# Presentation: 502673

## Single Top Quark Physics at the Tevatron and Beyond 

**Catalin Ciobanu, University of Illinois, CDF **
**For the LHC part, valuable help received from:**
**Sergey Sablospitsky, IHEP Protvino, CMS**
**Dugan O’Neil, Simon Fraser Univ., ATLAS**
**	****	****    ****WIN’03, October 7, 2003**

## Single Top Production

- Why look for single top:
- Cross section ~ | Vtb |2: the only way to directly measure CKM matrix el. Vtb
- Single top is background to other signals: e.g. Higgs searches.
- Test non-SM phenomena:
- Heavy W’ boson
- Anomalous Wtb couplings
- FCNC couplings tug or tcg
- At the Tevatron relevant channels are:
- t-channel W-gluon fusion:
- 1.98 pb at s=1.96 TeV
- Hard b-jet, W decay products, soft b (usually lost), light quark jet
- s-channel W*:
- 0.88 pb at s=1.96 TeV
- Two hard b-jets, W decay products
- _[B.W. Harris et al.: Phys. Rev. D 66, 054024 (theoretical calculations)](http://prola.aps.org/search.html)_

## From Run I to Run II

- Results at s = 1.8 TeV from CDF and D :
- Single top has not been observed; 95% CL limits were set:
- t-channel:  D limit: **22** pb, CDF limit: **13** pb (theoretical x-section: **1.40** pb)
- s-channel: D limit: **17** pb, CDF limit: **18** pb (theoretical x-section: **0.76** pb)
- combined s- and t- channels search: CDF limits: 14 pb (HT , Mlnb)
- most recent Run 1 CDF study finds a 2.2 excess and limit of 24 pb (7-inp NN)
- D:Phys. Lett. B 517, 282 (2001); CDF: PRD65, 091102 (2002), C. Ciobanu’s Ph.D. thesis
- For Run II:
- Higher rate: 32% increase in the combined s and t channel x-section:
- still small: 2.9 pb versus 6.7 pb for top pair production
- More luminosity – right now CDF has 180 pb-1 – that is 70% more data!
- Better detector acceptance
- Preliminary CDF analysis done for 107 pb-1 of data:
- Search for s- and t-channels combined production
- Seaparate search for t-channel

## Combined Search

- Look in the W+2 tight jets channel:
- “tight” jet: ET > 15 GeV, || < 2.8
- at least one SVX B-tag
- exactly one lepton with ET>20 GeV, ||<2.0
- missing energy: ET>20 GeV
- veto Z’s, dilepton events
- apply a top mass window cut: 140 < Mlnb <210 GeV/c2
- Monte Carlo samples:
- single top signal:
- Pythia (1M events)
- Madevent+Pythia (~200k)
- HERWIG tt background
- ALPGEN+HERWIG Wbb used to model the shapes of the non-top backgrounds: Wbb, Wcc, Wc, mistags, non-W events

## Expected Yield

- For L=107.1 pb-1:
- expect 2.4 signal events
- expect 16.1 backgrd events
- observe 19 (expect 18.5)
- The variable with the most discrimination power is HT, the total transverse energy in the event:
- Will fit the HT shape in data as the sum of weighted MC shapes.
- Use maximum likelihood method with Gaussian background constraints

## Fitting HT distributions

- HT distributions for signal and backgrounds:
- signal shapes similar for s- and t- channels
- fit yields 2.94.5 signal events (exp. 2.4 events).

## 95% C.L. limit

- Limit at 95% C.L. is 17.5 pb
- This accounts for the systematics:
- Jet ET scale: 20.9%
- ISR/FSR: 4.0%
- Top Mass: 6.2%
- PDF, signal generator, background model 1-2%
- Compare to Run I:
- Same luminosity 107 vs 106 (pb-1)
- 14 pb limit for 2.16 pb x-section
- Now: 17.5 pb limit for 2.86 pb x-s.
- 		*marginally better limit!*
- Several ways to improve this will be discussed later.

