# Presentation: 382741

## Search for Single Top Quark Production at the Tevatron

- Yurii Maravin for
- CDF and DØ Collaborations

## Single Top Production

- The standard model predicts production of a top quark via the weak current:
- Dominant processes at the Tevatron are:

- hep-ph/207055 (Harris, Laenen, Phaf, Sullivan, Weinzierl)

## Motivation

- Direct measurement of CKM **V****tb** element
- Sensitivity towards 4th generation!
- Physics with polarized t-quarks!
- Probe b-quark PDFs
- Search for New Physics
- H+ (2 pb)
- FCNC (1-4 pb)
- 4th generation

**W**

**b**

**t**

**V****tb**

- We need to observe the signal first!

- 3

## Single Top Production

- These processes have never been seen before
- Cross-sections are half of that for the top-pair production, but much more significant background...

**Anything with lepton + jets + missing E****T****: ****	****Wbb, tt-bar etc.**

**Backgrounds**

- 4

## Signal and Background Modeling

- Precise simulation of signal and background is crucial for the discovery
- Signal
- Benchmark: **ZTOP** NLO single top generator
- CDF and DØ use LO generators fixed to reproduce NLO simulation
- CDF:**MADEVENT**;
- DØ: **SingleTop** based on **CompHep**
- Background
**W+jets **(data/ALPGEN)
- Top-pair production
- Multi-jet events (data)
- WW, WZ, ZZ etc.

- CDF: Pythia
- DØ: ALPGEN

**http://www.fnal.gov/~zack/ZTOP/ZTOP.html**

## Searching for Single Top

- Clean up data, remove detector backgrounds
- CDF: restrictive selection
- DØ: a rather loose selection
- Apply **b-tagging**, understand shape variables, optimize S/B ratio
- CDF: optimize separation in one variable (Q · ), counting experiment
- DØ: use likelihood discriminants
- Perform binned Bayesian fit of the signal and background expectation compared to data

- Data selection

- Final criteria

- Data analysis

## Selecting Single Top

- s-channel
- W
- High-pT lepton (e or )
- Missing ET consistent with W production
- 2 b-jets
- At least one b-tagged jet
- t-channel
- Same as above + extra jet

## CDF Single Top Analysis

- W selection:
- pT() > 20 GeV/c, || < 1.0
- Missing ET > 20 GeV
- Jets selection:
- Exactly two jets, at least one is b-tagged
- Jet ET > 15 GeV, || < 2.8
- If only one jet is b-tagged, leading jet’sET > 30 GeV

**Event detection efficiency**	**s-channel ****	****	****1.06 ****± 0.08%****	****t-channel ****	****	**** 0.89 ± 0.07%**
**Main systematics**	**b-tag efficiency****	****7%****	****Luminosity****	****6%****	****JES****	****	****4%**

**Combined (s- and t-channels)**     **Expected signal ****	****	****4.3 ****± 0.5 events****     Expected background****	****33.8 ± 5.9 events****    Observed ****	****	****42 events**

**Use 162 pb****-1**

**Published:**** **_[PRD 71 012005](http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PRVDAQ000071000001012005000001&idtype=cvips)_

## CDF Analysis Strategy

- Use combined channels to search for single top quark production using HT distribution
- Separating channels:
**s-channel**: counting experiment
**2 b-tagged jets**
**t-channel**: (Q· ) distribution

## CDF Likelihood Results

**95% C.L. on the single top cross-section**

**Combined**

**t-channel**

**s-channel**

-  **17.8 pb****       13.6 pb**

-  **10.1 pb****      11.2 pb**

-  **13.6 pb****       12.1 pb**

## DØ Single Top Analysis

- DØ Single Top Analysis

**Preliminary!**

- Use 370 pb-1 of data
- W selection:
- pT() > 15 GeV/c
- |e| < 2.0 (1.1)
- Missing ET > 15 GeV
- Jets selection:
- Require 2-4 jets in the event withET > 15 GeV, and || < 3.4
- Important cross-check for background understanding
- Leading jet’s ET > 25 GeV, || < 2.5
- At least one jet is b-tagged

**D****Ø Preliminary****370 pb****-1**

**Combined (s- and t-channels, 1 tag)**     **Expected signal ****	****              19.4  ****±  2.0 events****     Expected background       372.9 ± 27.2 events****    Observed ****	****           367 events**

## DØ Analysis Strategy

- Study various kinematic observables that have a discriminating power against W+jj and tt-bar processes
- Example: (Q· ) and top mass
- Design16 likelihood discriminants for S/B separation:
- 4 signal/background pairs: s-channel and t-channel / W+jj and tt-bar
- 2 b-tagging schemes: 1-tag and 2-tags
- 2 lepton flavors: electron and muon

## DØ Likelihood Distributions

- DØ Likelihood Distributions

## DØ Results

**Combine results of likelihood discriminants in 2D histograms**

**No evidence for a signal, extract limits on cross-section!**

**s- and t-channel contours**

## DØ Results

- DØ Results

- Analysis is statistics limited!

- Reaching sensitivity for new physics (FCNC and 4th generation)

## Outlook

- Assume no improvement in analysis technique, methods, and resolution:
- It will take **1.5 fb****-1** of data to have an evidence for a single top production for one experiment!
- Both experiments have more than 1 fb-1 on tape!
- Lots of work to improve resolution and increase acceptances/efficiencies

**One experiment**

**1.5 fb****-1**** for observation****4.5 fb****-1**** for discovery**

**s+t combined**

- 16

## Summary

- Current analyses not only provide drastically improved limits on the single top cross-section, but set all necessary tools and methods toward discovery with larger data sample.
- Both collaborations aggressively work on improving the results:
- Improving ID algorithms (lepton ID, b-tagging algorithms etc.), acceptance
- Developing data analysis tools (Neural Nets, Boosted Decision trees, Maxtrix Element analysis etc.)
- Reducing systematic errors (JES, background modeling, etc.)

**Single Top Discovery is feasible in Run II, stay tuned!!!**