# Presentation: 084136

## Single-Top Production at CDF

- Rainer Wallny
- University of California, Los Angeles
- On behalf of the
- CDF Collaboration
- 2007 Europhysics Conference on
-  High Energy Physics

## The Tevatron Accelerator

- Design goal

- World’s highest energy collider (until 2008)
- Proton-antiproton Synchrotron
- Experiments CDF and DØ
- Run I (1992-1996)
- s = 1.8 TeV
- 100 pb-1 int. luminosity
- Major upgrade to accelerator complex
- Main Injector (x5)
- Pbar Recycler (x2)
- Run II (2001-2009)
- s = 1.96 TeV
- Current peak luminosity ~28.0 x 1031 cm-2s-1
- Both experiments have now > 2.5 fb-1 on tape.
- Aim for 4-9 fb-1 int. luminosity in Run II
**Only place in the world to produce top quarks.**

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Top Quark Production at the Tevatron

** **At the Tevatron, top quarks are primarily produced in pairs via the   strong interaction:
** **The Standard Model also predicts a    single top production via the    electroweak interaction: (st ~ 1⁄2 tt)

- NLO = 6.7± 0.8 pb

- The single top quark production mode was not yet observed – first evidence reported recently ...

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

**Notes:**
1 every 10 billion events – the needle in the haystack

## Why measure Single Top Production ?

** **Source of single ~100% polarized top   quarks:
-  Test V-A structure of W-t-b vertex
-  Access to the top quark spin
-  Allows direct Measurement of CKM-   Matrix Element Vtb:
-  st ~|Vtb|2
-  indirect determinations: Vtb ~0.999...
- verify

- Vtb

- s-channel

- t-channel

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Sensitivity to New Physics and WH

- Single top rate can be altered due to the presence of New Physics:
-  t-channel signature: Flavor changing       neutral currents (*t-Z/**γ/g**-c *couplings)
- - s-channel signature: Heavy *W* boson,       charged Higgs H+, Kaluza Klein excited *W*KK

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

-  Z

- c

- t

- W,H+

- s-channel single top has the same final state
- as WHlbb
- => benchmark for WH search!

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Single Top Candidate Selection

_**Event Selection:**_
**1 Lepton, **ET >15 GeV, ||< 2.0
**Missing E****T **(MET)** **> 25 GeV
**2 Jets, **ET > 15 GeV, ||< 2.8
**Veto** QCD, Conversions, Cosmics
**At least one b-tagged jet**, 			(secondary vertex tag)

- CDF W(l) + 2 jets Candidate Event:
- Close-up View of Layer 00 Silicon Detector

- 12mm

| Number of Events / 955 pb-1 | Single Top | Background |   S/B  |   S/B  |
| --- | --- | --- | --- | --- |
| W(l) + 2 jets | 74 | 15500 | ~1/210 | ~ 0.6 |
| W(l) + 2 jets + b-tag | 38 | 550 | ~1/15 | ~ 1.6 |

**Run: 205964,  Event: 337705**
- Electron ET= 39.6 GeV,
- Missing ET = 37.1 GeV
- Jet 1: ET = 62.8 GeV,  Lxy = 2.9mm
- Jet 2: ET = 42.7 GeV,  Lxy = 3.9mm

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Background Estimate

- Falsely tagged light quark or gluon jets
- Mistag probability parameterization obtained from inclusive jet data

_**W+HF jets (Wbb/Wcc/Wc)**_
- W+jets normalization from data and
- heavy flavor (HF) fraction from MC

- _Top/EWK (WW/WZ/Z__→__ττ, ttbar__)_
- MC normalized to theoretical cross-section

- _Non-__*W (QCD)*_
- Multijet events with semileptonic *b*-decays or mismeasured jets
- Fit low missing ET data and extrapolate into signal region

**Wbb**

**Wcc**

**Wc**

**non-*****W***

**Z/Dib**

**Mistags**

**tt**

- _W+HF jets (Wbb/Wcc/Wc)_
- W+jets normalization from data and heavy flavor (HF) fractions from ALPGEN Monte Carlo, calibrated in generic multijet data

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Signal and Background Event Yield

- CDF Run II Preliminary, L=955 pb-1 Event yield in W+2jets

| s-channel | 15.4 ± 2.2 |
| --- | --- |
| t-channel | 22.4 ± 3.6 |
| tt | 58.4 ±13.5 |
| Diboson | 13.7 ± 1.9 |
| Z + jets | 11.9 ± 4.4 |
| Wbb | 170.9 ± 50.7 |
| Wcc | 63.5 ± 19.9 |
| Wc | 68.6 ± 19.0 |
| Non-W | 26.2 ± 15.9 |
| Mistags | 136.1 ± 19.7 |
| Single top | 37.8 ± 5.9 |
| Total background | 549.3 ± 95.2 |
| Total prediction | 587.1 ± 96.6 |
| Observed | 644 |

