# Presentation: 261092

## Progress in Top Quark Physics

- Evelyn J Thomson
*University of Pennsylvania *
- XVII Particles and Nuclei International Conference
- Plenary session 28 October 2005* *

_**CDF+D0 parallel session talks:**_
**V.4 Peter Renkel *****“Top Quark Mass Measurement in Lepton+Jets Channel”***
**V.4 Tuula Maki *****“Top Quark Mass Measurement in Dilepton Channel”***
**V.4 Robert Kehoe *****“Top Quark Pair Production Cross Section Measurement”***
**V.4 Charles Plager *****“Measurements of Top Quark Decay Properties”***
**V.4 Valentin Necula *****“Search for Resonances in Top Quark Pair Production”***
**V.4 Yurii Maravin *****“Search for Single Top Quark Production”***
**VI.2 Ben Kilminster *****“Search for SM and MSSM Higgs Bosons”***

## Motivation

**Most massive elementary particle**
**Discovered in 1995 by CDF and D0**
**Only few dozen candidates in 0.1 fb****-1**
**Is it really Standard Model top?   Any effects from new physics?**
**Only CDF and D0 can study top until LHC**
**Large 1 fb****-1**** data sample for Winter 2006**
**Top quark mass is a fundamental parameter in the Standard Model and beyond...**
**Huge top quark mass induces significant radiative corrections to W boson mass**
**Reduced uncertainty on top quark mass imposes tighter constraints on unknowns, like Standard Model Higgs boson or SUSY**
**Significant background to many searches for new physics at LHC**

## Top Quark Production & Decay

| mt 
(GeV/c2) |          σ(pb)
Min  Central Max |  |  |
| --- | --- | --- | --- |
| 170 | 6.8 | 7.8 | 8.7 |
| 175 | 5.8 | 6.7 | 7.4 |

**Cacciari et al. **
**JHEP 0404:068 (2004)**
**Kidonakis & Vogt **
**PRD 68 114014 (2003)**

**At √s=1.96 TeV:  **
**85% qq **
**15% gg**

**At √s=14 TeV:**
**10% qq**
**90% gg**
-  **σ**** = ****833 ± 100 pb**

**Produce in pairs via strong interaction**

**Decay singly via electroweak interaction t→W****+****b**

**t→Wb has ~100% branching ratio**
**Width ~1.5 GeV so lifetime 10****-25****s**
**No top mesons or baryons!**
**Final state characterized by**
**number and type of charged leptons**
**from decay  of W****+**** and W****-**** bosons**

***Lepton***
***+ jets***

***Dilepton***

## Snapshot of Tevatron Operation

**Peak Luminosity (cm****-2****s****-1****)**

**Snapshot of Tevatron Operation**

- World Record Peak Luminosity yesterday!
- 1.58x10+32cm-2s-1
- Deliver 8 fb-1 if all upgrades succeed
- Note electron cooling upgrade making good progress!
- Deliver 4 fb-1 even if no further improvements
- Already delivered over 1fb-1 to experiments

**Run I**
**best**

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

**Produce ~3 top pairs every hour, or 6 billion collisions**

## Snapshot of CDF & DO Data

**Current top quark physics results from  ~350 pb****-1**** of data up to September 2004**
**2005 excellent year for CDF and D0!**
**Both experiments have collected over 1 fb****-1 ****of data at ****√s=****1.96 TeV**
**Watch out for top results with 1 fb****-1**** at Moriond 2006**

## Top Quark Production & Decay

| mt 
(GeV/c2) |          σ(pb)
Min  Central Max |  |  |
| --- | --- | --- | --- |
| 170 | 6.8 | 7.8 | 8.7 |
| 175 | 5.8 | 6.7 | 7.4 |

**Cacciari et al. **
**JHEP 0404:068 (2004)**
**Kidonakis & Vogt **
**PRD 68 114014 (2003)**

**At √s=1.96 TeV:  **
**85% qq **
**15% gg**

**At √s=14 TeV:**
**10% qq**
**90% gg**
-  **σ**** = ****833 ± 100 pb**

**Produce in pairs via strong interaction**

**Decay singly via electroweak interaction t→W****+****b**

**t→Wb has ~100% branching ratio**
**Width ~1.5 GeV so lifetime 10****-25****s**
**No top mesons or baryons!**
**Final state characterized by**
**number and type of charged leptons**
**from decay  of W****+**** and W****-**** bosons**

