# Presentation: 474468

## Top Production Cross Section from CDF

- Anyes Taffard

**University of Illinois**

**On behalf of CDF Collaboration**

**32nd International Conference on High Energy Physics**

## Why Study Top Quark Physics ?

- Experimentally, still know very little about the top quark
- Run I: ~110 pb-1, ~100 top candidates
- 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:
- SUSY: Large top mass causes EWSB
- In many dynamical symmetry breaking models, top interactions are modified (e.g technicolor)
- Probe for physics beyond the SM
- Non SM production (Xtt, Xll+jets+ET)
- Non-SM decay: Heavy enough to decay to exotic particles (tXb)
- On-sheel charged Higgs, SUSY
- Signal of today, background of tomorrow

## Tevatron & 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

**Notes:**
Antiproton reclycler (commissioning)
Electron cooling this year
Operational on June 05
40% increase in Luminosity

## Tevatron & CDF: Luminosity

- Record Peak Luminosity:
- July 16, 2004: 10.3 x 1031 cm-2sec-1
- With mixed pbar (recycler & accumulator)

- Acquired luminosity in 2004 already surpassed 2003 total
- CDF ~450 pb–1 total on tape
- 	In this talk:160 pb-1 <Ldt <200 pb-1

**Notes:**
DAQ runs with 5 to 10% dead time

## 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

- 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(tWb) 100 %
- Both W’s decay via Wl (l = e or ; 5%)
- final state l l bb : dilepton
- One W decays via Wl (l = e or ; 30%)
- final state l qq bb : lepton+jets
- Both W decays via Wqq (44%)
- final state: qq qq bb: all hadronic

- Top Quark Production & Decay

## 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

## Dilepton Channel

- Small sample, but highest S/B
- Backgrounds:
- Z/* l+l-
- WW, WZ, ZZ
- W+jets (fake leptons)

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

**Notes:**
Describe this:
Two high-momentum opposite sign leptons
 e,, some  from W decay
2 b-jets
Heavy flavor ID not used
Large missing energy (ETmiss)
Two undetected neutrinos

## Counting Experiments

- Higher purity, lower statistical significance

- Lower Purity, higher acceptance (~20% from )

- 1st Run II paper
- Combined 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

**Notes:**
Say that run I was not in so good agreement , now it is with more stat

## Inclusive 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 (Etmiss, Njet) plane

- Top Cross Section

- WW Cross Section

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

**Notes:**
The first error includes: statistical error, acceptance systematic, luminosity systematic error 
The second error is: Shape systematics from the Met-N jets shapes from various sources 
such as jet energy scale, jet multiplicity, pdf's and so on 

## Larger sample due to BR, but less pure 

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

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

- L+jets Channel

****

- Candidate Event Display

**Notes:**
Use event Display to describ topology:
One high-momentum lepton
 e, from W decay
2 b-jets
Large missing energy (ETmiss)
One undetected neutrino

Use the Ht plot:
To list background: W+jets, QCD
Quote S:B 	pretag 1:5
		after tagging 2:1, with HT 3:1 (significance on ttbar Xs improved by 7%)

## Using 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

**Notes:**
Hight light again, that the hard part is estimation of background, either using data (fake tag, QCD) or
MC (Method 2)

## Using 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

**Notes:**
Efficiency increase by 18%
ttbar expectation : increase by 50%
Background: 0.9 to 2.97
No HT cut here

## Using 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
- Punch though, decay in flight
- Top event tagging efficiency: 15%
- False tag rate (QCD jets): 	3%

## Using Kinematics Fits

- Determined signal fraction using kinematics shape

- Fit NN output (7 kin. var.)

- Fit leading jet ET, require
- @ least 1 b-tag

## All Hadronic Channel

- Final State: 4 jets from W, 2 b-jets (data sample: multi-jets trigger)
- Large statistics, but huge QCD background:
**S:B********1:2500**
- Increased S:B by requiring:
- ET>320GeV
- Topological cuts (aplanarity, centrality)
- Event selection efficiency 6.2%
**S:B ******** 1:24**
- @ least 1 b-tag jet: **S:B ******** 1:4**
- Measure Xs with  6 jets &  1 b-tag jet

**Notes:**
Huge QCD, Make S:B
Very challenging channel

## Single Top

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

- Strategy:
-  Isolate W+exactly 2 jets & 1 b-tag jet
- Non-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
-  Q of lepton,  of light quark jet

**Notes:**
FCNC: Flavor changing neutral current

What make it had:
2jet bin... W+jets background
2 background component: ttbar & non top
Non top, primary source W+jet (62%) mistags ~25 %
Top ~11%

Selection:
140<Mlvb<210
v momentum set to = to met, z component obtain from Mlv=Mw -> pick solution more central in eta. 
If non-zero imaginary part (due to jet e scale resolution) only use real part, if 2 b tag, use b jet with highest Qxeta 

Combined Search (t- and s-channel are signal)
Signal statistics is better, better S/B.
The two channels share the same kinematical properties, for the most part.
Both cross-sections proportional to |Vtb|2 .

Separate SearchTest individual cross-section predictionsDifferent sensitivities to new physics:
t-channel: FCNC couplings, anomalous V+A contributions to the W-t-b vertex, etc.
s-channel: heavy charged vector bosons W’, CP-violation effects within MSSM, Kaluza-Klein excited W-boson within MSSM 
Apply 2 additional cut: subdivide 1 & only 1 btag & 2 btag, higher threshold Etjet>30.

## Single Top cont.

**Notes:**
Combined: Expect 38 see 42, worst limit than expected.
Separate search.
33 evts in 1 tag bin, 6 in double tag bin
	 1 tag bin eff 0.86% t-channel, 0.78% s-channel
	 2 tag bin, 0.007 t-channel, 0.23 s-channel: 2 tag yield to distinction between t/s channel
There are of course always two b-quarks involved in the t-channel. Independent of the Feynman graph. The strong interaction conserves flavor. Feynman graphs are prescription of the perturbative calculation. The 2 -> 2 Graph uses a b-quark structure function. This is a perturbative approximation, see for example the papers by Willenbrock et al. which describe this story. In Pythia the second b-quark is obtained from backward evolution. Anyhow, it turns out that the second b-quark is rarely seen in our acceptance. It is at low ET and high pseudo-rapidity. That is why there are no t-channel events in the double-b-tag bin. 

Single Top: expect observation with 2fb-1
Hard to say, have more advance techniques.
We can increase the b-taggin eff.

## Summary

- 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

**Notes:**
More data on tape & coming: improve and expend horizon (searches, kinematics study of top event etc...)
Can really start to understand top 

Why bb & cc Xs not well predicetd, but tt is?
	bb,cc comes from qq, tt comes from qq. SM less precise for lighter quark