objects, acceptance increases Backgrounds Data events in Run II Cross-section results

Future directions for dilepton top analyses

Outline


Top quark was expected in the Standard Model (SM) of electroweak interactions as a partner of b quark in SU(2) doublet of weak isospin for the third generation of quarks. In each generation, complete doublet is required

for cancellation of [triangle/axial/chiral] anomaly.• Without cancellation, SM is not renormalizable

or unitary.

The role of top in the SM

Zaxial

f

f

fgfa


(1964) Cronin & Fitch discover CP violation…3x3 CKM matrix suggested a top quark…

b quark (5 GeV !) discovered in 1977 at Fermilab

(1979) Searches at PETRA e+e- collider (ps up to ~47 GeV) (1981) SppS hadron collider turns on (√s = 630 GeV): (1984):

PETRA Mtop > 23.3 GeV

UA1 evidence for top Mtop » 40 GeV ! (based on ~200 nb-1)

(1987) TRISTAN e+e- collider turns on (√s up to ~60 GeV) (1988) Tevatron hadron collider turns on (√s 1800 GeV)

(1988): UA1 subsequent results: Mtop > ~60 GeV (~700 nb-1)

Backgrounds very important to get right ! CDF Mtop > 77 GeV

(1989) SLC & LEP turn on (√s at Z pole)

History


(1990): TRISTAN Mtop > 30.2 GeV CDF Mtop > 85 GeV UA2 Mtop > 69 GeV

(1992): CDF Mtop > 91 GeV

(1993): SLC/LEP indirect evidence Mtop even

higher (130 GeV +)

(1994): D0 Mtop > 131 GeV, CDF silent… CDF & D0 evidence for top in 1994

(1995): Discovery! Top finally found (175 GeV); mass near electroweak scale, ~40 times

heavier than the b quark!

History

16 years, 5 colliders, 7 collaborations, 103 physicists !


All hadronic (BR: 36/81 , 44%) final state: 6 jets (QCD

background very high).

Lepton+jets [e,μ] (BR: 24/81, 30%) final state: 4 jets, 1 lepton,

missing transverse energy.

Dilepton [e,μ] (BR: 4/81 , 5%) final state: 2 jets, 2 leptons,

missing transverse energy “Golden mode”.

top decay modes

‘s produce missing transverse energy in the detector

angle


vs. missing energy plot from Run I dilepton analysis intriguing…

Short lifetime and high mass of top (relative to b quark) will allow for precise tests of top CKM values, W polarization studies.

Motivation for top studies


Motivation for top studies

CDF/D02 fb-1goal!

CDF/D02 fb-1goal!


The Tevatron is a proton-antiproton collider with 980 GeV/beam

36 p and p bunches 396 ns between bunch crossing Increased from 6x6 bunches with

3.5s in Run I

Increased instantaneous luminosity: Run II goal 30 x 1031 cm–2 s-1

Current: 3 - 4.5 x 1031 cm–2 s-1

(Run I peak inst. lum. ~2 cm-2 s-1)

Tevatron collider in Run II

RunI)(1.8TeV RunII in1.96TeV s


Tevatron collider in Run II

March 2001 March 2002 March 2003


The upgraded CDF detector

New bigger silicon, new drift chamber.

Upgraded calorimeter, muon coverage.

Upgraded DAQ/trigger, esp. displaced track trigger.


CDF silicon detectors & b tagging

Silicon Vertex Tag Signature of a b decay is a

displaced vertex: Long lifetime of b hadrons

(c ~ 450 m)+ boost B hadrons travel Lxy~3mm

before decay with large charged track multiplicity

B-tagging at hadron machines established:

•crucial for top discovery in Run I •essential for Run II physics program

Achieving optimum resolution requires relative alignment of silicon detectors stable to ~10 microns. Monitor with RASNIK system...


RASNIK locations in situ


RASNIK principle

Illustration of the RASNIK principle:

Portion of the ‘coded mask’ from a system in situ after digitization:

A system with cover removed:

20.0 m


RASNIK data

RASNIK data from a recent month ( ‘SVX’ system).

CDF data taking periods indicated in blue.

RMS of motions during data taking within tolerance for b tagging resolution.

Systems return to previous position after large disturbances.

hours0 2412 186


Measuring σttbar in dilepton channel

Top already observed, goal for summer 2003 analysis:• Increase acceptance as much as possible.• Add higher angle lepton detection.


Physics objects in the analysis Two leptons:

‘Primary’• Must be isolated (sum of energy in cone around lepton must

be less than 10% of lepton energy).• Must have associated track.

‘Secondary’• Some allowed to be non-isolated.• Some allowed to not have an associated track.

Never both of the above.

Missing energy (’s)

At least two jets: b tag not required.


Physics objects in the detector An electron at

CDF…

A jet at CDF…

A muon at CDF…

An electron in plug calorimeter


Analysis overview

Data for the dilepton cross-section analysis collected between March 2002 - May 2003 (126 pb-1).

Three primary data sets: Central electrons (1 main central electron trigger), ~700K events. Central muons (3 main central muon triggers), ~600K events. Plug electrons (1 main trigger [plug EM + missing transverse

energy]), ~2.6M events.

Analysis increased acceptance over Run I measurement by »30%:1. Included CMIO (Central Minimum Ionizing Object) muons2. Recovered events previously vetoed by Z mass window cut3. Included electrons using Run II upgrade endplug detectors


Acceptance increases (#1,2)

CMIO (Central Minimum Ionizing Object) muons... Muon detector coverage at CDF not hermetic. Can recover significant

acceptance by including CMIO’s which are not fiducial to muon chambers.

• Effectively isolated tracks + some (central) calorimeter cuts.

Recovering events under the Z mass window…


Acceptance increases (#2 cont.)

