# Presentation: 506727

## Physics with Top Quarks

- Prof. Robin Erbacher
- University of California, Davis
- Lepton-Photon 2007

## Something about top history

- Top Discovery!
- Tevatron Run 1
- 1994-5

- R. Erbacher - LP07

## Top Rediscovered in Run 2

**Top Rediscovered in Run 2!**

- R. Erbacher - LP07

## Periodic Table of the Particles

**Periodic Table of the Particles**

- R. Erbacher - LP07

## Top Quark Production

- _Top Quark Production_
-  Mechanism
-  Top Pair Cross Section
-  Ewk Production (single top)
-  Forward-backward asymmetry
-  Resonances decaying to top
-  stop production

- _Top Event Decays_
-  W helicity (V-A)
-  Branching ratios
-  Top to charged higgs
-  Top sample (W+c)
-  FCNC

- _Top Properties_
-  Top Mass
-  Top Quark Width
-  Charge of Top Quark

- R. Erbacher - LP07

## Events Characterized by W Decays

** ****t********W****b**** ~ 100% **

- R. Erbacher - LP07

## Dilepton Decay Mode

- R. Erbacher - LP07

## Lepton+Jets Decays 

- Lepton+Jets Decays

- R. Erbacher - LP07

## All-Hadronic Decays

- R. Erbacher - LP07

## Muon + jets event with 

- Muon + jets event with
- 2 tagged b-quark jets

- R. Erbacher - LP07

## How is top produced?

**Strong Pair Production at the Tevatron**

**Rarely!!**

**~85%**

**~15%**

- R. Erbacher - LP07

## Electroweak 

- Electroweak
- Single Top Production

- New
- Resonance
- Production?

**s-channel ~1 pb**

**t-channel ~2 pb**

- R. Erbacher - LP07

## Fermilab Tevatron

**Fermilab Tevatron**

- R. Erbacher - LP07

## Slide 14

- R. Erbacher - LP07

**Notes:**
General purpose detectors capable of many different physics measurements
Top physics uses almost all detector systems

## Top Quark Production

- _Top Quark Production_
-  Production mechanism
-  Top pair cross section
-  EWK production (single top)
-  Forward-backward charge asymmetry
-  Resonances decaying to top

- R. Erbacher - LP07

## Does t-tbar production match NLO prediction?

- (ggtt)/(pptt) = 0.07 ± 0.16
*(N**_x0005_**LO: ****gluon**~15%, ****quark**~85%)*
*(neural net analysis: f**g** < 0.33 @ 68% CL)*

- R. Erbacher - LP07

## Top Pair Production

- Cross Section:
-  As QCD predicts?
-  Only SM top?
-  By heavy particles?

- t-tbar!

- R. Erbacher - LP07

## Top pair

- Event topology
- Discriminant:
- No b-jet
- tagging

- Requiring two
- identified b-jets:
- Ultra pure top
- pair sample

- R. Erbacher - LP07

**Notes:**
Lepton + Jets

## Many new dilepton measurements!

| Dilepton Selection | Cross Section (Mtop =175 GeV) |
| --- | --- |
| 2 Tight Leptons (1.2 fb-1) | tt=  6.2 ± 1.1(stat)± 0.7 (sys) pb |
| Lepton+Track   (1.1 fb-1) | tt=  8.3 ± 1.3(stat)± 0.9 (sys) pb |
| Lepton+Track+btag (1.0 fb-1) | tt= 10.1 ± 1.8(stat)± 1.3(sys) pb |
| 2 Tight Leptons (1.0 fb-1) | tt=  6.8 ± 1.5(stat)± 1.0_x0010_(sys) pb |
| Lepton+Track+btag (1.0 fb-1) | tt=  4.9 ± 1.5(stat)± 1.0_x0010_(sys) pb |
| Lepton+Tau+btag (1.0 fb-1) | tt=   8.3 ± 1.9(stat)± 1.4 (sys) pb |

- Tau Lepton
- channels
- difficult!

- *BR(ttl++2+2b) =
- 0.19±0.08(stat) ± 0.07(syst) !

- R. Erbacher - LP07

**Notes:**
Expected xs*BR = 0.12 from theory

## Single Top Production:

- Backgrounds!
-  Best channels S/B~1/20
-  Signal smaller than
- background uncertainty!

