Search for t q

FermilabMarch 28-29, 2006

Sarah Demers

Outline

Motivation

CDF and the Tevatron

Event Selection

Tau Identification

Result

The Standard Model

Encompasses 3 of 4 fundamental forces

Fundamental particles:

6 quarks6 leptons

Interactions mediated by force carriers

The Standard Model: What’s Missing?

Why are there three generations?

Why the large variation in quark masses?

Why is there so little antimatter in the universe?

What about gravity?

Top quarks and tau leptons

The Higgs BosonExtensions to the standard model may be neededMSSM Charged Higgs: t -> H+b, H+ ->gives identical final state

Result from CDF Run 1 (mid-1990s)Expected 3.2 events (0.7 signal events) with theoretical cross section

Found four events with three b tagged events

Need to improve signal to noise

Top quarks and tau leptons

Experience with tau leptons in the environment of a hadron collider

Room for new physicsThe “recently” discovered top quarkThe heavy third generation

The Tevatron

The Tevatron (and the LHC)

Tevatron LHC

proton-antiproton proton-proton

Beam Energy 1 TeV 7 TeV

Radius 1 km 4.24 km

Interactions/Crossing

3 20-30

Instantaneous Lum.

1032 cm-2s-1 1034 cm-2s-1

Time between bunch crossings

396 ns 25 ns

Fermilab’s Accelerators

Cockroft-WaltonHydrogen gas ionizedIons accelerated to 750 keV

Linac500 ft longOscillating electric fields accelerate protons to 400 MeV

BoosterCircular acceleratorProtons make 20,000 lapsAccelerated to 8 GeV

Fermilab’s Accelerators

Main InjectorAccelerates protons and anti-protons to 150 GeVInjects the particles into the Tevatron

Anti-protonsFrom 120 GeV proton beam extracted from the Main Injector

TevatronAccelerates to almost 1 TeVParticles move only 200 mph slower than speed of light

Collisions

Main Injector

Tevatron

DØCDF

Chicago

p source

Booster

Detectors

Event Selection: Decay Chain

top, anti-top events needed for statistics

t t

W+

b bW-

jet jet e e

Final State

t t

W+

b bW-

jet jet e e

hadrons(1 or 3 prong)

Strategy for this analysis

First CDF Run II top -specific analysisClosely follow {e+} dilepton analysis (but)

admit only the lowest background categories with tight, central electron and muon requirementsplace a premium on ensuring non-tau top final states are excluded

t t

W+

b bW-

jet jet e e

hadrons(1 or 3 prong)

Event Selection

Reconstructed tau passing all ID cuts, Et > 15 GeV

t t

W+

b bW-

jet jet e e

hadrons(1 or 3 prong)

CMS CR 2005/018

elemuon1 prong (had)3 prong (had)5 prong (had)

Tau decay modes

Event Selection

Tau identification requirements

No impact parameter

t t

W+

b bW-

jet jet e e

hadrons(1 or 3 prong)

Event Selection

Corrected Missing Et

greater than 20 GeV

Opposite sign tau and electron (muon)

t t

W+

b bW-

jet jet e e

hadrons(1 or 3 prong)

Event Selection

>= 2 jets, < 2.0

1st jet > 25 GeV

2nd jet > 15 GeV

Ht > 205 GeV

Z Mass veto

t t

W+

b bW-

jet jet e e

hadrons(1 or 3 prong)

Optimization

The HT and lead jet ET cuts are chosen by a formal optimization procedure

2D optimization with MC signal & data+MC bkgndMinimize S/sqrt(B), the stat. uncertainty in Gaussian limit in “no signal observed” caseMaximize likelihood ratio: LS+B/LB

Acceptance

Pythia Monte Carlo

Before scale factors:

eh : 50%

h : 42%

eh : 4%

h : 4%

50%

42%

4% 4%

Acceptance

35% ID efficiency from Monte Carlo

With W->, compare data to Monte Carlo

W+

Acceptance Summary

0.076 ± 0.005 (stat) ± 0.013 (sys) %

BR ~ 3% with ~2.5% efficiency

Expected signal:

1.00 ± 0.06 ± 0.16 events

Expected background:

1.29 ± 0.14 ± 0.21 events

Background Summary

jet fakes

We measure the jet to tau fake rate in 4 datasets:

20 GeV jet 50 GeV jet 70 GeV jet Large total event

Energy

Rates from 0.1% to 10%

e fakes

Measure e tau fake rate in data with Z ee

Electron veto variable (HadE/SumP) shown for loose leg Zee

We calculate a (1.2±0.3)% etau fake rate at our 0.15 cut

fakes

Z MC predicts background

Cross-check in data

Fakes are extremely rare, so data statistics only allow a cross-check

Agreement at level of 30%

Z

First require of missing energy, “taus” consistent with Z Then reconstruct mass by assigning MET to “taus”

Z

65 GeV < Mass < 115 GeV88% reduction of BG, 4% reduction of signal

The Result

We could report this result as a cross-section, as is done with other rate analyses

However, clearly this analysis has little to contribute to a cross-section average

The motivation for the analysis is a universality testWe quote:

Conclusions

We predict 2.3 events and see 2 events.

