# Presentation: 214069

## Top Physics at CDF

- Julia Thom
- Fermilab
- XVIII Rencontres de Physique de La Vallee d’Aoste
- La Thuile
- 3/5/2004

## Motivations for Studying Top

- Only known fermion with a mass at the natural electroweak scale
-    Window into the problem of EWSB?
- New physics may appear in production (e.g. topcolor) or in decay (e.g. charged Higgs).
- Can only be studied at Tevatron prior to LHC.

## Run I  Top Studies

- Observed in 1995 in first ~70 pb-1 of Run I data.
- Final Run I top analyses based on ~110 pb-1.
- Production cross sections in many channels
- Mass: 174.3  5.1 GeV (CDF/DØ combined)
- Event kinematics
- W helicity, limits on single top production..
- overall consistency with the Standard Model.
- but only ~100 top candidates
- 	→ analyses statistics-limited.

## Improvements for Run II

- Accelerator:
-                    sqrt(s) =  1.96 TeV (was 1.8 TeV in RunI)
-                    ->30-40% increase in top cross section
- CDF Detector:
-       -New DAQ
-       -New Silicon system
-          ->improved b-tagging
-       -Extended muon systems
-       -Calorimeter endplug for
-            forward coverage
-       -New central drift chamber

**Notes:**
To turn these initial measurements into  precision top physics the following improvements were implemented:

## Tevatron Peak Luminosity

- Record Luminosity: 6.3x1031 (3x better than Run I)
- First store w/antiprotons from recycler
- Current to tape: 350pb-1, for this talk: up to 200pb-1

## Tevatron Luminosity

- Base
-   Design
-      Measured

- Predicted for 2004: 380 pb-1 delivered (design)

## Pair-production and Decay Basics

- Pair Production:

- 85%

- 15%

- NB: qq, gg fractions reversed at LHC

- t

- W

- b

- t

- W

- b

- BR( *t **→ **Wb*) ≈ 100%

- σtheory ≈ 7 pb

- Event topology determined by the decay modes of the W’s

- b-jet: identify via secondary vertex or soft lepton tag

**Notes:**
Note that quarks produce jets in the detector

## t-tbar Final States

- Dilepton
- BR = 11%
- 2 high-PT leptons + 2 b-jets + missing-ET
- Lepton + jets
- BR = 44%
- single lepton + 4 jets(2 b-jets) + missing-ET
- All-hadronic
- BR = 45%
- six jets, no missing-ET

- Highest signal:noise
- High pt decay products
- Central/spherical topology

- More challenging backgrounds (QCD multijet)

- Lepton ID (tracking, detector coverage)
- Calorimetry (calibration)
- B identification (tagging)
- Simulation

-     Tools:

**Notes:**
Final states: Dilepton with highest s:N (signature) but lowest BR. No need to tag jets as B

## Programme:

- Top cross-section
-            dilepton channel
-                Lepton+jets channel
- Single top physics
- Top Mass
- W helicity in top decay

- New results

- New results

- New results

## Measuring the ttbar Cross Section

- starting point for all top physics
- Requires detailed understanding of backgrounds and selection efficiencies.
- Test of QCD
- Latest calculations: NNLO + NNNLL
- Departures from prediction could indicate nonstandard production mechanisms, i.e. production through decays of SUSY states.

##  Dilepton Cross Section: lepton+track

- Signature: 1 lepton+1 isolated track,
-                   missing Et, ≥ 2 central jets
- Acceptance: ~2x better than Run I
- ~20% from 
- Background:
- Drell-Yan              WW, ZZ, WZ                W+jets
-                                                                    (“fakes”)
** **

- Relative weight depends on number of jets

**Notes:**
Lepton is e or mu, increase background, DY: MC and data, WW MC, W+jets data

## Dilepton Cross Section: lepton+track

- New result

- Measured cross section
- for different jet Et
-  and track pt thresholds

## Different background composition, higher S:N, lower acceptance
 Events with 1 “tight” and 1 “loose” e or 

           lepton composition: 1 ee, 3 , 9 e

 2 “tight” leptons (e, )

     lepton composition: 1 ee, 2 , 4 e

-  Events with 1 “tight” and 1 “loose” e or 
-            lepton composition: 1 ee, 3 , 9 e
-  2 “tight” leptons (e, )
-      lepton composition: 1 ee, 2 , 4 e

- Dilepton cross section: ee, e, 

- New results

## Jet Multiplicity in Dilepton Events

- ttbar signal bin

- Lepton + track:

- ee, e, :

## Dilepton Kinematics

- Ht:Scalar summed ET of jets, leptons, and missing ET

- RunI: had seen hints of discrepancy in kinematic distribution:

- Missing ET

- Leptons transverse momentum

- With higher statistics in Run II see good agreement with SM

## Dilepton event display

**jet**

**jet**

****

**e****1**

**e****2**

-  2 electrons (ET1=73 GeV, PT2=63 GeV)
-  Missing ET = 59 GeV
-  2 central jets + 1 forward jet

**e****1**

**e****1**

**e****1**

**e****2**

**jet**

**jet**

## Cross Section – lepton+jetsusing kinematic fits

- Isolate signal from large W+jets background using **kinematic shapes: H****t**** (scalar sum of energy in the event)**

