# Presentation: 491029

## Elizaveta Shabalina

University of Illinois (Chicago)

- University of Illinois (Chicago)

- Top Quark Physics at the Tevatron

- From discovery to precision measurements

## Motivations for Studying Top

- Special place in the Standard Model (SM):
**Only known fermion with a mass at the natural electroweak scale (40 times larger than its isospin partner, *****b*****-quark) **
**Large Yukawa coupling to Higgs boson (G****t****~1) **
**Special role in precision electroweak physics**
**Window into the problem of ElectoWeak Symmetry Breaking (EWSB)?**
- Top quark mass sets severe constraints on SM extensions
- 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.

## A Brief Top History

- Observed by CDF and D0 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 measurement**
**Limits on single top production, rare/non-SM decays****.**

- Top quark was expected in the SM of electroweak interactions as a partner of b-quark in SU(2) doublet of weak isospin for the third family of quarks

- Overall consistency with the Standard Model.
- But all our knowledge is based on ~100 analyzable top events    → **analyses statistics-limited**

## 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 3.5s in Run I 
Increased instantaneous luminosity:
Run II goal 30 x 1031 cm–2 s-1
Current: 4÷5 x 1031 cm–2 s-1

**                                          **
- 36 p and p bunches 396 ns between bunch crossing
**Increased from 6x6 bunches with 3.5********s in Run I **
- Increased instantaneous luminosity:
**Run II goal 30 x 10****31**** cm****–2**** s****-1**
**Current: 4÷5 x 10****31**** cm****–2**** s****-1**

- Tevatron collider in Run II

## Tevatron Peak Luminosity

- Typical recent stores: 4-51031

- Run IIa goal: **8********10****31**

- 71031

## Tevaton performance

- -

**Start of Fiscal Year**

**Integrated Luminosity (fb****-1****)**

- Performance above design/base

## Integrated luminosity at D0

-  **Most of the results presented today are obtained with ****120********200**** pb****-1**
-  **Goal for 2004**: additional 310-380 pb-1 delivered.

- delivered

- on tape

- ~403 pb-1

- ~314 pb-1

- Data collection efficiency ~90%

- Start of physics-quality data – spring 2002

## The upgraded detectors

- New tracking: silicon and fibers in 2 T magnetic field
- Upgraded muon system
- Upgraded DAQ/trigger (displaced track soon

- New bigger silicon, new faster drift chambers, TOF
- New scintillating tile and end-plug calorimeter and  increased coverage of the muon system
- Upgraded DAQ/trigger,. displaced-track trigger

**D0**

**CDF**

## Top physics in Run II

- Run II:
-     with high precision we hope to answer questions such as:
- Why is top so heavy ?
- Is it or the third generation special ?
- Is top involved with EWSB ?
- Is it connected to new physics ?

- _New results_
-  Top quark pair production cross section
-  Single top quark production
-  new top mass world average

**(FCNC)**

## Cross section  basic number, starting point for all top physics
In Run II (1.96 TeV) is expected to be ~30% higher than in Run I (1.8 TeV)

**85%**

**15%**

- Top Quark production at the Tevatron

**B(t********Wb) = 100%**

- Cross section  basic number, starting point for all top physics
- In Run II (1.96 TeV) is expected to be ~30% higher than in Run I (1.8 TeV)

- Requires detailed understanding of detector, backgrounds and  selection efficiencies

## Test of QCD
Latest calculations (NNLO + CTEQ6M parton densities):

Departures from QCD could indicate non SM physics: 
decays of SUSY states
Top-color objects

**CDF **
**D0**

**Test of QCD**
- Latest calculations (NNLO + CTEQ6M parton densities):
- Departures from QCD could indicate non SM physics:
- decays of SUSY states
- Top-color objects

- Production cross section

| luminosity | 0.1 fb-1 | 2 fb-1 | 8 fb-1 |
| --- | --- | --- | --- |
| 
N top produced | 5 pb
500 | 7 pb
14000 | 7 pb
56000 |

- RunII (2 fb-1)

**   ****Current uncertainty is statistics limited**

## Top quark decays

***All jets***

**

*e+jet*

***+jet*

*  **14.8%*

*1.2%    1.2%*

*2.5%*

*44.4%*

*21.1%*

*           *

*           *

*  **14.8%*

- W decay modes determine top quark final state
- Dilepton (ee, μμ, eμ)
**Both W’s decay leptonically**
**BR = 5%**
- Lepton (e or μ) + jets
**One W decays leptonically, another one hadronically**
**BR = 30%**
- All-hadronic
**Both W’s decay hadronically**
**BR = 44%**
- τhad +X
**BR = 23%**