We set a limit on:

r < 5.2 at the 95% confidence levelFrequentist Method: profile likelihood (Rolke et al)

Acknowledgements

Thank you to the Fermilab group for inviting me!www-cdf.fnal.govwww.fnal.govwww.particleadventure.orglhc.web.cern.ch/lhc/

backup slides

CDF (and CMS)

CMS Tracker25,000 Silicon Strip CensorsTotal Area of 210 m2

9600000 readout channels

Crystal Electromagnetic CalorimeterSampling Hadronic Calorimeter with

copper Absorbing PlatesTrigger

Store ~100 events per second (out of 40 million +)

CDF Tracker405,504 silicon readout channelsOpen cell drift chamber (30,240 readout channels)

Lead/Scintillator Sampling Electromagnetic CalorimeterIron/Scintillator Sampling Hadronic CalorimeterTrigger

Three level system~8 s decision time at Level 1

Systematics

Data

• Fake background from jets and electrons.

• Electrons pass isolation cuts but fail electron veto (lower left corner)

• Jets fill plot but tend to fail the track and 0 isolation

Candidate events

• Two events survive all cuts.

• Jet 1 of Candidate 1 is tagged as a b quark jet.

Jet Multiplicity

We have background predictions in hand

As an a priori test:

predict rates in 0 and 1 jet multiplicity bins(no HT or Z Mass cut)

did not look at 2+ jet bin until satisfied

result more likely than 41% of pseudoexperiments

identification cuts

Our ID cuts are similar to other tau analysis

We have tighter a tighter calorimeter isolation cut

Our electron veto is bracketed by cuts in other analyses

Our W+jets background is reduced at the expense of reduced tau ID efficiency

1D version, fixing ET(1) cut…

Optimization (cont’d)

HT

HT

Signal/SQRT(Bkgnd)

Likelihood Ratio

Signal/BkgndHT

HT

Optimization (cont’d)

HT and lead jet ET cuts can distinguish signal from background

Integral distributions above cuts shown

HT

ET(1)

HT

Signal

Background

identification cuts

Tightening the calorimeter isolation cut is a concern because it is the worst modeled

Using the “tight” sample fromCDF 6010, thisis a 5%(relative) scalefactor effect

W analysis

CDF 6010 cut

Our cut

MC

Data

Acceptance

Pythia ttopei MCNcand includes jet fakesBefore scale factors:

eh : 50%

h : 42%

eh : 4%

h : 4%

Efficiencies and Scale Factors

Zvertex 0.948 +/- 0.003 6917

Ele trigger 0.966 +/- 0.001 6234

CMUP trigger

0.904 +/- 0.012 6293

CMX trigger 0.901 +/- 0.016 6293

Ele ID SF 0.965 +/- 0.006 6590

CMUP ID SF 0.94 +/- 0.01 6825

CMX ID SF 1.015 +/- 0.007 6825

value CDF Note

Systematics: Techniques

•Jet Energy Corrections:

•Level 5, half of difference between +1 and –1

•Monte Carlo Generator Dependence (half of difference):

• ttop2e (pythia) with no QED FSR

• weighted for BR ttopli (herwig)

•ISR: ttopei (ISR on) compared to ttop0e (ISR off)

•FSR: ttopei (tune A) compared to ttop5e (tune B)

•Statistical uncertainty dominates

Systematics

•PDFs: Compare # expected events in ttopei with:

• ttop3e (MRST PDFs)

• ttop4e (MRST PDFs, lower ISR)

• ttop6e (MRST PDFs, lower FSR)

•For ttop2e and ttop4e comparisons our systematics are limited by statistics

jet fakes

• Cross-checking samples yeilds a maximum difference of 26%, which we take as our systematic error

• jet50 is closest in Et to spectrum in data fakes so we use jet50 to determine our backgrounds

Candidate Event

• Run 167229

• Event 2376337

Candidate Event

Plot optimization variable in 2D vs cuts

Choose cuts at lower left corner of “mouth”

highest acceptance for same optimization

Optimization

HT

ET(1)

HT

Signal/SQRT(Bkgnd)

ET(1)

Likelihood Ratio

e fakes

Form an electron veto sample in the data with tau candidates that pass all ID cuts but fail electron veto

Apply 1.2% fake rate

Plotted on the right is our veto sample before Ht cut

Peak at zero gives us confidence that we have electrons in this sample

WW and WZ

WW and WZ are sources of real taus in our background

Cross Section * Branching ratio combined with low (relative to e and mu) tau reconstruction reduces BG

These two backgrounds combined are <15% total BG

WW: atop4x (Herwig + Alpgen +0 parton) atop5x (Herwig + Alpgen +1 parton)

WZ: atop0y (Herwig + Alpgen + 0 parton) Statistical error is 100%

Jet Multiplicity Study

Summary of all bins with probability of our data,and distribution of pseudoexperiment results

predicted

seen

0j, e, OS 23±3 18

1j, e OS 4.4±0.6 5

0j, , OS 23±3 14

1j, , OS 2.7±0.6 4

0j, e, SS 6.5±1.7 5

1j, e, SS 1.8±0.5 1

0j, , SS 4.3±1.3 3

1j, , SS 0.6±0.2 0

Jet Multiplicity Study

If you believe there are patterns of discrepancy, can study subsets

worst is muon 0j OS bin. 7% probable