- New results

- Largest systematic: jet energy scale

- W+>=3 jets: observe 519 events
- Top fraction from fit: 0.13+-0.04

## Tagging high-pt jets: Silicon vertex tag (SVX-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

- Top event efficiency: 55%
- False tag rate (QCD jets): 0.5%

**Notes:**
It helps a lot in bkgd suppression to require at least one of the jets to be identified as coming from a B

## Cross Section – lepton+jetsusing kinematic fits + SVX-tag

- In addition to shape information: require at least one b-tag

- W+>=3 jets: observe 35 events
- Top fraction from fit:

- Using 108 pb-1

- Templates:

## Cross Section – lepton+jetsusing SVX-tag

- Cross Section – lepton+jetsusing SVX-tag

- Using 107 pb-1

- 2d displacement of tagged jets:

- Number of jets per event:

- W+>=3 jets: 35 positive tags
- Expected background (mistags, QCD,..): 15.1+-2

**Notes:**
Check counting vs fitting? Silicon vertex tag!

## Cross Section – lepton+jetsusing “Soft Lepton Tag”

- tag semi-leptonic decays of B
- leptons have a softer pT spectrum than W/Z leptons
- They are less isolated
- Identify low-pt muon

- Using 125 pb-1

- New results

**Top Event (>2 jets) **
**Tag Efficiency:15%**
**False Tag Rate **
**(QCD jets): 3.6%**

- Number of jets per event:

**Notes:**
Background in blue, with signal contribution in red

## Summary of Cross Section Results

*195 pb**-1*

*200 pb**-1*

*200 pb**-1*

*108 pb**-1*

*126 pb**-1*

- Main data driven systematics (jet energy scale, ISR, btag) scale with 1/N

- RunII(2fb-1) ******tt****/********tt** <10%

- √s-Dependence:

## Single Top Physics

- Probe top EW coupling
-     direct determination of Vtb
- Sensitivity to new physics:
-               t-channel:anomalous couplings,
-               FCNC
-               s-channel: new charged gauge
-               bosons
- Strategy:
-       Isolate W+ exactly 2 jets
-        and tag one jet
-   Likelihood Fit to Q* (t-channel)
-   Likelihood Fit to Ht (combined)

- s-channel

- t-channel

-  0.880.11 pb

- 1.980.24 pb

- Q: charge of lepton, pseudorap of forward jet

- B.W.Harris et al

**Notes:**
This is harder to do, smaller x-sec and less decay products

## Single Top Physics

- Templates from MC (combined):

- t-channel:

##     Search for Single Top 

- t(combined)<13.7pb @95% C.L.

- t(t-channel)<8.5pb @95% C.L.

- New results

- Using 162 pb-1 of data:

- Fit to the data (combined search):

- t-channel search:

## Top Mass Measurement

- Mtop is a precision electroweak parameter that helps constrain the mass of the Higgs.

- LEP EWWG summer ‘03

## Top Mass: Lepton + 4 jets with  SVX-tag

-  22 vertex-tagged events from lepton+4 jet sample

- 6 parton/jet matching assignments possible
- test for consistency with top using kinematic constraints
- pick lowest χ2
- fit resulting mass distribution to background + signal templates at different values of Mtop

- Dominant syst: jet energy scale, expecting significant impr. soon

## Top Mass: Dilepton Channel

- 6 events
- 124 pb-1

- Underconstrained system
- Use Pttbar,z to weight the mass fit distribution
- Likelihood fit to top mass templates

- Improved tools are underway:

- “Dynamic Likelihood” method (matrix element convoluted likelihood) K.Kondo 1988 J.Phys.Soc.57 4126
-  results expected soon

## W Helicity Measurement

- Top decays before it can hadronize, because width *Γ*t = 1.4 GeV > ΛQCD.
- Decay products preserve information about the underlying Lagrangian.
- Unique opportunity to study the weak interactions of a bare quark, with a mass at the natural electroweak scale!
- SM Prediction:
- W helicity in top decays is fixed by Mtop, MW, and V-A structure of the tWb vertex.
- W helicity reflected in kinematics: W lepton pt,...

## Helicity affects lepton PT in lab frame

- Templates from MC:

- SM V-A predicts W helicity:
- F0 = 70% longitudinal
- F- = 30% left-handed
- [V+A: 70% long., 30% r.-h.]

- CDFII result soon

## Conclusion and Outlook

-    Now using 2x the RunI data set
- Improving measurements of cross section, mass
- W helicity, single top... are making progress.
- We expect ~50x more data compared to Run I!
- What’s ahead: top  H+
-                           Study of  channels
-                           measure Vtb
-                           ttbar resonant production
-                           rare decays

## Backup Slides

## Slide 33

## Matrix Element Method

***f(q)*** is the probability distribution than a parton will have a momentum ***q***

***W(y,x)*** is the probability that a parton
- level set of variables ***y*** will be measured
- as a set of variables ***x***

***d******n******σ ***is the differential cross section

-  Leading-Order ttbar->lepton+jets matrix element, PDFs
-  12 jet permutations, all values of P()
-  Phase space of 6-object final state
-  Detector resolutions

-  Only *W*+jets, 80%
-  VECBOS subroutines for *W*+jets
-  Same detector resolutions as for signal
-  All permutations, all values of P()
-  Integration done over the jet energies

** **Convolute probability to  include all conditions for accepting or rejecting an event
-  Form a Likelihood as a function of: Top Mass, F0 (longitudinal fraction of W bosons)

## W Helicity Measurement, contd.

- The angular dependence of the semileptonic decay in the W rest frame is given by

- where = MW2/Mtop2

- SM predictions (for mb=0):

- left

- right

- long.