# Presentation: 415393

## Status of Single Top Searches at the Tevatron

- Reinhard Schwienhorst

- TEV4LHC Workshop, Fermilab, September 16 2004

## Outline

- Introduction
- Standard Model Single Top Quark Production
- Event Signature
- Backgrounds
- Current Searches for Single Top Quark Production at DØ and CDF
- Sensitive Variables
- Current results
- Analysis Status and Outlook
- The Phenomenological Connection
- Conclusions

- Notes:
-  I will not discuss LHC Single Top searches
-  I will not discuss non-SM Single Top

## SM Single Top Quark Production at the Tevatron

- Electroweak (charged current) production of top quarks                                                     *s-channel                        t-channel          s+t channel*
- NLO cross-sections:              0.88pb ± 8%                    1.98pb ± 11%
- Run I 95% CL limits, DØ:         <17pb                                 <22pb                                CDF:         <18pb                                 <13pb               <14pb
- Tevatron Goal: Measure CKM matrix element *V**tb* (test CKM unitarity)
- Observe top polarization
- Discover new Physics
- Additional production mode at the LHC: Wt
- Not discussed here (TEV=0.09pb)

***V******tb***** **

- q

- q'

- W

- t

- b

- q

- g

- b

- b

- W

- t

- q'

- b

- W

- g

- t

-   Wt

- s-channel

-  t-channel

## Identifying Single Top Events

- Event signature:
- Similar to top pair production in the lepton+jets channel
- But fewer jets
- Larger Backgrounds
- Similar to associated Higgs production
- Single top s-channel is an important background to Higgs searches at the Tevatron
- Important Tools:
- Good lepton ID
- Reduce fake-lepton backgrounds

- t

- b

- b

*l*

- 

- 1 high-ET lepton
-  ET
- 2 jets
- 1 b-tagged jets

**/**

- Excellent b-quark tagging
- Good jet ID and resolution

## Standard Model Backgrounds

- W/Z+jets production (real-*l*)
- Wjj, Wcc, Wbb, Zjj, Zcc, Zbb, ...
- Estimated from _data_ (DØ)
- Estimated from _MC/data_ (CDF)
- Mis-reconstructed multi-jet events (fake-*l*)
- Jet mis-identified as isolated electron or muon
- Estimated from _data_
- Top-pair production
- Estimated from _MC_
- Other (WZ, WW, Z, cosmic rays,...)
- Estimated from _MC_ (CDF)
- Included in _data_ W/Z+jets estimate (DØ)

- q

- q'

- q (b)

- W

- g

- q

- g

*l*

- 

- b

- q

- q

- b

- t

- q'

- t

- q (b)

*l*

- 

## Loose pre-selection cuts
Lepton, MET 
ET>15GeV (DØ), ET>20GeV (CDF)
Jets: ET>15GeV (DØ and CDF)
Large  range, maximize acceptance
B-tag requirement
Check background model
Final Event Selection
Tight cuts to separate single top signal from backgrounds
Separately for s-channel and t-channel
and s+t combined
Cross Section (limit) derivation
Event counting (DØ)
Template fit (CDF)

- Loose pre-selection cuts
- Lepton, MET
- ET>15GeV (DØ), ET>20GeV (CDF)
- Jets: ET>15GeV (DØ and CDF)
- Large  range, maximize acceptance
- B-tag requirement
- Check background model
- Final Event Selection
- Tight cuts to separate single top signal from backgrounds
- Separately for s-channel and t-channel
- and s+t combined
- Cross Section (limit) derivation
- Event counting (DØ)
- Template fit (CDF)

- _Analysis Strategy_

## Initial analyses are using simplevariables for cuts or fits
 HT=ET(lepton) + MET +iET(jeti) 
 M(top)
t-channel: pseudorapidity of the light quark jet × lepton (top) charge

-  *H**T**=E**T**(lepton) + MET +****i**E**T**(jet**i**) *
* **M(top)*
- t-channel: pseudorapidity of the light quark jet × lepton (top) charge

