# Presentation: 253293

## Frontiers in Contemporary Physics III

**Frontiers in Contemporary Physics III**
**May 23-28 Vanderbilt University, Tennessee**

**Julien Donini**
- University of Padova and INFN
*On behalf of the CDF collaboration*

## Search for Single Top

**Top has been discovered at Tevatron in tt pairs**
**Single top production predicted by theory: **Wtb coupling

**s-channel**
******Tev**** = 0.88 ****± 0.11 pb**

**t-channel**
******Tev**** = 1.98 ****± 0.25 pb**

**Wt-associated**
**Tev****< 0.1**** pb**
**negligible at Tevatron**

***Z. Sullivan hep-ph/0408049, T. Tait hep-ph/9909352***

**Observation of single top:**
-  cross section is about 40% of ttbar
-  more difficult to observe due to large background
- - important W+jets background

***(for M******top****** = 175 GeV)***

## Single Top Physics

**Observation of single top allows direct access to V****tb **** CKM matrix element**
-  cross section ~ |Vtb|2
-  study top-polarization and EWK top interaction
**Probe b-quark PDF (t-channel)**
**Test non-SM phenomena**
-  heavy W’ boson
-  anomalous Wtb couplings
-  FCNC couplings like t  Z/ c
-  4th generation
**Potentially useful for Higgs searches**
-  single top has same final state as Higgs+W (associated) production

**Why look for single top ?**

## Run I Results

**Tevatron Run I results at ********s = 1.8 TeV**
- Theoretical cross-sections were 30% smaller
-  t = 1.40 pb
-  s = 0.76 pb
- Single top has not been observed: both CDF and DØ set 95% limits
-  t-channel:	22 pb (DØ)	13 pb (CDF)
-  s-channel:	17 pb (DØ)	18 pb (CDF)
-  combined:			14 pb (CDF)
**Run II**
-  higher beam energy (1.96 TeV)
-  higher rate
-  more luminosity: CDF/DØ have more than 800 pb-1 on tape
-  better detector acceptance

## Signal Topology

**General strategy:**
-  Separated search of signal of s and t-channels single top production in 1-tag and 2-tag samples
-  Combined search of s+t signal
**Event selection: we look for W+2 jets channel**
-  one **high p****T**** isolated lepton** (from W): Et > 20 GeV, || < 1
-  **veto** Z, dilepton, convertion events
-  **missing E****t** ( from W): MET > 20 GeV
-  exactly **two jets** w/ Et > 15 GeV, || < 2.8
-  ask for at least **1 b-tag**
- Note: s and t-channel both produce 2 b-jets but bbar from t-channel usually lost into beam pipe
**Additional cuts**
-  cut on **m****t ****mass** window: 140 < Mlb < 210 GeV
-  for 1-tag sample, **leading jet** Et > 30 GeV

## Invariant Mass Mlb Calculation

**b choosing**
-  in 1-tag events, choose b-tag jet as b from top
-  in 2-tags evt, choose tight jet with larger Q. as the b from top
-  Recipe is 97% successful for t-channel
-                     53% (75%) successful for s-channel 1-tag sample (2-tags)
**Choosing correct neutrino solution**
-  Et() = MET
-  pz () is obtained from the constraint Ml = MW:
- Two solutions: pick the one with lower |pz ()|
-     i.e most central solution.
- Successful matching rate: 74% t-channel, 71% s-channel

## b-tagging at CDF

- Three methods are currently used and well tested in high pt physics
** ****Soft lepton tagging**
- 	 Use **semi-leptonic decay** of b-quarks:
- 	     b  lc (BR ~ 20%), b  c  ls (BR ~ 20%).
- 	 Leptons with softer pt spectrum than W/Z, less isolated.
-  **Secondary vertex identification**
- 	 **Iterative fit on tracks with significant impact parameter**
- 	 Efficiency is **45-50%** for central top b-jets
- 	 Mistag rates are kept typically at **4-5%**
-  ***Jet-probability***
- 	 Probability ***P*** that tracks associated with a jet come from primary vertex** **
-        Heavy flavors: *P *tends to 0.

**b-tagging at CDF**

## Signal and Background Modeling

**Signal modeling**
-  Pythia (or Herwig) do not reproduce correct NLO pt distribution for the b jets in the t-channel
-  Use **MadEvent generator**
-  generate b+q  t+q’ and g+q  t+b+q’ separately and merge them to reproduce the b pt spectrum from NLO calculations (ZTOP)

**Background modeling**
- ttbar and non-top background
-  Top pairs: Pythia, (tt) = 6.7 pb.
-  Non-top:
-  W + heavy flavor (62%): Alpgen
-  mistags (25%) and non-W (10%): estimated from data
-  diboson (WW, WZ, ZZ) production (3%): Pythia

## Signal and Background Modeling

**Signal and Background Modeling**

**Expected number of signal and background events compared with observation for *****L *****= 162 pb****-1**

-  Combined and 1-tag signal mostly dominated by t-channel (65-70%)
-  2-tags signal: only s-channel
-  Total background is dominated by non-top events (89%)
** ****Observations are in good agreement with predictions**

**Published CDF results: ****Phys. Rev. D 71, 012005 (2005)**

## Separate Search for Single Top

**Separate Search for Single Top**

**Maximum likelihood technique to extract the signal content from the data**
- Perform search to separate s-channel and t-channel events: use variables that help discriminate the two channels.

