# Presentation: 550620

## Top Quark Physics

**Pierre Savard**
**University of Toronto and TRIUMF**
**APS Meeting **
**Denver**
**May  2004**

## History and Theoretical Overview
Review of Experimental Results
Run I Results
Run II Results
cross section measurements
mass measurements
other results
Conclusions and Outlook

- Review of Experimental Results
- Run I Results
- Run II Results
- cross section measurements
- mass measurements
- other results
- Conclusions and Outlook

**Outline**

## Discovery of b quark in 1977 
isospin analysis shows that b  should have SU(2) partner

Indirect evidence of top through loop contributions:
BB Mixing
Z      bb rate
 MW/MZ ratio 

95 CDF and DØ announced  discovery                

**isospin analysis shows that b  should have SU(2) partner**
**Indirect evidence of top through loop contributions:**
- BB Mixing
- Z      bb rate
-  MW/MZ ratio
**95 CDF and D**Ø **announced  discovery****                **

**Some History**

**-**

**-**

-       If we assume CKM matrix unitarity and measured mass ~ 180 GeV/c2  , then top properties are well understood within context of SM:
- Spin = 1⁄2
- couplings:
**      ****- **+2/3e
-          - color triplet
-          - weak (T3)L
**      ****- **Yukawa coupling  ~1** **
- Width:  ~ 1.4 GeV
- lifetime   ~ 5 x 10-25  (no top hadrons!)

**Top Quark in the SM (I)**

- Mt at the ewk scale
- Only quark decaying to W
- Large Mt-Mb difference implies large ewk loop corrections

- Vtb = 0.99
- branching ratios:
- t      W b	  BR ~ 1
- t      W s	  BR ~ 10-3
- t      W d  BR ~ 5 x 10-5
- t        c,u BR ~ 10-8
- t      Z c,u BR ~ 10-12
- Fraction of longitudinal W bosons:

**Top Quark in the SM (II)**

- Experimental tt signatures depend on how W decays:

**~70% of W bosons longitudinally polarised for Mt of 175 GeV/c****2**** **

**-**

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

In supersymmetric models, large top mass causes EWSB:

In many dynamical symmetry breaking models, top interactions are modified:
Examples: technicolour models like Top Flavour, Top Seesaw, Topcolour-assisted technicolour
Need to test all aspects of top production and decay. Experimentally we still know very little about the top quark  

**In supersymmetric models, large top mass causes EWSB:**
**In many dynamical symmetry breaking models, top interactions are modified:**
**Examples: technicolour models like Top Flavour, Top Seesaw, Topcolour-assisted technicolour**
**Need to test all aspects of top production and decay.**** ****Experimentally we still know very little about the top quark****  **

**Top Quark Beyond the SM **

**Run 1, 100 pb-1: **
- collisions every ~ 3****sec
- beam energy 900 GeV
- inst. Luminosity 10**31**
**Run 2:**
- collisions every ~400ns
- beam energy 980 GeV
- inst.  Luminosity 10**32**
- CDF and DØ detectors underwent major upgrades for Run II

-  Experimental Top Quark Physics

**Tevatron Collider is world’s only top quark production machine **

## Run I Experimental Results

- Samples collected by identifying strong production of pairs of top quarks (have also looked for ewk production)
- To help isolate signal, some analyses look for evidence of a B hadron decay:
- Secondary Vertex Tagging (SVT or SVX)
- Soft Lepton Tagging (SLT):

##  Run I Results: Production Properties 

- Test of QCD_ _
- Overall discrepancy could indicate non-SM production mechanisms
- Inconsistencies between channels could indicate non-SM  decay mechanisms
- Run I results consistent with SM but with large statistical uncertainties

##  Run I Results: Top Mass  

- Top mass important ewk parameter (due to t-b mass difference)
- Uncertainty on top mass currently limiting factor in indirect determination of Higgs mass
- Accurate measurement needed for self-consistency tests of SM
- New Run I DØ  l+jet result using matrix element technique

## Improved Method by DØ 

- Use Probability density:
- Background probability
- Main component W+jets (85% of background)
- Pbkg calculated from leading order matrix element from VECBOS
- 22 events remain: 12 signal, 10 background
- Dominant systematic is jet energy scale: 3.3 GeV/c2

- x : reconstructed
- 4-vectors

- LO Matrix element + phase space

- PDF’s

- Transfer function:
- parton values to
- measured quantities

- Mt = 180.1 ± 3.6 (stat) ± 3.9 (syst) GeV/c2 =
- 180.1 ± 5.3 GeV/c2

## New Run I Top Mass Result and implications on  Higgs Mass

- New DØ combined mass:
- Mt = 179.0 ± 5.1 GeV/c2
- New world average:
- Mt = 178.0 ± 4.3 GeV/c2
- Global fit to electroweak data                            using this top mass
- Method of LEPEWWG                                                  (hep-ex 0312023)
- Best-fit MH  113 GeV/c2
- 95% C.L. upper limit 237 GeV/c2