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Jet Flavor Separation

|  | Background Estimate | Neural Network Fit |
| --- | --- | --- |
| W+bottom | 299.0  56.8 | 292.8  26.3 |
| W+charm | 148.1  39.4 | 171.6  53.8 |
| Mistags | 140.0  19.8 | 179.5  42.5 |
| Sum | 587.1  96.6 | 644.0 |

**mistags** **/** **charm**** ....................... ****beauty**

- Even with a reconstructed secondary vertex required,  about 50% of the background in the W + 2 jets sample do NOT contain b quarks
- Train Neural Network with secondary vertex tracking information (25 input variables) to distinguish b/c/light quark jets
- Replaces Yes-No decision by a continuous variable  - used in all analyses
- Obtain good jet-flavor separation!
- Improves sensitivity by ~15-20%!

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Analysis Techniques

- Likelihood Analysis
- Neural Network Analysis
- Matrix Element Analysis

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## The Likelihood Function Analysis

*N**sig*

*N**bkg*

*i, indexes input variable*

- Background

- Signal

- Unit Area

- tchan
- schan

- Wbb
- ttbar

- Expected signal significance: 2.0

- Leading Jet ET (GeV)

- Uses 8 (5) kinematic variables for t-channel
- (s-channel) Likelihood Function
- i.e. *M(Wb), H**T**, QxEta, Neural Network flavor separator, *
*Madgraph Matrix Elements etc.*

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Kinematic Variables

- Background

- Background

- Signal

- Signal

- Background

- Signal

- Unit area

- Wbb
- ttbar

- Wbb
- ttbar

- tchan
- schan

- tchan
- schan

- tchan
- schan

- Mass(W,b)

- HT =ET(lepton,MET,Jets)

- QxEta(un-tagged jet)

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Analysis Techniques

- Likelihood Analysis
**Neural Network Analysis**
- Matrix Element Analysis

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Neural Network Analysis

-   

- combine 18 input variables into one  powerful discriminant (i.e b-tagging flavor separator, M(W,b), M(j1,j2), HT, number of soft jets, angular variables, etc..)
- Continuous output between -1 (bkg like) and +1 (sig like)
- Three networks: s-channel,  t-channel and  combined s+t channel

- Expected signal
- significance: 2.6

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Neural Network Analysis - Separate Search

- Two NN’s trained separately for s-channel and t-channel  (same input variables)

- t-channel

*Wbb*

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Analysis Techniques

- Likelihood Analysis
**Neural Network Analysis**
**Matrix Element Analysis**

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Matrix Element Method

- Parton distribution functions

- Leading Order matrix element (MadEvent)

***W(E******jet******,E******part******)*** gives the probability of measuring a jet energy Ejet when Epart was produced

- Integration over part of the phase space *Φ**4*

- Pioneered by top quark
- mass analyses
- Now applied to a search!
- Calculate event probability density for signal and background

- Inputs only lepton
- and 2 jets 4-vectors!

- Eparton

- Ejet

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Kinematic Variables Used in Matrix Element Analysis

- Jet1

- Jet2

- Input to the Matrix Element Analysis are the measured four-vectors
- of the Lepton, Jet1 and Jet2 in the W+2jets data (>=1 b-tagged jet)

- Lepton

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Event Probability Discriminant (EPD)

*;b* = Neural Network *b*-tagger output

- We compute probabilities for signal and background hypothesis per event
- Use full kinematic correlation between signal and background events
- Define ratio of probabilities as event probability discriminant (EPD):

- Expected signal
- significance: 2.5

*Signal-like*

*Background-like*

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

- Results

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## CDF Single Top Results

- Likelihood

- Neural Networks

- No evidence of signal
- s+t<2.7pb at 95% C.L.
- Expected signal significance 2.0σ

- No evidence of signal
- s+t<2.6pb at 95% C.L.
- Expected signal significance 2.6σ

- p-value = 1.0% (2.3)
- s+t=2.7(+1.5/-1.3)pb
- Expected signal significance 2.5σ

- Matrix Element

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Single Top Candidate Event

**Central Electron Candidate**
- Charge: -1, Eta=-0.72    MET=41.6 GeV
- Jet1: Et=46.7  GeV  Eta=-0.6    b-tag=1
- Jet2: Et=16.6  GeV  Eta=-2.9    b-tag=0
**QxEta = 2.9 (t-channel signature)**
- EPD=0.95

- t-channel single top production has a kinematic peculiarity:
-  Distinct asymmetry in  Q x  distribution: lepton charge (Q) x pseudo-rapidity  =-log (tan/2) of untagged  jet

**Run: 211883, Event: 1911511**

- Signal region EPD>0.9

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Compatibility of CDF Results

- Performed common pseudo-experiments
- Use identical events
- ME uses 4-vectors of lepton, Jet1/Jet2 as inputs
- LF/NN uses several sensitive variables
- Correlation among analyses: ~60-70%
- 1.2% of these pseudo-experiments fluctuated as unlucky as the observed data

- Extensive cross-checks performed
- Next round of analysis will shed some  light on this issue – coming soon!