***Lepton***
***+ jets***

***Dilepton***

## Dilepton

**Dilepton**

| Events | ee | μμ | eμ | Total |
| --- | --- | --- | --- | --- |
| Bkg | 1.0±0.3 | 1.3±0.4 | 4.5±2.2 | 6.8±2.2 |
| Data | 5 | 2 | 21 | 28 |

**2 isolated electrons/muons p****T****>15 GeV/*****c***
**At least 2 jets p****T****>20 GeV/*****c***
**Reduce backgrounds:**
**Z/****γ*****→****ee with MET and sphericity**
**Z/****γ*****→****μμ**** ****with MET and**** ****χ****2 ****consistency with Z mass**
**Z/****γ*****→****ττ→****e****υ****e****υ****τ****μυ****μ****υ****τ**** ****with**** ****Σ****p****T**** of jets and leading lepton**
**Instrumental with multivariate likelihood electron id in ee channel**

## Lepton+Jets

**Lepton+Jets**

**1 isolated electron/muon p****T****>20 GeV/c**
**At least 3 jets p****T****>15 GeV/c**
**MET>20 GeV**
**Need more discrimination against same final state from W+jets processes!**

_**Kinematic event observables**_
**Decay products of massive top quarks more energetic and central than W+jets **
**Combine several kinematic observables in optimal artificial neural network**
**Fit observed data to expected distributions from signal and backgrounds **

## Lepton+Jets with b-tagging

**Lepton+Jets with b-tagging**

**Each top quark decay produces one energetic central b-quark,  however, only few % W+jets have b or c quarks**
**Distinctive experimental signature from long lifetimes of massive B hadrons**
**Reconstruct significantly displaced secondary vertex from ****charged B   decay products**** inside jet**
**Efficiency per b-jet about 50%**
**False positive rate about 1%**

**CDF Run II**
** ****Preliminary**

## Lepton+Jets with b-tagging

**Lepton+Jets with b-tagging**

| Events | Control region |  | Signal region |  |
| --- | --- | --- | --- | --- |
| Nbtag=1 | W+1 jet | W+2 jets | W+3 jets | W+≥4 jets |
| Bkg | 254±38 | 228±31 | 71±9 | 22±2 |
| Data | 251 | 215 | 121 | 88 |

**Single tag**
**N****btag****==1**

**Double tag**
**N****btags****≥2**

| Events | Control  | Signal region |  |
| --- | --- | --- | --- |
| Nbtags≥2 | W+2 jets | W+3 jets | W+≥4 jets |
| Bkg | 17±3 | 7±1 | 1.9±0.3 |
| Data | 22 | 11 | 21 |

## eτh and  μτh                  Neutrino+jets

**1 isolated electron/muon p****T****>20 GeV/c**
**1 isolated *****τ******→******υ******τ******+******hadrons***** p****T****>15 GeV/c**
**MET>20 GeV**
**At least 2 jets p****T****>20 GeV/c**
**Reduce backgrounds**
**Total transverse energy >205 GeV**
**Not compatible with Z****→****ττ**

**Zero isolated electrons/muons!**
**At least 4 jets p****T****>15 GeV/c**
**MET significance > 4 GeV****1⁄2**
**MET not collinear with jets**
**At least 1 b-tag**
**In future: explicit tau identification!**

| Events (195 pb-1) | eτh | μτh |
| --- | --- | --- |
| Bkg | 0.8±0.1 | 0.5±0.1 |
| Data | 2 | 0 |

**CDF set limit on anomalous decay rate**

**CONTROL**

**CDF Run II**
**Preliminary**

## All-hadronic

**At least 6 jets with p****T****>15 GeV/c**
**Reduce huge background from QCD processes at a hadron collider!**
**At least one b-tag**
**Combine kinematic observables in artificial neural network**
**Require NN>0.9**

| Events | All-hadronic |
| --- | --- |
| Raw Bkg | 494±5 |
| Corrected Bkg | 482±5 |
| Data | 541 |