**For t-channel use kinematical boost**
** ****In proton-antiproton collisions:**
-  top production: light quark jet goes in *p* direction
-  anti-top: light quark jet goes in *p-bar* direction
** ****Correlation between**
-  pseudorapidity of untagged jet 
-  lepton charge Q
** ****Q. ******** distribution asymmetric for t-channel**
**s-channel**
** ****Count double b-tags**

- g

- u

- d

- b

- W+

- t

**z+ direction**

##      Q. Distributions for Separate Search

- Data and stacked MC templates weighted by the expected number of events in the 1-tag sample.

## Likelihood Function

- Joint likelihood function for Q. distribution in the 1-tag sample and number of events in 2-tag samples:

- Poisson

- bg gauss. constraints

- Syst. Uncert.

-  **4 processes**: t-channel (j=1), s-channel (j=2), ttpair and non top (j=3, 4)
-  ******k**: mean number of events in bin k of Q. distribution
-  ******d**: mean number of events in the 2-tags sample
-  **n****k****, n****d**: observed number in data
-  the background is allowed to float but is **constrained to SM predictions**
-  7 sources of **systematic uncertainties**

- t or s-channel

- k: bin index
- j: process index

## Systematic Uncertainties

- Systematic acceptance uncertainties for t-channel, s-channel and combined signal

**Main systematics**:

-  b-tagging (7%)
-  luminosity (6%)
-  top mass (4%)
-  JES (4%)

##   Expected Sensitivity for Separate Search

**Perform N MC experiments to estimate expected sensitivity:**
-  For each exp. we integrate out all nuisance parameters (all variables except sig) from  Lsig, and calculate the upper limit at 95% C.L.
-  Calculate the median of N individual upper limits

_**t-channel**_**                                                      **_**s-channel**_

##      Posterior probability densities for data

_**t-channel**_**                                                      **_**s-channel**_

- The maxima of the probability densities give the **most probables values** for the cross sections:
- Since all these results are compatible with zero, we set **upper-limits (at 95% CL):**

******t-ch** = 0.0+2.4-0.0 pb
******s-ch** = 4.6 ± 3.8 pb

******t-ch** < 10.1 pb
******s-ch** < 13.6 pb

## Combined Search for Single Top

**Measurement of the combined t-channel plus s-channel signal in the data**
-  use kinematic variable that discriminates s+t channels from the background
-  total transverse energy: HT distribution (scalar sum of MET, Et of lepton and jets)
-  same HT distribution for s and t-channels
-  different distributions for ttbar and non-top background

**Make a likelihood function similar to that in separate search**
- Use smoothed HT distributions

##       HT Distributions for Combined Search

**Data Distributions versus SM Expectation ****for combined sample**

## Limits for Combined Search

_**Expected sensitivity**_

_**Probability density from data**_

******s+t**** **(MPV)      = 7.7 +5.1-4.9 pb
******s+t**** **(95% CL) < 17.8 pb

******s+t**** **(a-priori) < 13.6 pb at 95% CL

## Summary

** ****No significant evidence for single top quark production**
-  set first limits on single top cross section
-  compared with Run I improvement of upper limits of 28% (t-channel) and 20% (s-channel)
** ****Technical improvements of the analysis**
-  improved MC modeling for single top events
-  full Bayesian treatment of systematic uncertainties

**95% CL limits and most probable x-sections values (pb), with 162 ****± 10 pb****-1**

## Multivariate Analysis

**New analysis improvements**
** ****Resolution of M****l********b**** :**
-  Factors contributing to poor resolution:
- 	Jet energy, b-jet choosing, neutrino choosing, lepton energy.
-  use a kinematic fitter to find the neutrino solution most consistent with measured values of the b-jet and MET

** ****Multivariate likelihoods**
-  use a combination of variables to optimize signal and background separation
-  variables include Q*, dijet invariant mass, cos lepton – other jet, HT, ET
-  likelihood function computed from distributions of each variable
-  compute sensitivities (i.e how much luminosity for discovery ?)

## Conclusion

**CDF accumulated more events than in entire Run I**
- Single top search at the Tevatron is more challenging than anticipated
**CDF prepares advanced analysis techniques**
-  multivariate (neural-network) analysis
**CDF expects 3******** signal evidence with 1.5 fb****-1**
**Many improvement are needed to observe single top in the next few years but it is feasible in Run II !**

## Backup slides

## Slide 23

## Slide 24