Top Production Cross Section from CDF

Anyes TaffardUniversity of Illinois

On behalf of CDF Collaboration

32nd International Conference on High Energy Physics

2Why Study Top Quark Physics ? Experimentally, still know very little about the top

quark Run I: ~110 pb-1, ~100 top candidates

o Overall consistency with SM prediction, statistic limited Only know fermion with mass near electroweak scale

Theoretical models proposed to solve the problems of the SM often have top playing a leading role:

o SUSY: Large top mass causes EWSBo In many dynamical symmetry breaking models, top interactions are

modified (e.g technicolor) Probe for physics beyond the SM

o Non SM production (Xtt, Xll+jets+ET)o Non-SM decay: Heavy enough to decay to exotic particles (tXb)

On-sheel charged Higgs, SUSY

Signal of today, background of tomorrow

3Tevatron & CDF Proton-antiproton collisions

s=1.96 TeV Main injector

150 GeV proton storage ring

•New DAQ•New Track Trigger

•New Silicon ||<2•Improved b-tagging

•New Drift Chamber•New Plug Calorimeter

•Increase acceptance•Upgrade Muon Detectors

4Tevatron & CDF: LuminosityRecord Peak Luminosity: July 16, 2004: 10.3 x 1031 cm-2sec-1

With mixed pbar (recycler & accumulator)

Acquired luminosity in 2004 already surpassed 2003 totalCDF ~450 pb–1 total on tape

In this talk:160 pb-1 <Ldt <200 pb-1

days

CD

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quire

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Store Number

2002 2003 2004

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Pair Production 85% qq, 15 % gg (fractions reversed @

LHC) Central, spherical events Large transverse energy

Cross section increases 30% with Tevatron s increase to 1.96 TeV

Single top production is a factor of 2 smaller

BR(tWb) 100 % Both W’s decay via Wl (l = e or ; 5%)

o final state l l bb : dilepton One W decays via Wl (l = e or ; 30%)

o final state l qq bb : lepton+jets Both W decays via Wqq (44%)

o final state: qq qq bb: all hadronic

Top Quark Production & Decay

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)

Bonciani et al., Nucl. Phys. B529, 424 (1998)Kidonakis and Vogt, Phys. Rev. D68, 114014

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Measuring tt Cross Section Starting point for all top physics

Events triggered on one high momentum lepton or multi-jets Optimized event selection for top physics and new physics Define top sub-samples by counting lepton and jets

2 jets in dilepton channel 3 jets in l+jets channel 6 jets in all hadronic channel

Efficiency & Acceptance: Measured from MC (Pythia) & Data

Background estimate:most challenging part

Improve S:B withb-tagging

7Dilepton Channel Small sample, but highest S/B Backgrounds:

Z/* l+l- WW, WZ, ZZ W+jets (fake leptons)

Background can be furtherreduced with an HT cut.Ht: Scalar summed ET of jets, leptons, and missing ET

8Counting Experiments

Higher purity, lower statistical significance

Lower Purity, higher acceptance (~20% from )

1st Run II paperCombined Result: hep-ex/0404036

Standard Run I method (ee,,e)Looser selection: e/ + track

With higher statistics in Run II observe good agreement with SM

13 candidates: 1 ee, 3 9 e

9Inclusive Dilepton Analysis No cuts other than two identified leptons

If same flavor, Z e+e-/+- dominates: require significant Etmiss

Advantages: increase acceptance Goal: search for new physics Fit data to tt,WW, Z +- contribution in 2D (Et

miss, Njet) plane

Top Cross Section

WW Cross Section

CDF RunII Preliminary

J.M.Campbell and R.K.Ellis, Phys.Rev.D60 113006 (1999)

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Larger sample due to BR, but less pure

Improve S:B by requiring b-tagging 2nd vertex semileptonic decay

L+jets Channel

Candidate Event Display

3 Jets

11Using Vertex Tagging B decay signature: displaced

vertex Long life time c ~ 450 m Travels Lxy~3mm before decay

Top event tagging efficiency: 52% False tag rate per QCD jets: 0.5%

@least 1 b-tag & HT>200 GeV

12Using Double b-Tag Double b-tag event essential for top mass

measurement: reduces combinatorics Improve b-tagger:

Increase per jet tagging: 10.8 % 12.0% False tag rate: x 3.4

Significance up by 18%: tt expectation increases from 8.7 13.0 3x more background

Loose SecVtxStandard SecVtx

13Using Soft Lepton Tag Tagging B may decay semileptonically Leptons id challenges:

softer spectrum than leptons from W/Z non-isolated (cannot use calorimetry information)

Id low pT muon Background dominated by fake tag

o Punch though, decay in flight

Top event tagging efficiency: 15% False tag rate (QCD jets): 3%

14Using Kinematics Fits Determined signal fraction using kinematics

shape

Fit NN output (7 kin. var.) Fit leading jet ET, require @ least 1 b-tag

15All Hadronic Channel Final State: 4 jets from W, 2 b-jets (data sample: multi-jets

trigger) Large statistics, but huge QCD background:

S:B1:2500

Increased S:B by requiring: ET>320GeV Topological cuts (aplanarity, centrality)

o Event selection efficiency 6.2%o S:B 1:24

@ least 1 b-tag jet: S:B 1:4

Measure Xs with 6 jets & 1 b-tag jet

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Probe EW coupling, direct determination of Vtb

Sensitive to new physics t-channel: anomalous couplings, FCNC s-channel: new charged gauge boson

Single Top

t-channel production (Wg fusion) = 1.98 pb

s-channel production (W*)

= 0.88 pbB.W. Harris et al.Strategy:

Isolate W+exactly 2 jets & 1 b-tag jetNon-top 89%: W+jets (62%), false tags (25%), QCD (10%) Top: 11%

Likelihood Fit to Ht (combined): to discover Likelihood Fit to Q* (t-channel) : to see new physics

o Q of lepton, of light quark jet

17Single Top cont.

CDF Run II Limits at 95% C.L.:Combined: 17.8 pb (β95=6.2/4.8)t-channel: 10.1 pb (β95=5.1/5.6)s-channel: 13.6 pb (β95=15.4/13.7)(Format: β95=observed/expected)

Entries 42

18Summary x2 Run I dataset

Observed consistent with SM prediction @ mtop=175 GeV/c2

1st paper out, more coming soon. Near future: already x2 more data on tape