## Is this the standard model Top Quark?

**Top always decays to *****W******+******b?***
**Any Charged Higgs from *****t******→******H******+******b?***
**Top electric charge is +2/3?**
**W helicity “right’?**
**Anomalous FCNC *****t******→******Zc, gc, ******γ******cb?***

**Search for Single Top Quark Production**

**Pair Production Rate**
**New massive resonance *****X******→******tt? ***
**Top spin**
**Tests of NLO kinematics**

**Test Top Quark Decay**

**Test Top Quark Pair Production**

**Observe Top Quark Pair Production**
**in all final states**

**Precision measurement **
**of top quark mass:**
**30% improvement this year!**

## Is this the standard model Top Quark?

**Top always decays to *****W******+******b?***
**Any Charged Higgs from *****t******→******H******+******b?***
**Top electric charge is +2/3?**
**W helicity “right’?**
**Anomalous FCNC *****t******→******Zc, gc, ******γ******cb?***

**Search for Single Top Quark Production**

**Pair Production Rate**
**New massive resonance *****X******→******tt? ***
**Top spin**
**Tests of NLO kinematics**

**Test Top Quark Decay**

**Test Top Quark Pair Production**

**Observe Top Quark Pair Production**
**in all final states**

**Precision measurement **
**of top quark mass:**
**30% improvement this year!**

## Does top always decay to W+b? Part (b)

**Does top always decay to W****+****b? Part (b)**

- If BR(t→Wb) is lower than SM prediction of ~100%, or     if b-tag efficiency is lower than estimated value
- observe fewer double b-tag events
- observe more events without any b-tags
- Fit R=BR(t→Wb) / BR(t→Wq)  times b-tag efficiency from observed number and estimated composition of 0,1,2-tag dilepton and lepton+jets events

**CDF 161 pb****-1**

**Δε****= ****ε****b****- ****ε****light****= ****0.44 ****± 0.03**
**from independent estimate**

**R>0.62 @ 95% C.L.**

## Does top always decay to W+b? Part (W)

**Does top always decay to W****+****b? Part (W)**

- Branching ratio for t→H+b significant (>10%) for small and large tanβ
- H+ decays differently than W+
- H+→τ+υτ enhanced if high tanβ: observe more taus!
- H+→t*b→W+bb for high m(H+) if low tanβ: mimics SM signature but observe more b-tags
- Compare number of observed events in 4 final states: dilepton, eτh + μτh, lepton+jets with single b-tag, and lepton+jets with double b-tags

**Set limits in several MSSM scenarios with NLO corrections**

## Does top always decay to W+b? Part (W+)

**Does top always decay to W****+****b? Part (W****+****)**

- Electric charge of +2/3 implies t→W+b
- Electric charge of  -4/3 implies t→W-b
- How to tell the difference experimentally?

- Measure Λ=11.5
- Exclude Q= -4/3 @ 94% C.L.

**First result!**

**Select 21 double b-tag lepton+≥4 jets**
**Very pure sample with only 5% bkg**
**Statistical estimate b charge from jet-charge**
**Pick best lepton and b-jet combination with kinematic fit for fixed m****top****=175 GeV/c****2**** hypothesis**
**17 double b-tag events pass**
**Correct assignment 79****±2% **
**Calculate magnitude of “top” charges**
**Q****1****=| lepton charge + b****1****-jet charge|**
**Q****2****=|-lepton charge + b****2****-jet charge|**
**Define ****Λ**** as ratio of unbinned likelihoods for ****SM (Q=+2/3)**** and Exotic (Q=-4/3) hypotheses**

## Decay consistent with standard model so far!

**Top always decays to *****W******+******b?***
**Any Charged Higgs from *****t******→******H******+******b?***
**Top electric charge is +2/3?**
**W helicity “right’?**
**Anomalous FCNC *****t******→******Zc, gc, ******γ******cb?***

**Search for Single Top Quark Production**

**Pair Production Rate**
**New massive resonance *****X******→******tt? ***
**Top spin**
**Tests of NLO kinematics**

**Test Top Quark Decay**

**Test Top Quark Pair Production**

**Observe Top Quark Pair Production**
**in all final states**

**Precision measurement **
**of top quark mass:**
**30% improvement this year!**

## Top Pair Production Rate

**Top Pair Production Rate**

- Are measurements in different final states consistent with each other and with theory?