- Most favorable channels for top physics

- More challenging backgrounds, but measurements still possible

## Top cross section: dilepton channels

- Event selection:
- 2 high PT isolated charged leptons (e,)
- Neutrinos: large missing ET
- At least 2 jets (*b*-jets)
- Large scalar sum of all measured “objects” ET’s (leptons, jets)
- Backgrounds
- Physics:** **determined from Monte Carlo
- WW/WZ/ZZ, Z
- Instrumental: determined from data
- fake leptons in W+jets and QCD
- Drell Yan (Z/* ee, with fake missing ET  (eμ is not affected)

## D0: ee, eμ, μμ final states

- D0: ee, eμ, μμ final states

- 156 pb-1

- 143 pb-1

- 140 pb-1

- Top contribution is calculated assuming

## D0: ee, eμ, μμ final states

-  13.14.7+5.9 (*stat)*1.7+1.7 (*sys*)  0.9(*lumi*) pb

-  19.19.6+13.0 (*stat*) 2.6+3.1 (*sys*)  1.2(*lumi*) pb

-  11.714.1+19.7 (*stat)*8.2+4.1(*sys*)  0.8(*lumi*) pb

-  14.34.3+5.1 (*stat)*2.0 +1.9 (*sys*) * *0.9 (*lumi*) pb

## D0: dilepton channels – eμ candidate

- Soft lepton tag

- Isolated muon

- Secondary vertex

## CDF: ee, eμ, μμ final states

-  tt = 8.7+3.9(*stat*)1.4(*sys*)   0.5(*lumi*) pb

-  tt = 8.1+4.4(*stat*) 1.6(*sys*) 0.5(*lumi*) pb

- Events with 2 “tight” leptons:                       1ee, 2 μμ, 4eμ

- CDF: ee, eμ, μμ final states

- Events with 1 “tight” and 1 “loose” e or μ : 1ee, 3 μμ, 9eμ

- -3.4

- -2.6

## Require 1 isolated lepton and 1 isolated track (instead of isolated lepton)
~20% higher acceptance but lower S/B 
different background composition

- ~20% higher acceptance but lower S/B
- different background composition

- Measured cross-section for different jet ET and track pT

-  tt = 6.9+2.7 (*stat*) 1.2(*sys*) (*lumi*) pb

- CDF: lepton+track final states

- 2.4

## Top cross section: lepton+jets channels

**b**

****

**p**

- t(→W±b) t(→W±b)

- e±,±

- qq

*jet*

*jet*

*jet*

*jet*

-   Event preselection:
-   1 high PT isolated charge
-     lepton (e,)
-   Neutrinos: large missing ET
-   Large jet multiplicity
-   Backgrounds:
-   W+jets and multijet production (QCD) with fake isolated lepton or fake missing ET
-   Features:
-   Larger yield but higher background
-   Additional techniques
-   make use of event topology
-   tag *b* jets

## Lepton+jets channels: overview

## Lepton+jets: topological variables 

- Sphericity

- Aplanarity

- HT2/HZ

- KTmin

- Topological variables to separate signal from background
- Minimize systematic uncertainty by reducing sensitivity to Jet Energy Scale
- Small correlations
- Angular quantities
- sphericity
- aplanarity
- Ratios of energy-dependent quantities
- H’T2
- K’Tmin

## D0: lepton+jets topological cross section

## D0: lepton+jets topological cross section

-  8.8 +4.1 (*stat*) +1.6 (*sys*)  **0.57(*lumi*) pb

- -3.7

- -2.1

- 2.4

- 1.7

- 3.7

- 2.1

- 3.0

- 1.6

-  6.0+3.4 (*stat*) +1.6 (*sys*)  **0.39(*lumi*) pb

## Combine 7 kinematic variables in Neural Network to separate signal from background

-  Nj3

-  Nj4

- CDF: lepton+jets topological cross section

## CDF: lepton+jets with Soft Lepton Tag

- Exploits the *b* quarks semi-leptonic decays
- These leptons are soft and  not isolated
- Probability to tag a     event – 15%, fake rate (QCD jet) – 3.6%

- same preselection as topological analysis
- ≥ 3 jets

**Cross section:**

-  126 pb-1

-  tt = 4.1+4.0 (*stat*) 1.9(*sys*) pb

- 2.8

- D0 has presented this analysis with 92 pb-1; to be updated soon with ~150 pb-1

##  Top cross section: lifetime b-tagging

- jet is tagged as *b* jet
**If signed decay length significance **Lxy/(Lxy)>cut

- S = IP/(IP)
- Jet is positively tagged if it has
**at least two tracks with S>3 or**
**at least three tracks with S>2**

-  Top cross section: lifetime b-tagging

- Signature of a *b* decay is a displaced vertex
- Long lifetime of *b*-hadrons (c ~ 450μ) + boost
- B hadrons travel Lxy ~ 3mm before decay with large charge track multiplicity

***b*****-tagging at hadron machines is proved to be:**
-  crucial for top discovery in Run I
-  essential for Run II physics program