-  Rate  |Vtb|2 in SM
-  Sensitive to H+, 4th gen,
- 		W’, FCNC, ...
-  Signature ~ SM Higgs
-  SM cross section ~3 pb

- R. Erbacher - LP07

**Notes:**
1/2 SM cross section
Add tevatron xs plot w/ st normalized to one

##   multivariate techniques 

**neural network**

-   multivariate techniques
-   can coax signal out
-   from large backgrounds

**Boosted **
**decision **
**tree**

- boosted decision trees, matrix
- element reconstruction, bayesian
- neural networks, likelihood discriminants

- R. Erbacher - LP07

## D0 Single top results

- First direct limit on Vtb:
- 0.68 <|Vtb|< 1 @ 95%CL

- s+t= 4.9 ±1.4 pb

- Expected sensitivity: 2.1

**Boosted **
**decision trees**

-  D0 Results: Search for Single Top

- R. Erbacher - LP07

**Notes:**
Taken from users meeting talk-- clean it up a bit.  

## Results for Single Top from CDF

- Expected  p-value = 0.13% / 3.0

- New CDF Results: Search for Single Top

- s+t= 3.0 ± 1.2 pb
- s= 1.1, t =1.9 pb

- Expected sensitivity: 2.9
- Observed significance: 2.7

- Expected sensitivity: 3.0

- s+t= 2.7 ± 1.2 pb
- s= 1.1, t =1.3 pb

- R. Erbacher - LP07

**Notes:**
Came much earlier than predicted due to experimental creativity and hard work.
LF:
Observed p-value = 0.31%  / 2.7
Expected  p-value = 0.20% / 2.9
Upper limit: 5.14 pb at 95% CL  (Bayesian calculation).

Median expected limit in background-only pseudoexperiments:  _x0001_2.06 pb

Matrix element:
Observed p-value = 0.09%  / 3.1
Expected  p-value = 0.13% / 3.0

## |Vtb|= 1.02 ± 0.18 (expt) ± 0.07 (theory)

- |Vtb|= 1.02 ± 0.18 (expt) ± 0.07 (theory)

- Z. Sullivan, Phys.Rev. D70 (2004) 114012

*D0 |V**tb**|>0.68*

- R. Erbacher - LP07

## Forward-Backward Production Asymmetry

- Forward-Backward Production Asymmetry Afb

- Diagram interferences for qq

- No asymmetry expected at LO
- 4-6% expected at NLO in parton frame

- J. Kuhn, *et al.*

- Reduced Asymmetry in tt+jet -- Uwer, *et al.*

*Smaller asymmetry in lab frame*

- R. Erbacher - LP07

**Notes:**
No asymmetry for gg initial states.  Qq states with the same final state interfere due to different C eigenstates
Reduced asymmetry for tt+jets.  Can only see this at the Tevatron?

## Afb 

- Afb

- How would new physics look?

- For MZ' = 750 GeV:
- F < 0.44 (expected)
- F < 0.81 (observed)

- Afb= 12 ± 8(stat) ± 1(syst) %
*(Uncorrected for reconstruction) *

- First Afb Result from D0

- F: fraction of top pair events
- produced via Z' resonance

- R. Erbacher - LP07

**Notes:**
Replace a Z’ for the gluon in the diagram

## Afb CDF 

- First Afb Results from CDF

*NLO: (4-7%) in ****y*Q****l***

- Afb=(28 ± 13(stat) ± 5(syst) ) %
*(Fully corrected) *

- R. Erbacher - LP07

**Notes:**
Numbers of events measured in two bins: positive and negative values of y.Ql
The asymmetry value is background subtracted and corrected for smearing effects of ttbar pair reconstruction with the inversion of 2x2 matrix

## Resonances  decaying to ttbar

- New
- Resonance
- Production?

- Bump-hunting for Xttbar!

- New D0 Result!
- See Talk by K. Tollefson Today:
- Tevatron Striking Results

- R. Erbacher - LP07

## Stop Search

- Single-variable separation poor;
- Construct multivariate discriminant.

- Can SUSY
*stop* hide alongside
- top?

- No evidence for stop
*(Limits on BR v. stop mass)*

- R. Erbacher - LP07

## Top Properties

-  Top Mass
-  Top Quark Width
-  Charge of Top Quark

- R. Erbacher - LP07

## Top Quark Mass: Important EWK Parameter

- Top Quark Mass
-  Important EWK parameter
-  Key role in BSM physics models
-  Constrains the Higgs mass
-  Heavy: Unexpected role in EWSB?