~25% of e/e or μ/μ dilepton top events have an invariant mass within the Z window, (76 < M < 106) GeV.

Lose ~12% of the overall acceptance by enforcing Z window cut.

Lepton invariant mass distribution from top Monte Carlo.

The problem...



Under the Z mass window: Using cuts on jet significance

and on the phi angle between the missing transverse energy in the event and the nearest jet:

• Recover 90% of e/e and μ/μ top events.

• Reject 80% of Z events.

The solution... Z Monte

Carlo

tt Monte Carlo


Acceptance increases (#3)

Plug electrons…

Developing calorimeter definition of electrons in new endplug detectors buys large gain in acceptance for analysis (~30% over winter 2003 tdl analysis).

Adding tracks to plug electrons:• Tracks necessary for stripping plug-based data set.

Cannot use wire chamber (COT) Stand-alone silicon tracking difficult.

• Solution ! develop calorimeter-seeded silicon tracks.


Acceptance increases (#3 cont.) Calorimeter based plug

electron ID variables developed by hand-scans of events from test beam data, MC samples, Run II data.

Cut values tuned using central/plug Z decays (data) and background samples from jet data.

Calorimeter-only plug electrons give ‘secondary lepton’ for the analysis.


Acceptance increases (#3 cont.) Calorimeter-seeded tracking:

Two points + a curvature define a unique helix.• Beam spot + precise EM shower coordinates from plug shower

maximum detector provide 2 points.• Plug EM calorimeter gives shower energy … curvature.

Define two hypothetical track trajectories corresponding to positive and negative charges.

Look for hits in the silicon detectors which support one or both of these hypotheses.

• Choose best fit track if basic quality cuts satisfied.

Calorimeter plug electron cuts + calorimeter-seeded track gives ‘primary lepton’ for the analysis.


Acceptance increases (#3 cont.) Check that plug definitions are

working by measuring W and Z boson cross-sections AS CROSS CHECK in the endplugs:

¢B(W!e) = (2.43§0.25) nb

SM prediction at 1.96 TeV: ¢B(W!e) = (2.72§0.13) nb

¢B(Z!e+e-) = (259§25) pb SM prediction at 1.96 TeV:

¢B(Z!e+e-) = (252§9) pbNote: Plug/plug Z’s


Acceptance summary

With increases, # reconstructed dilepton decays all MC top events

Of the 0.74%: By decay channel:

• e/e: 24% • e/μ: 53%• μ/μ: 23%

By detector region:• Central/central: 78%• Central/plug: 21%• Plug/plug: 1%

Sensitivity to new physics an important goal!

£ factors = 0.74% (~ 5% max.)


Events in Run II data (summary)

HT = Σ (jet ET’s + lepton ET’s + missing transverse energy)Require ¸ 2 jets with at least 15 GeV transverse energyTopological cuts on missing tranverse energyJet significance

Z)


Systematic errors

10.6Total

3.9MC Generators

7.7PDF’s

1.6ISR/FSR

5.6Jet Corrections

2.0Lepton ID Scale Factors + Trigger Efficiencies

Uncertainty (%)Source

Statistical error is 30%


Run II cross-section summary

(syst)(stat)7.6 1.51.9

3.83.1

1.7(syst)3.4(stat)7.3

0.9(syst)1.9(stat)5.3

2.1(syst)1.8(stat)5.1

0.9(lum)(syst)(stat)8.7 2.72.0

6.44.7

0.8(lum)(syst)(stat)8.1 1.61.4

2.22.0

0.5(lum)(syst)(stat)4.6 2.12.0

3.12.7

1.1(lum)(syst)(stat)11.4 2.01.8

4.13.5

0.8(lum)(syst)(stat)8.0 1.71.5

2.42.1

0.7(lum)(syst)(stat)7.4 2.11.8

4.43.6

1.1(lum)(syst)(stat)10.8 2.12.0

4.94.0

Theoretical prediction @NLO ≈ 6.7 pb

NNLL


Events in Run II data


Kinematic distributions from Run II Dilepton analysis


Kinematic distributions from Run II Dilepton analysis

HT = Σ (jet ET’s + lepton ET’s + missing transverse energy)


Conclusion

Question suggested by Run I vs. MET plot answered.

Top cross-section measured in the dilepton channel is consistent with Standard Model predictions at NLO.

Dilepton kinematic distributions are not suggestive of new physics so far.

Focus of dilepton channel analyses switching to a priori minimization of uncertainties, detailed analyses of kinematics.


Backup slides:


Backup slides:


Backup slides:

Believe NI leptons not faked by b’s because of ET cut on electron Not many b’s with

electrons > 20 GeV• Et spectrum of

electrons from b’s in Wbb plotted at right

• This plot is just to give a qualitative sense for this contribution

If this is true, shouldn’t we start to see b’s if we lower the cut?


Backup slides:

muons

electrons




Systematic errors (backgrounds)

21-50

32

50

17

40

32

10

Uncertainty (%)

Jet energy scale

MethodDY (ee, )

MethodFakes

WW/WZ

Z →

Background

Jet energy scale

MC Generator

Jet energy scale

2-jet efficiency

Source


Backup slides:

Rescale lepton ID efficiencies to match those observed in Z data; Scale Factors applied: CMUP: 0.94 +/- 0.01 CMX: 1.00 +/- 0.01 CMU: 0.97 +/- 0.02 CMP: 0.96 +/- 0.02 TCE: 0.98 +/- 0.01 NI TCE: 0.78 +/- 0.07 PEM: 0.96 +/- 0.05 NI MUONS: 0.85 +/- 0.09

Apply track efficiencies Decreases overall acceptance by 6.6%


Backup slides:

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Measurement of the tt Cross Section in the Dilepton Channel at Fermilab’s Run II

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