- Secondary Vertex Tag (SVT)

- Counting Signed Impact
- Parameter tag (CSIP – D0)

## B-tagging analysis at D0

-      cross section is determined from the excess of the observed number of tagged events w.r.t. the predicted background for njets≥3

- after tagging

- before tagging

- lepton+ MET + njets

- QCD

- W+ njets

- N bckg tag

- from data

## D0: signal and background summary

- Event tagging probability requires:
*b*-tagging efficiency
*c*-tagging efficiency
- light jet tagging efficiency (mistag rate)

- Performance highlights:
- tt tagging probability ~ 56%
- Single Top (s channel) ~ 52%
- Wbb ~ 52%
- Wj ~ 0.3% (mistag rate)

**Number of events**

**Number of events**

- e+jets, 1 tag

- e+jets, ≥2 tag

- excess

## D0: signal and background summary

- lepton + jets

**1 tag**

**≥****2 tags**

**tt contribution is calculated assuming  ******** = 7 pb**

##      event b-tagging efficiency  ~ 55%, fake tag rate (QCD jets) ~0.5%

-  tt = 5.6+1.2(*stat*) 1.0(*sys*) pb

- 1.0

- 0.7

-   162 pb-1

- CDF: cross section with *b*-tagging

- 162 pb-1

## Use event shape information in tagged events to further separate signal from background
Examine various kinematic quantities: leading jet ET, subleading jet ET, their sum 

- Use event shape information in tagged events to further separate signal from background
- Examine various kinematic quantities: leading jet ET, subleading jet ET, their sum

-  tt = 6.0+1.5(*stat*) *sys*) pb

- -1.8

-       tt

-   Wbb

- CDF: cross section with *b*-tagging and kinematic fits

- Fit templates to extract signal and background fractions
- Signal templates from tt MC, background templates from data: W+3jets events without *b*-tag, QCD contribution from non-isolated lepton

- 1.8

## D0: all jets channel with SVT

- Final state: 6 jets, 2 *b*-quark jets
- Overwhelmed by QCD multijet background – impossible to extract signal without tagging *b*-jet(s)
- Three NNs combine various kinematic variables: apply successive cuts on the outputs

- Derive SVT tag rate function in the same multijet events. Apply to untagged sample to predict background shape

## All jets channel: discriminating variables

- Energy Scale – HT, s
- Soft non-leading Jets – H3jT, ET5,6,<Nj>
- Event Shape – sphericity, aplanarity

- Rapidity – Centrality, <h2>
- Top Properties – Top and W Mass Likelihood, MWW, Mtt, min dijet masses

- Variables are designed to address different aspects of the background

## D0: all jets channel cross section

- 0.75

- 220 events pass all cuts
- Expect 186 ± 5(stat) ± 7.5(sys)

## CDF: all jets channel cross section 

## Top pair production cross section summary

## Electroweak production of the top quark

- Not  observed yet, despite the expected
- “large rate”  (ss+t ~ 40% stt )
- Existing Run I upper limits (@ 95% CL):
*    **CDF*:   ss < 18 pb, st < 13 pb, ss+t < 14 pb
-     *D**Ø*:     ss < 17 pb, st < 22 pb

-   Access to Wtb vertex
-   Only  vertex which can be sensitively probed at hadron colliders
-   Measure Vtb directly, test CKM unitarity
-   Test VA structure of the Wtb vertex

## Search for single top quark production

- Experimental signature: similar to ttl+jets
- Same signature as SM Higgs associated production:
**W+2 jets bin!**
- Single top samples have less objects in the final state:
** ****larger background**

- Backgrounds: W+jets, tt, multijets (misidentified leptons), diboson
- Experimental strategy:
- Optimize event preselection to maximize signal acceptance while reducing backgrounds/misreconstructed events as much as possible.
***b*****-tagging is extremely important**.
- Make use of topological information to further discriminate 		      signal from background

## CDF: single top quark search

- Event preselection:
- 1 high PT isolated charge lepton (e,m)
-  large missing ET
-  two jets, at least one with a *b*-tag
- Topological selection:
- 140 GeV < Mlb < 210 GeV
- Leading jet: pT < 30 GeV (only t-channel search
- Good agreement between observed number of events and predicted sum of background and signal
- Consider discriminating variables
**Combined (s+t channels) search: H****T**** = p****T**** (lepton)+MET+******** p****T****(jet)**
**t-channel search: Q(lepton)******** (untagged jet)**

## CDF: single top quark search

- Maximum likelihood fit to data HT or Q distributions using a sum of templates determined from MC: single top (PYTHIA), tt (HERWIG), non-top: Wbb (ALPGEN)