- What a theorist sees...

- What an experimentalist sees

- Challenges: combinatorics, b-tagging efficiencies, jet energy scale.

- Solutions: sophisticated analyses,
- in-situ Wjj calibration

- R. Erbacher - LP07

## New for summer 2007!

*(not included in *
*March combination)*

- Top mass:Exciting Program
- of measurements
- at the Tevatron

***Most precise!***

- R. Erbacher - LP07

## Top mass dilepton

- Matrix Element Weighting

- Neutrino-weighting

- Many new
- top dilepton
- mass results!

- Lepton Pt

- Combining channels helps:
- D0 matrix weighting + D0
- neutrino weighting  ~4%
- better for same luminosity

- R. Erbacher - LP07

## New Ideas

- Top mass with cross
- section constraint: trades stat uncertainty for theory

- Top mass extracted from cross sections: Compare to theory and across channels! Consistent with kinematic measurement?

**dilepton**

- New Ideas!

- Cacciari, Mangano, et al
- hep-ph/0303085

- R. Erbacher - LP07

**Notes:**
Cross section constraint gains you precision in stat at the expense of theory.  You gain only until stat~sys
NLO QCD calcs including higher order resummations.  (Put the mass dependence in here?)

## Top mass Best per channel

- Snapshot:
- most precise
- per channel

- Most precise!

**M****top****=171.6 ± 2.0 GeV/c****2**

**M****t****=170.4 ± 3.1****(stat)**** ± 3.0****(sys)****GeV/c****2**

- all-hadronic from
- winter 2007:

**M****t****=170.4 ± 3.7****(stat+JES)**** ± 2.1****(sys)****GeV/c****2**

- R. Erbacher - LP07

## Top mass summary and combination

- World Average March 2007:

**M****top****=170.9 ± 1.9 GeV/c****2**

- See P. Petroff’s Talk (next)
- for electroweak implications

- D0-CDF Joint Systematics Effort Underway!
- New combinations will follow...

- R. Erbacher - LP07

**Notes:**
Note that no new averages will be made until CDF/D0 systematics effort is finished with some studies

## Top quark width

- Top
- Quark
- Width

- t < 12.7 @ 95% CL
- Mt =  175 GeV

- R. Erbacher - LP07

**Notes:**
< 7.5 @ Mtop=170
Blessed July 26th!  First direct measurement of top width in run 2!  (Lifetime limit with 300 pb-1 from CDF set indirect measurement)

## Top Quark Charge: -2/3?

- D0 result with 300 pb-1:
- See no evidence for exotic
- model so far...

- Exclude top charge exotic
- model XM of -4/3* with 87% C.L.

- f+ = 0.87
- P-value = 0.31

*******Chang,Chang,Ma ‘99**

- R. Erbacher - LP07

**Notes:**
Are we observing Standard Model top?
Standard Model top has charge +2/3
Alternative hypothesis: exotic quark with charge -4/3*
Difficult to measure         (“t”W+b or W-b)
W charge measured through the lepton (straightforward)
Bottom charge inferred from jet (difficult)
 Correctly pair the lepton and b jet (difficult)
Select and fully reconstruct top events in lepton+jets (2 b-tagged jets) and dilepton channel (≥1 tagged jet)
Determine:
   flavor of b-jet
   charge of W (lepton)
   pairing between W and b (2 fit and Mlb)

## Top Event Decays

-  W helicity (V-A)
-  Toplight charged higgs
-  Branching Ratios
-  Top sample (W+c)
-  FCNC

- R. Erbacher - LP07

## t-W-b Coupling

- V-A?

- The V-A character of the decay
- makes the helicity of the W only
- F0 = 0.70, F- = 0.30, F+ = 0
*(left-handed, longitudinal, right-handed)*

- cos* = angle between lepton and top in W rest frame

- R. Erbacher - LP07

## W helicity

- t-W-b Coupling: W Helicity

***1.7 fb******-1***

- _1-d fit: Fix F__0__=0.7, fit for F__+_
- D0(1 fb-1) : f+=0.02 ± 0.05 ± 0.05
-                   f+<0.14 @ 95%CL
- CDF1(1.7 fb-1) : f+=0.01 ± 0.05 ± 0.03
-                          f+<0.12 @ 95%CL
- CDF2(1.7 fb-1) : f+=-0.04 ± 0.04 ± 0.03
-                          f+<0.07 @ 95%CL
- _2-d fit: Fit for F__0__, F__+ __together_
- CDF1(1.7 fb-1) : f0=0.38 ± 0.22 ± 0.07
- 	            f+=0.15 ± 0.10 ± 0.04
- CDF2(1.7 fb-1) : f0=0.61 ± 0.20 ± 0.03
- 	            f+=-0.02 ± 0.08 ± 0.03