**Χ****2****/dof=****4.3/5**

**D0 Run 2 Preliminary**

## Does something new produce ttbar?

**Does something new produce ttbar?**

- Search for new massive resonance decaying to top pairs
- Lepton+≥4 jets with ≥ 1 b-tags
- Kinematic fit to ttbar hypothesis to improve experimental resolution on invariant mass of ttbar system
- Fix SM backgrounds to expected rate
- Use theory prediction of 6.7pb for SM top pair production

**Interpret in terms of **
**one **
**of many possible models: **
**topcolor assisted technicolor Z’**

## What does CDF observe?

**What does CDF observe?**

- Lepton+≥4 jets (no b-tagging)
- Matrix element technique to increase sensitivity
- Fix top pair, diboson, QCD to expected rates
- Assume everything else is W+jets
- Also see excess around 500 GeV/*c*2
- Only 2 std. dev. now...could be interesting result with 3xdata for Moriond 2006

## Does something new produce Single Top Quarks?

**Does something new produce Single Top Quarks?**

**Single top quark production via electroweak interaction**
**Cross section proportional to |V****tb****|****2**

**0.88 ± 0.11 pb**

**1.98 ± 0.25 pb**

**Harris et al PRD 66 (02) 054024**
**Cao et al hep-ph/0409040**
**Campbell et al PRD 70 (04) 094012**

**See Tait, Yuan **
**PRD63, 014018 (2001)**

**t-channel**
**Sensitive to FCNCs**

**s-channel**
**Sensitive to new resonances**

**Interesting to measure both channels – sensitive to different physics**

**<0.1 pb**

**Tait PRD 61 (00) 034001**
** ****Belyaev, Boos  **
**PRD 63 (01) 034012**

**Trigger on lepton from t****→Wb→****l****υ****b**
**                 ****2 b-jets for s-channel              1 b-jet and 1 light jet for t-channel **

## Search for Single Top Quark Production

**Search for Single Top Quark Production**

- Why is it difficult?
- Signal swamped by W+jets
- Signal sandwiched between W+jets and top pair production
- Dedicated likelihood to discriminate between each signal and each background
- Kinematic observables
- Show likelihoods for t-channel
- Rely on good MC modeling of W+jets background composition and kinematics
- Big challenge for discovery!
- 3σ evidence expected with <2 fb-1

| D0
370 pb-1 | Expected
 95% C.L. (pb) | Observed 
95% C.L. (pb) |
| --- | --- | --- |
| s-channel | 3.3 | 5.0 |
| t-channel | 4.3 | 4.4 |

**D0 Preliminary: World’s best limits!**
**Factor of 2-3 away from standard model**

## Production & Decay consistent with standard model

**Top always decays to *****W******+******b?***
**Any Charged Higgs from *****t******→******H******+******b?***
**Top electric charge is +2/3?**
**W helicity “right’?**
**Anomalous FCNC *****t******→******Zc, gc, ******γ******cb?***

**Search for Single Top Quark Production**

**Pair Production Rate**
**New massive resonance *****X******→******tt? ***
**Top spin**
**Tests of NLO kinematics**

**Test Top Quark Decay**

**Test Top Quark Pair Production**

**Observe Top Quark Pair Production**
**in all final states**

**Precision measurement **
**of top quark mass:**
**30% improvement this year!**

## Top Quark Mass:Reconstruction

**Top Quark Mass:Reconstruction**

- Kinematic fit to top pair production and decay hypothesis
- Obtain improved resolution on reconstructed top mass
- Choose most consistent solution for t→jjb and t→lυb
- 24 possibilities for 0 b-tags
- 12 possibilities for 1 b-tag
-   4 possibilities for 2 b-tags
- Fit data to reconstructed top mass distributions from MC
- Need excellent calibration of jet energy between data and MC!
- 3% systematic uncertainty on jet energy scale gives ~3 GeV/c2 systematic uncertainty on top quark mass

**e****/****μ**

## Top Quark Mass: in situ jet energy calibration

**Top Quark Mass: *****in situ***** jet energy calibration**

- New for 2005! Simultaneous fit of invariant mass of jets from W→jj *in lepton+jets data*
- Determine global jet energy correction factor
- Use to correct energy of all jets
- Uncertainty dominated by data W→jj statistics
- Will decrease <1 GeV/*c*2 with more data!