-   NN

-   LF

-   NN

-   ME

-   ME

-   LF

- (NN)

- (LF)

- (ME)

- SM Theory

- CDF Run II Preliminary

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Conclusions

- Single top production
- measures |Vtb| directly
- probes physics beyond the Standard Model
-  serves as a benchmark for Higgs boson searches at the Tevatron
- Presented three multi-variate analysis techniques to improve Signal/Background separation
- Individual sensitivity ~ 2.0-2.5σ
- Combined sensitivity 3σ using all three discriminants
- Two of the three analyses show deficit in the signal region, one analysis observes a 2.3σ hint of the signal at almost the Standard Model rate
- Compatibility of the analyses is ~1%
- We have ~ twice the data available, we are improving our analyses and we expect the results to converge for the next update
- 	=> Expect news on CDF single top analyses soon !

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

- BACKUP
- slides

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Search for Heavy W Boson

**Limit at 95% C.L. M(W**** ́) > 760 GeV/c****2**** for M(W ́) > M(****ν****R****) **
**M(W ́) > 790 GeV/c****2**** for M(W ́) < M(****ν****R****)**

- W

- Search for heavy W boson in W + 2, 3 jets
- Assume Standard Model coupling strengths
- (Z. Sullivan, Phys. Rev. D 66, 075011, 2002)
- Perform fit to MWjj distribution

- Previous Limits:
- CDF Run I: M(WR) > 566 GeV/c2 at 95% C.L.
- D0 Run II: M(WR) > 630 GeV/c2 at 95% C.L.

- New
- Result!

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Four Fold Ambiguity in building m(W,b)

| Parton/Jet matching | t-channel | s-channel |
| --- | --- | --- |
| 1 tag | 96.6% | 51.1% |
| 2 tags | 61.9% | 68.5% |

_**2) M**__**l**__**ν**__**b**__** reconstruction:**_
- Assumption that tagged jet is from top
- In double tagged events, take jet with
- larger Qlept x ηjet
- Works well for t-channel only

_**1) Neutrino Pz:**_
- Neutrino px, py from MET
- Pz from W-mass constraint
- This yields two solutions:
- Smaller solution is correct 67.6%

**M****l****ν****j2****~170 GeV/c****2**

- Mtop

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Hypothesis Test (ME-Analysis)

- Hypothesis Test (ME-Analysis)

- Most sensitive bins:

- We use the CLs/CLb Method developed at LEP
- Define Likelihood ratio test statistic:

- CDF RunII Preliminary, L=955pb-1

| Median (‘expected’)  p-value = 0.6%  (2.5) |
| --- |
|                   Observed p-value = 1.0%  (2.3) |

- b

- s+b

*L. Read*, J. Phys. G **28**, 2693 (2002)
*T. Junk*, Nucl. Instrum. Meth. A 434, 435 (1999)

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Cross-Checks in Data Control Regions (ME-Analysis)

- Validate method using data without
- looking at single top candidates
- Compare the Monte Carlo prediction of
- the shape of the discriminant to various
- control samples in data
- W+2 jets data (require no b-tagged jet)

- CDF Run II Preliminary

- CDF Run II Preliminary

- b-tagged dilepton+2jets
- data (99% ttbar)
- b-tagged lepton+4jets
- data (85% ttbar)

- Dilepton+2jets

- Lepton+4jets

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007

## Sources of Systematic Uncertainty

| Systematic (-1/+1) | s-channel | t-channel | All Single top | Shape Variations |
| --- | --- | --- | --- | --- |
| Jet Energy Scale | -1.4% / 1.3% | -2.4% / 1.8% | -2.0% / 1.6% |  |
| Initial State Radiation | 1.1% / -2.0% | 2.6% / 2.0% | 2.0% / 0.3% |  |
| Final State Radiation | 1.3% / 1.4% | 3.4% / 2.2% | 2.6% / 1.9% |  |
| Parton Dist. Function | 1.0% / -0.6% | 1.7% / -0.3% | 1.4% / -0.4% |  |
| Monte Carlo Generator | 1% | 2% | 1.6% |  |
| Event Det. Efficiency | 6.1% | 7.8% | 7.4% |  |
| Luminosity | 6% | 6% | 6% |  |
| Neural Net b-tagger | N/A | N/A | N/A |  |
| Mistag Model | N/A | N/A | N/A |  |
| Non-W Model | N/A | N/A | N/A |  |
| Q2 Scale in Alpgen MC | N/A | N/A | N/A |  |
| Total Rate Uncertainty | 9.1% | 11.3% | 10.5% | N/A |

- CDF RunII Preliminary, L=955pb-1

*All rate and shape systematic uncertainties are included as *
*nuisance parameters in the analyses!*

- Rainer Wallny - Single Top Production at CDF  - 2007 EPS Conference Manchester

- July  19th, 2007