- _Variables used in the analysis_

## Current Results (160-170pb-1)

- _Current Results (160-170pb__-1__)_

- 	_*Yields*_*	**   **	*SLT	  SVT	  JLIP
- Signals
-   *s-channel**	*1.3±0.3	3.1±0.8	3.2±0.7
-   *t-channel*	1.7±0.4	5.1±1.3	5.3±1.3
- Background sum	76±13	137±21	166±26
- Observed 	97	138	148

- Observed/Expected limit at 95%CL:

-  s+t < 23/20 pb

-  t < 25 / 23 pb

- s < 19 / 16 pb

- Systematic Uncertainties
- MC signal: JES, trigger and tagger modeling, (DØ: ~20%, CDF: ~10%acc, 20-30% shape)
- MC backgrounds: also normalization, ~25%  nuisance parameter in template fit
- Data backgrounds: DØ: dominated by tagging probability  estimate, ~20%

-  s+t < 18 /14 pb

-  t < 10 /11 pb

- s < 14 /12 pb

*D**Ø** Run II preliminiary*

- b-tagging algorithm:

## Analysis Status and Outlook

- Initial analysis started after Run II top pair production observation
- Current Results are already an improvement over Run I limit
- More data, better detector
- Current analyses would need several fb-1 for observation
- Particle ID, b-tagging not as efficient as predicted
- Improvements in progress
- Large systematic uncertainties
- Background modeling, detector understanding, ...
- Analyses are not yet optimized
- Working on multivariate analysis techniques (NN, Matrix Element, ...)
- Required to make observation soon
- Need accurate models for signal and background
- Benefit from recent NLO calculations
- Identify variables that give good signal-background separation

## The Phenomenological Connection

- Phenomenological studies provide important information
- Cross sections, Distributions, correlations,...
- How to combine the different t-channel diagrams in a generator
- Several recent phenomenology papers on single top at NLO at the Tevatron, more papers in preparation
- Address signal and background issues
- Studies of single top production (Z. Sullivan)
- Single top production and decay at next-to-leading order (J. Campbell, R.K. Ellis, F. Tramontano)
- Next-to-leading Order Corrections to Single Top Production and Decay at the Tevatron: 1. s-channel Process (Q. Cao, RS, C.-P. Yuan)

- Phenomenology

- LHC

- Tevatron

## Example: Cross Section and Jet multiplicity

- _Example: Cross Section and Jet multiplicity_

- s-channel

- s-channel

-  t-channel

-  t-channel

- total cross section

- fraction of 3-jet events

## NLO distributions of 3rd jet

- _NLO distributions of 3__rd__ jet_

- s-channel

- s-channel

-  t-channel

-  t-channel

## Conclusions

- Tevatron single top analyses have started
- Focus on understanding data
- First results are improvements over Run I limits
- Improvements to the analysis are needed for observation
- Implement more advanced final analysis steps
- Reduce systematic uncertainties
- Improve efficiencies: ID, b-tagging, trigger, ...
- In progress
- Cooperation with Phenomenologists is important
- Model signal accurately
- Model backgrounds accurately
- New ideas for sensitive variables

-  Single Top Physics at the Tevatron is challenging
-  and will be a very active field for the next few years

## Backup Slides

## Event pre-selection

- Loose cuts to select events with lepton, neutrino, jets                     _D__Ø_                                                    _CDF_
- Lepton:      ET>15GeV                                       ET>20GeV      |det|<1.1 (e); |det|<2.0 ()                          ||<1.0
- Neutrino:  ET>15GeV                                        ET>20GeV
- Jets:         2njets4                                            njets=2                 ET>15GeV                                        ET>15GeV              |det|<3.4                                              ||<2.8          ET(jet 1)>25GeV
- b-tag:     use 3 different taggers                   require secondary vertex tag

**/**

**/**

- secondary
- vertex

- impact parameter

- muon-in-jet

## Final Event Selection

- Separate s-channel production (2 b-tagged jets)from t-channel production (1 b-tagged jet)
- Reconstruct top quark
- Cut on invariant mass of *l*b system
- Cut on variable that separates signal and dominant background
- Total event transverse energy HT

- Mlb         [GeV]

- CDF Run II preliminary