- Fitted signal content compatible with zero

- t < 8.5 pb @ 95% CL

- s+t < 13.7 pb @ 95% CL

- L=162 pb-1

## D0: single top quark search

- Event preselection:
- 1 high PT isolated charge lepton (e,m)
-  large missing ET
-  2< # jets<4, leading jet pT < 25 GeV, other pT < 15 GeV
- At least 1 *b*-tag: SVT or soft muon tag
- No topologocal selection applied yet – large room for improvement
- 4 orthogonal analyses:
- e+jets: SVT, soft muon tag
- μ+jets: SVT, soft muon tag

- Background estimates:
- Multijets (QCD): from data
- W+jets: from data using measured inclusive tag rate functions on a multijets sample.
- tt: from ALPGEN MC

- +jets/SVT

## D0: single top quark search

|  | SLT | SVT | JLIP | SLT | SVT |
| --- | --- | --- | --- | --- | --- |
| s-channel | 0.67 0.14 | 1.87 0.46 | 1.88 0.46 | 0.63 0.13 | 1.38 0.35 |
| t-channel | 0.95 0.20 | 3.14 0.76 | 3.20 0.82 | 0.88 0.19 | 2.19 0.56 |
| tt | 9.60 1.65 | 23.31 4.90 | 24.50 5.48 | 8.43 1.44 | 18.57 3.75 |
| non-top | 31.1 5.2 | 63.5 12.3 | 71.7 13.7 | 34.3 5.1 | 67.0 11.8 |
| Total expected | 42.4 5.4 | 91.9 13.3 | 96.3 14.8 | 44.2 5.3 | 89.2 12.4 |
| Observed | 49 | 88 | 99 | 48 | 94 |

|  | Without Systematics | With Systematics |
| --- | --- | --- |
| s (pb) | <6.4 | <13.8 |
| t (pb) | <9.0 | <19.8 |
| s+t (pb) | <7.9 | <15.8 |

- Expected cross-section limits @95% CL from combination of e+jets and μ+jets for SVT and SLT analysis channels:

-  Sensitivity already better than Run I but...**this is just half of the analysis!!**
-  Significantly increased statistical sensitivity expected  from use of topological information

## The Top Quark Mass

- Fundamental SM parameter
**needed to determine ttH coupling **
**Enters as a parameter in the calculation of radiative corrections to other SM observbles**
- Experimental handles:
**B tagging: reduce background & combinatorial**
**Data driven systematics scale with 1/********N (energy scale, gluon radiation) **

- CDF/D0
- 2 fb-1goal!

**With 2 fb****-1**** data (Run II): ********M****t**** = 3%                      constrain ********M****h****/M****h**** to 35%**

## D0: Run I top quark mass update

- Old approach
**Lowest ********2****  solution from kinematic fit**
**Topological discriminant is used to separate signal from background**
**Mass estimate is made by 2D fit in fitted mass and descriminant and compared to MC template**
- New approach
**Calculate event probability to be a tt event**
**Consider all combinations, assign each a weight and sum their probabilities**
**Increase purity by cutting on background probability: 22 events (out of 71 4 jet events survive) **

**M****top**** = 180.1 ± 3.6 (stat) ± 4.0 (syst) GeV**

**Improvement in statistocal error is equivalent to 2.4 times more data**

- New Run I D0 top mass average:
- Mt = 179.0 ± 5.1 GeV

## New top quark mass average

- Two updates:
**New D0 Run I l+jets mass measurement**
**Latest  CDF results (PRD63 032003, 2001)  **

**New Tevatron CDF+D0 result: **
**M****top****=178 ± 4.1 GeV**
- Breakdown of errors:
**2.7** statistical, **3.3** systematical error

**2.6 **  jet energy scale **1.65** signal modelling **0.12** Monte Carlo **0.88** background

**0.83** U-noise and multiple interactions **0.35 **fit

## Consequences for SM Higgs

## Conclusions and Outlook

- The top quark is back!
- First Run II results cover a variety of channels and topics and are improving rapidly. All measurements consistent with the Standard Model – but still statistics limited
- The Tevatron is the unique top quark factory until LHC
- We still expect at least 50x more data compared to Run I!

- It is the start of a program of precision top physics  and hopefully top surprises  at the Tevatron

## Backup slides

## New Run I Top  Mass (D0)

- _Template Method_

- _Optimized Method (ME) _

- All the events are presented to the _**same template**_.
- The template corresponds to a probability distribution for the entire sample, using _**several variables**_ calculated from MC simulations.
- The features of individual events are integrated _**(averaged) over the variables not considered**_ in the template.

- Each event has its _**own probability distribution.**_
- The probability depends on _**all measured quantities**_ (except for unclustered energy).
- The full information contained in each event is contributed to the probability: _**well-measured events contribute more than poorly-measured events.**_

## Matrix Element Method

***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 Matrix element

-  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)

- Measured, reconstructed objects in the event

-  = Mtop is to be estimated

- Probability of measuring the event