- V-A: F0=0.7, F-=0.3
- V+A: F0=0.7, F+=0.3

- R. Erbacher - LP07

## Simultaneous measurement of  and Branching Ratio

******tt**** = 8.10****-0.82**** (stat+syst) ****±**** 0.49 (lumi) pb**

**+0.87**

**R= 0.991****-0.085**** (stat+syst)**

- Simultaneous measurement of  and Branching Ratio

- Simultaneous measurement of  and Branching Ratio

- New measurement by D0!
- See Talk by K. Tollefson Today:
- Tevatron Striking Results

- R. Erbacher - LP07

**Notes:**
one analysis is performing the simultaneous xs and R measurement. From that we extract V_tb. Let me call this R=R_b since it's the top decay Br into a b-quark over the top decay Br into all quarks. It is thus a measurement which is sensitive on b disappearance.

## Limits on charged higgs

- Ratio of Cross Sections:
- Limit on Charged Higgs!

- R=(pptt)l+jets/(pptt)ll

- R=1.21 ± 0.26 (stat+sys)

- Assume tH+b, H+cs only.
- If MH=MW (not ruled out by LEP):

- B(tH+b)<0.35 @95% CL

*Previous CDF result with 200 pb**-1** explores other parameter spaces.*

- Expected: B(tH+b)<0.35 @95% CL

- R. Erbacher - LP07

**Notes:**
If you get questions about our assumptions on the charged Higgs model:a charged Higgs at 80 GeV which decays predominantly hadronically could be realised in the following scenarios:- general multi-Higgs-doublet model (MHDM). 

It was demonstrated that such a leptophobic charged Higgs boson with a mass of 80 GeV could lead to noticable effects at the Tevatron if tanbeta < 3.5.- within MSSM:
 large radiative corrections from SUSY-breaking effects can lead to a suppression of Hア -> tau nu compared to Hア -> cs.
Ah maybe interesting as well: LEP was not sensitive to a 80 GeV charged Higgs because it was in the W mass peak.

## Search for tZc: FCNC

- No FCNCs in SM at tree level
- Allowed in higher order penguins
- Light quark penguins observed
- e.g. b→sγ observed by CLEO in 1995, BR O(10-4)
- Not yet observed for top
- SM BR: O(10-12)
- New Physics models predict BRs up to O(10-2)
- SUSY, Higgs doublet, Warped extra dimensions *(J. A. Aguilar-Saavedra, Acta Phys. Polon. B35 (2004) 2695)*

**Tree level FCNC**

**Penguin**

- Search for tZc: FCNC

- CDF: First Run 2 limits, better than LEP!
- See Talk by K. Tollefson Today:
- Tevatron Striking Results

- R. Erbacher - LP07

## Summary

- R. Erbacher - LP07

## Backups

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## D0 single top expected and observed

- R. Erbacher - LP07

## How to extract top width

- R. Erbacher - LP07

## Afb and higher orders

- R. Erbacher - LP07

## CDF Single Top: What Changed ?

- _Likelihood_
- improved  treatment of kinematic ambiguities
- Bug fix in matrix element
- More MC statistic allows refined training
- Overall expected sensitivity gain:
-   (as measured on 955 pb-1 analysis) :35%

- _Matrix Element_
- Separate treatment of single and double tag events
- More refined transfer functions
- Overall expected sensitivity gain:
- (as measured on 955 pb-1 analysis) :~10%

- _Common Improvements_
-  new ALPGEN Monte Carlo
-  W + Heavy Flavor normalization from W + 1 jet eve nts
-  Define event selection on **hadron level jets:**
- CDF Top group wide change
- More meaningful to theorists
- Better understood  (MET resolution,  nonW model..)

- Causes _event migrations:_
-  LF loses  1 gains 7
-  ME loses 5 gains 4
-          for highest discriminant          region

- R. Erbacher - LP07

## Cos(theta*) kinematics

- R. Erbacher - LP07