## Separate Search

- Looking for the two signal channels individually is desired:
- Different sensitivities to new physics:
- s-channel: heavy charged vector bosons W’, CP-violation effects within MSSM, Kaluza-Klein excited W-boson within MSSM
- t-channel: FCNC couplings, anomalous V+A contributions to the W-t-b vertex, etc.
- To go back to extracting Vtb we will have to know the individual rates
- We use the same selection as in the combined search.
- However, using HT is no longer appropriate
- A variable with good W* - W-gluon separation potential is Q x , where Q is the lepton charge and  the pseudorapidity of the non-b jet.
- Now treat W* as a background, therefore perform a 4-component fit

## Fitting Q

- t-channel exhibits an asymmetry toward Qx positive values (twice as many events than in the negative Qx range)
-  fit to data yields: 1.9± 3.7 t-channel signal events (exp. 1.6 events).

## 95% C.L. limit

- Limit at 95% C.L. is 15.4 pb
- This accounts for the systematics:
- Jet ET scale: 21.5%
- Signal generator: 21.1%
- Background model: 43.0%
- ISR/FSR: 6.9%
- Top Mass: 6.7%
- PDF 4.3%
- Compare to Run I:
- 13 pb limit for 1.40 pb x-section
- Now: 15.4 pb limit for 1.98 pb x-s.
- 		*again a better limit!*

## Future Prospects

- In progress: Neural Network with 5 input variables: HT, ET(j1), ET(j2), ET, PT(j1-j2),  Looking in the W+2j channel, no Mlnb cutAssuming the background uncertainties will scale as L-1/2

- Right now: 3 sigma with 2.5 fb-1
- Improvements to come. We try:
- Soft lepton tagger, Jet Prob. tagger
- Use of forward electrons
- Better discriminant variables:
- Matrix elements (or ratios of these)
- Use the top mass hypothesis to constrain the event
- Will lead to better Mlnb resolution
- Better understanding of the systematic uncertainties is crucial.

## Single Top at the LHC

- LHC will be a top quark factory:
- 8 million top pairs per experiment per year (10 fb-1 / year)
- Some cross section values for s = 14 TeV:
- top pair production: ~ 800 pb (mostly via gluon-gluon fusion)
- t-channel single top: 153 (top) and 90 (antitop) = 243 pb
**e.g. per day ~6000 events, at 10****33****cm****-2****s****-1**
- s-channel single top: 6.6 (top) and 4.8 (antitop) = 11 pb
- associated Wt production: 50-60 pb
**Negligible at the Tevatron**
- Will discuss each of the three modes in what follows

## t-channel single top (CMS)

- CMS study (hep-ph/0003033):
- Full calorimeter simulation + b-tag efficiency parametrization
- Pythia 5.72 signal, tt, WZ; Vecbos+Herwig W+jets. Selection:
- One isolated lepton with PT>20 GeV/c, ||<2.5
- Missing ET>20 GeV, 50 < Ml <100 GeV/c2
- Two jets with ET > 20 GeV, || < 4.0
**One jet: E****T ****> 20 GeV, |********| < 2.5, the other E****T ****> 50 GeV, 2.5<|********| < 4.0**
**Leading jet E****T****<100 GeV (to reduce tt)**
**Exactly one b-tagged jet – the central one**
- Reject WZ candidates with: 80 < Mjj <100 GeV/c2
- Signal peak visible in reconstructed Mtop distribution
- With 10 fb-1, (S+B)/S = 1.4%
- The quest continues – newer det. simulation
- use fancier generators: TopRex, Single Top
- Work-in-progress, unofficial results  /  ~ 10%

- V.Abramov, CMS Week 9/17/03

## ATLAS study of W-gluon single top (Dugan O’Neil’s Ph.D. thesis):
ATLFAST parametrized detector simulation
Onetop+Pythia 5.72 signal, tt; M.E.+HERWIG Wbb, Wjj
Selections are rather similar to CMS study, plus:
150<Mtop<200 GeV/c2
Result: (S+B)/S = 0.9% with 10 fb-1.