- Yellow region excluded:
-  MH < 114.4 GeV/c2 @95% CL

**Notes:**
96 +60 -38
123 +76 -50

##  Other Run I Results: Single Top and Branching Ratios 

**Single top D****Ø** **cross section (s-channel)**
**Single top D****Ø**** cross section (t-channel) **
**Single top CDF cross section (s-channel) **
**Single top CDF cross section (t-channel)**
**Fraction of longitudinal W bosons (D**Ø**):**
**Fraction of longitudinal W bosons (CDF):**
- Branching ratios (CDF):** **

-  **F****0**** = 0.91**** **** 0.37  0.13 **

-  **F****0**** = 0.56**** **** 0.31  0.04 **

-  ******< **** 17 pb @ 95% c.l**

-  ******< **** 22 pb @ 95% c.l**

-  ******< **** 15.8 pb @ 95% c.l**

-  ******< **** 15.4 pb @ 95% c.l**

## Integrated luminosity between 100 and 200 pb-1
Focus on new cross section and mass results
Theoretical cross section: ~ 7 pb 

- Focus on new cross section and mass results
- Theoretical cross section: ~ 7 pb

**Run II Results**

## Dilepton cross-section: lepton+track (CDF)

- Signature: 1 lepton + 1 isolated track, missing ET , 2 central jets
- Higher acceptance reduced purity relative to  Run 1,
- Backgrounds:  Z/ * l+l-, WW, WZ, ZZ, W+jets (fake leptons)

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

- 19 events on 7.1 ± 1.2 background
- 11 e-track, 8 -track

-  **7.0****+2.7****-2.3****(*****stat*****) ****+1.5****-1.3****(*****sys*****) ********* *****0.4 (*****lumi*****) pb**

## Lepton + track 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, we observe good agreement with SM

## Dilepton cross-section: ee,,e final states (CDF)

- Different background composition; Lower acceptance, but higher S/B

- 13 events (1 ee, 3 , 9 e, expect 10.6 SM with 2.4 ± 0.7 events.  Result:

**Combined result, (hep-ex/0404036, 1st Run II top paper):**

-  **7.0****+2.4****-2.1****(*****stat*****) ****+1.6****-1.1****(*****sys*****) ********* *****0.4 (*****lumi*****) pb**

-  **8.4****+3.2****-2.7****(*****stat*****) ****+1.5****-1.1****(*****sys*****) ********* *****0.5 (*****lumi*****) pb**

## Physics background Z/* l+l, W+W- estimated using MC
Instrumental backgrounds determined from data: 
fake missing ET  in ee channel
isolated fake e/ in all three channels 

- Instrumental backgrounds determined from data:
- fake missing ET  in ee channel
- isolated fake e/ in all three channels

**ee: 156 pb****-1  ****e********: 140 pb****-1  ********: 143 pb****-1**

- Dilepton cross-section: ee,****,e**** final states (DØ)

## Dilepton cross-section: ee,,e final states (DØ)

- Dilepton cross-section: ee,****,e**** final states (DØ)

-  **13.1****+5.9****-4.7****(*****stat*****) ****+2.2****-1.7****(*****sys*****) ********* *****0.9 (*****lumi*****) pb**

-  **11.7****+19.7****-14.1****(*****stat*****) ****+7.9****-5.0****(*****sys*****) ********* *****0.8 (*****lumi*****) pb**

- Kinematic distributions below: Ht (left) and lepton Pt (right)

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

## Lepton+jets cross-section using event topology DØ

- Signature: high-pT isolated lepton, missing ET and   jets
- Combine topological variables in event Likelihood. Choose variables with
- Good signal-to-background discrimination
- Small correlations
- Low sensitivity to jet energy scale (e.g. sphericity, energy ratios)
- Fit data to signal and background templatesextract tt fraction

## Lepton+jets cross-section using event topology DØ

- +jets
- 144 pb-1

- e+jets
- 141 pb-1

-  **8.8****+4.1****-3.7****(*****stat*****) ****+1.6****-2.1****(*****sys*****) ********* *****0.6 (*****lumi*****) pb**

-  **7.2****+2.6****-2.4****(*****stat*****) ****+1.6****-1.7****(*****sys*****) ********* *****0.4 (*****lumi*****) pb**