**(-9%)**
**(-3%)**
**(+3%)**
**(+9%)**

## CDF Top Mass Measurement: Lepton+Jets

**CDF Top Mass Measurement: Lepton+Jets**

| Systematic Source | Uncertainty
(GeV/c2) |
| --- | --- |
| ISR/FSR | 0.7 |
| Model | 0.7 |
| b-jet | 0.6 |
| Method | 0.6 |
| PDF | 0.3 |
| Total | 1.3 |
| Jet Energy | 2.5 |

- Simultaneous fit of reconstructed top mass and W→jj mass
- Include Gaussian constraint on jet energy scale from *a priori* determination
- Best single measurement! Better than previous Run I CDF+D0 average!

**Correction approx. -0.3%**
**Uncertainty 20% smaller**

***Submitted last week! ***
***PRD: hep-ex/0510048***
***PRL:  hep-ex/0510049***

**CDF Run II**
**L**** = 320pb****-1**

## D0 Top Mass Measurement: Lepton+Jets

**D0 Top Mass Measurement: Lepton+Jets**

- LO Matrix element technique of Run I
- Exactly 4 observed jets (150 events, 32±5% top)
- Use LO Matrix element for ttbar and W+jets
- Weight all 24 possible solutions (no b-tagging)
- New for 2005: W→jj jet energy calibration
- Fit jet energy scale as well as top mass
- No *a priori* jet energy determination

| Systematic Source | Uncertainty
(GeV/c2) |
| --- | --- |
| ISR/FSR | 0.3 |
| Model | 0.7 |
| b-jet | 1.1 |
| Method | 0.9 |
| PDF | 0.1 |
| Total | 1.7 |
| Jet Energy | 3.2 |

**Correction  +3.4%**
**Uncertainty ****±3.4%**

**Check: apply JES and fit**

## Tevatron Top Quark Mass

**Tevatron Top Quark Mass**

**First application of matrix element technique to dilepton channel:**
**20% improvement over previous techniques!**

**Now final: 173.5 ****± 3.9**

**Tevatron Run-I/II***
**Summer 2005**

**172.7 ****± 2.9**

## Bright Future with Inverse Femtobarns!

**Bright Future with Inverse Femtobarns!**

| Adapted from
A. Freitas et al
hep/ph-0311148 | Experiment
δMtop 
(GeV/c2) | Prediction
δMW 
 (MeV/c2)   |
| --- | --- | --- |
| CDF+D0  Run I | 4.3 | 26 |
| CDF+D0  2005 | 2.9 | 18 |
| CDF+D0 1 fb-1 | 2.0 | 12 |
| CDF+D0  4 fb-1 | 1.5 | 9 |
| LHC | 1.3 | 8 |

**CDF+D0 will achieve ****±****2.5 GeV/c****2**** in 2006! Will reach ****±****1.5 GeV/c**** **** with 4 fb****-1**** base!**
**Shown is only lepton+jets channel with W****→jj jet energy calibration**
**Conservative estimate of other systematics, will get smarter with more data!**

**Quantum loops make W mass sensitive to top and Higgs mass**
**Recent theoretical calculation of full two-loop electroweak corrections **
**Precise prediction of W mass in standard model limited by  uncertainty on experimental measurement of top mass**

**Run II Goal**

## Test of Standard Model

**Impact of CDF+D0 Top Quark Mass = 172.7 ****± 2.9 ****GeV**

**Good agreement between **
**direct measurements**
**and **
**indirect SM prediction**

**<219 GeV with LEP Excluded**

**future**
**CDF+D0 **
**(4 fb****-1****)**

## Conclusions

**Observed top quark consistent **
**with standard model **
**so far**
**Achieved **
**1.7% precision **
**top quark mass measurement**
**Future is bright!**
**Excellent performance of Tevatron & CDF & D0**
**delivering high statistics samples of top quarks**
**Watch out for interesting results with 1 fb****-1**
**at Moriond 2006!**