What about Vtb? Extracting |Vtb|2 from x-section picks up uncertainties:
x-section uncertainty:
Statistical: 5% is a perhaps reasonable/optimistic guess for L=10 fb-1
Systematic <20%. Here things can easily be off by a lot, especially in the 1st year...
Luminosity uncertainty: traditionally 4-5%
Theoretical uncertainty: 
From factorization and renormalization scale dependence (3% from NLO calculation)

- ATLFAST parametrized detector simulation
- Onetop+Pythia 5.72 signal, tt; M.E.+HERWIG Wbb, Wjj
- Selections are rather similar to CMS study, plus:
- 150<Mtop<200 GeV/c2
- Result: (S+B)/S = 0.9% with 10 fb-1.
- What about Vtb? Extracting |Vtb|2 from x-section picks up uncertainties:
- x-section uncertainty:
- Statistical: 5% is a perhaps reasonable/optimistic guess for L=10 fb-1
- Systematic <20%. Here things can easily be off by a lot, especially in the 1st year...
- Luminosity uncertainty: traditionally 4-5%
- Theoretical uncertainty:
- From factorization and renormalization scale dependence (3% from NLO calculation)

- t-channel single top (ATLAS)

## CMS study (S. Sablospitsky, CMS week 9/17/03)
disclaimer: work-in-progress, not yet ‘official’ results
use TopRex to generate single top, tt, Wbb. Pythia W+jets. Use CMSJET 4.801
selection:
one lepton with ET>10 GeV, ||<2.5
2 B-tags with ET>20 GeV, no other jets with ET>20GeV
Vector sum all final state PT: |PT|<15 GeV
Reconst. top mass: 150 GeV<Mtop<200 GeV/c2
Results: (S+B)/S = 12% , S/B = 8% for L = 30 fb-1 
which implies |Vtb|=8.3%

- disclaimer: work-in-progress, not yet ‘official’ results
- use TopRex to generate single top, tt, Wbb. Pythia W+jets. Use CMSJET 4.801
- selection:
- one lepton with ET>10 GeV, ||<2.5
- 2 B-tags with ET>20 GeV, no other jets with ET>20GeV
- Vector sum all final state PT: |**P****T**|<15 GeV
- Reconst. top mass: 150 GeV<Mtop<200 GeV/c2
- Results: (S+B)/S = 12% , S/B = 8% for L = 30 fb-1
- which implies |Vtb|=8.3%

- s-channel single top (CMS)

## s-channel single top (ATLAS)

- ATLAS study (hep-ph/0003033)
- Onetop+Pythia 5.72 signal, tt; M.E.+HERWIG Wbb, Wjj
- Selection:
- One isolated lepton with PT>20 GeV/c, ||<2.5
- 2 b-jets with ET>75 GeV
- Scalar sum all final state PT: |PT|<175 GeV
- Reconst. top mass: 150 GeV<Mtop<200 GeV/c2
- Results: (S+B)/S = 5.5% for L = 30 fb-1,  |Vtb|~1%
- Wt single top production at ATLAS:
- 65< Mjj < 95 GeV/c2
- Event invariant mass < 300 GeV/c2
- (S+B)/S=4.4% for L = 30 fb-1

## Top Polarization in Single Top Events

- EW-produced top quarks are highly polarized:
- In the top rest frame, its spin points along the direction of the down (d) quark.
- Restrict to t-channel single top:80% (69%) of the top (antitop) events have the ‘d’ quark in the final state. The angular distrib. of the charged lepton:
- used signal samples with P=+1 and P=-1. Consider only W+jet background.
- Chisquare fit, letting the two P contributions float.
- The error on polarization measurement is 1.6% for 10 fb-1 (Dugan’sPh.D.thesis)
- W boson helicity measurement:
- Again restrict to t-channel signal.
- Measure cos(l) distribution. l between the direction of the lepton in W rest frame and the direction of the W in the top rest frame.
- Three component fit: fL , fR , fLong , with fL+fR+fLong=1
- Uncertainties of the order 2-3% for 30 fb-1 of data.

## Conclusions

- CDF and Dø accumulated more events than in entire Run I (factor of 2).
- Preliminary CDF study from 107 pb-1 sets promising single top limits
- However, many improvements needed if we are to observe single top in the next 2-3 years.
- Precision measurements of |Vtb|, as well as top polarization are not within (immediate) sight
- LHC will see very exciting results in the first (few) years of running:
- 10 –30 fb-1 needed for observation in most searches.
- Higher collider energy = considerable higher cross sections
- ATLAS and CMS single top studies are very mature
- The Monte Carlo efforts are particularly strong: Top Rex, SingleTop.
- CDF/Dø can help with insight into systematics. Search strategies are already similar between Tevatron and CERN, but big improvements are just an idea away!