-  **6.0****+3.4****-3.0****(*****stat*****) ****+1.6****-1.6****(*****sys*****) ********* *****0.4 (*****lumi*****) pb**

## Lepton+jets cross-section using SVX tag CDF

- Analysis requirements at least 1 displaced vertex tag (SVX)
- Event b-tagging efficiency  ~ 55%, fake tag rate (QCD jets) ~0.5%
- Main backgrounds: W + heavy flavour, W + fake tag, QCD
- Count events with 3 or more jets and Ht > 200 GeV

-   162 pb-1

- (l+jets, SVX) =
**5.6****+1.2****-1.0****(*****stat*****) ****+1.0****-0.7****(*****sys*****) ****pb**
**Double Tag Analysis Result:**
**5.4 **** ****2.2(*****stat*****) ******** ****1.1 (*****sys*****) ****pb**

## All jets cross-section using SVX Tags (CDF)

-  Final state: 6 jets, 2 b-quark jets (top needle in a haystack of QCD)
- Use dedicated trigger (4 jets > 15 GeV and sumEt >125 GeV)
- S/B of 1/2500 increased to 1/24 with sumEt> 320 GeV and topo. cuts: aplanarity, centrality
- Require 6 to 8 jets, and SVX tags
- Dominant systematic uncertainty due to jet energy scale

- (all jets, SVX) =
**7.8****+2.5****-1.0****(*****stat*****) ****+4.7****-2.3****(*****sys*****) ****pb**

## All jets cross-section using NN and SVX Tag (DØ)

-  Final state: 6 jets, 2 b-quark jets
- Require 1 SVT tag rate and use  untagged  sample to predict background shape
-  Three NNs combine various kinematic variables: Ht, sphericity, aplanarity, centrality etc.
- 220 observed with expected background of 186** **** 5  12**

- (all jets, SVX) =
**7.7****+3.4****-3.3****(*****stat*****) ****+4.7****-3.8****(*****sys*****) **** 0.5 (*****lum*****) ****pb**

## Run II Top Cross Section Summaries

## Top mass: l + jets template (CDF)

- find top mass that best fits event
- loops over jet-parton assignments 
- Impose constraints: Mt=Mt , M(j,j) = M(*l*,) = MW, with inputs: MW, W, t
- loop over two solutions for pz  of 
-  2-C fit performed
- Perform likelihood fit:
- find top mass template that best fits data with background templates
- background normalisation constrained

## Top mass: l + jets (template)

- Choose events with 4 jets, 1 vertex tag
- 28 events in 162 pb-1 with estimated bckg of 7.0 **±** 0.8
- Syst. uncertainty dominated by jet-energy scale.
- Result:

**m****t**** = 174.9 ****+7.1****-7.7 ****(stat) ± 6.5 (sys) GeV**

## Top Mass: l + jets (DLM)  

-  Dynamic Likelihood Method:
- Likelihood defined as d(Mt)  per unit phase volume of final partons times the transfer function (jets to partons):
- See original paper by K.Kondo J.Phys. Soc. 57, 4126 (1988)
- use 162/pb data sample: 22 events  with 4.2 **±** 0.8 background predicted.

## Top Mass: l + jets (DLM)  

- Result:

**m****t**** = 177.8 ****+4.5****-5.0 ****(stat) ± 6.2 (sys) GeV**

- Systematic Uncertainties:

- Result:

##  Some other Run II Results (CDF) :

- Single top cross section (t-channel 162 pb-1)
- Single top cross section (channels combined, 162 pb-1)
- top mass in dilepton channel (126 pb-1)
- cross section ratio ll)/lj) (125 pb-1)
- Longitudinal W fraction (162-193 pb-1)

-  ** ****1.45**** ****+**** 0.83**** ****- 0.55**** **

-  ********** ****< ****8.5 pb @ 95% c.l**

-  **********< ****13.7 pb @ 95% c.l**

**175 ± 17****stat**** ± 8****sys**** GeV**

-  ** ****0.27**** ****+**** 0.35**** ****- 0.21**** **

## Conclusions and Outlook (I)

- New Run I measurement from D0 shifts world average top mass to 178 GeV/c2
- Run II measurements up to date are consistent with SM expectations for Mt ~ 180 GeV/c2
- We are now improving upon many Run I measurements but we are still at a very early stage of the Run II top physics program

## Conclusions and Outlook (II)

- “Precision” top quark measurements in sight at Tevatron
- Future looks very bright:
- Top factory (LHC) will turn on in a few years.
- Fantastic top physics to be done with ATLAS and CMS (e.g. see hep-ph/0003033)

- Some measurement targets to aim for in Run II