# Presentation: 253294

## Top Quark Propertiesand Search for Single Top Quarkat the Tevatron

- Meenakshi Narain
- Boston University
- Presented at EPS 2005

## Top Quark at the Tevatron

- Top quark discovered a decade ago
- (in 1995)!
- Run I (1992-1996)
-   s = 1.8 TeV
- Integrated luminosity
- 120 pb-1
- Run II (2001-present)
-   s = 1.96 TeV
- 3 fold increase performance since June’03
- Integrated luminosity by June ‘05:
- Delivered >1fb-1
- On tape ~800pb-1
- Analyzed up to ~350 pb-1

- World’s only top factory!

## Top Quark  Physics

- Top is very massive
-  It probes physics at much higher energy scale than the other fermions.
- Top decays before hadronizing
- momentum  and spin information is passed to its decay products.
- No hadron spectroscopy.
- Top mass constrains the Higgs mass
- Mtop, enters as a parameter in the
-    calculation  of  radiative  corrections to other
-    Standard Model observables
- it is also related, along with the mass of
-     the W boson, to the that of the Higgs* *boson.

**M****top ****(world average)**= 172.7  2.9 GeV

** ********top**** ~ **10-24 sec

## The Top Properties Tour

- W helicity

- Anomalous Couplings

- CP Violation

- Top Mass

- Top Charge

- Top Spin

- Top Width

- Production X-Section

- Production Kinematics

- Top Spin Polarization

- Resonance Production

- Rare/non SM decays

- Branching Fractions

- |Vtb|

- Y

## Top Quark Decay Properties

- Does top quark decay 100% of the times to Wb?
- B(t Wb)
- Search for exotic decay modes of the top quark
- t H+b
- Properties of the W-t-b vertex
- W Helicity
- Top quark Charge

## Is B(t Wb) ~ 100%?

- Within the SM, assuming unitarity of the CKM matrix, B(tWb)~1.
- An observation of a B(tWb) significantly different than unity would be a clear indication of new physics:
-  non-SM top decay, non-SM background to top decay, fourth fermion generation,..

## B(tWb) can be accessed directly in single top production. 
    Top decays give access to B(tWb)/B(tWq):

R can be measured by comparing the number of ttbar candidates with 0, 1 and 2 jets tagged.
In the 0-tag bin, a discriminant variable exploiting the differences in event kinematics between ttbar and background is used.	

- B(tWb) can be accessed directly in single top production.
-     Top decays give access to B(tWb)/B(tWq):
- R can be measured by comparing the number of ttbar candidates with 0, 1 and 2 jets tagged.
- In the 0-tag bin, a discriminant variable exploiting the differences in event kinematics between ttbar and background is used.

- DØ Run II Preliminary

- Lepton+jets (~230 pb-1)

- Lepton+jets and dilepton (~160 pb-1)

- In the SM

- hep-ex/0505091

- Results consistent with the SM prediction

- Measurement of B(tWb)/B(tWq)

## Exotic Decays of the top quark

- Since R is about 1
- Top quark decays to a b-quark     t X+b
- Is X = W+ ?
- OR
- could X = H+   ?.
- as predicted by generic 2Higgs Doublet Models?

## Search for tH+b

- If MH±<mt-mb
- then t H+b competes with t W+b
- results in B(tWb)<1.
- H decays are different than W decays
-  affect (tt) measurements in different channels (dileptons, lepton+jets, lepton+tau).
- Perform simultaneous fit
- to the observation in all channels and
- determine model-dependent exclusion region in (tan, MH±).

## Are the other Properties of the Top Quark as Expected?W-t-b Vertex: W helicityTop Charge

## W helicity in Top quark Decays

- Large top quark mass:
- Are there new interactions at energy scales near EWSB?
- helicity of the ***W ***** **boson:
- examines the nature of the ***tbW****  vertex *
- provides a stringent test of Standard Model

- V-A coupling

**V-A SUPPRESSED**

- gWtb  |Vtb| (V-A)

***F******0******= 0.7        F******-******= 0.3       F******+******=0***

## W helicity

- In the Standard Model (with mb=0):
- The PT of the lepton has information about the helicity of the W boson:
**longitudinal**: leptons are emitted perpendicular to the W (**harder lepton P****T**)
**left-handed**: leptons are emitted opposite to W boson (**softer lepton P****T**)

**Longitudinal**

**Left-handed**

**Right-handed**

***SM: F******-******= 0.3        F******0******= 0.7       F******+******=0***

## W Helicity

- Likelihood analysis of PT spectrum
- Consider dilepton channels
- Fix  F0=0.7, measure F+ (F-=1-F0-F+)
- Binned likelihood and estimate F+ using Bayesian method
-  Run I best (DØ 125 pb-1): F0=0.560.31

- Likelihood analysis of cos *
- Consider lepton+jets channels
- Fix F0=0.7, measure F+ (F-=1-F0-F+)
- Two analysis: topological and b-tag
- Run I best (CDF 109 pb-1): F+<0.18 @ 95% CL

- Results consistent with the SM prediction: F0=0.7, F+=0

- Lepton+jets (1 b-tag)

## Measurement of top quark Charge

- Is it the Standard Model top ?
- OR
- An exotic doublet of quarks (Q1, Q4)
- with charges  (-1/3,-4/3) and M ~ 175 GeV/c2
- while M(top) ~ 274 GeV/c2
- W.-F. Chang et al.,hep-ph/9810531
- q = -4/3 is consistent with EW data,
- new b-couplings  improve the EW fit
- (E. Ma et al. , hep-ph/9909537)

## Top Quark Charge Measurement

- Goal: discriminate between
-  |Qtop| = 2e/3 and |Q”top”| = 4e/3
- Top quark charge is given by the sum of the charge of its decay products
- Determine:
- Charge of W (lepton)
- Charge of b-jet Qjet = qi pTia/  pTia
- (here, a=0.6)
- Associate b-jets to correct W (charged lepton)
- The charge of the quark is correlated with the charge of the highest pT hadron during  hadronization

## Top Quark Charge 

- We need an observable and an expectation for the ”2/3” and ”4/3” scenarios
- Consider only lepton+jets  double-tagged events
- Two top quarks in the event  measure the charge ”twice”
- The exotic scenario is obtained by permuting the charge of the tagged jets
- qb and qB are taken from the data derived jet charge templates
- Results coming soon...

**q****B**

**q****B**

**q****b**

**q****B**

**q****b**

## Top Quark Production Properties

- Since top decay properties look quite consistent with SM predictions....
- What about its production?
- Could it be a “t-prime”?
- Search for t’t’ production (t’ Wq)
- Could the ttbar pair originate from the decay of a resonance?
- Model independent search for narrowresonance Xtt used to exclude a leptophobic X boson:
- What about single top production?

- Run I search of X with G=1.2%M:
- MX>560 GeV @ 95% CL  (DØ)  and MX>480 GeV @ 95% CL (CDF)

## Search for Single Top Quark

## Search for Single Top

- Electroweak Production of top quark:
- Measure production cross sections
- Direct measurement of |Vtb| (s  |Vtb|2)
- Top spin physics (~100% polarized top quark)
- s- and t-channels sensitive to different New Physics
- Irreducible background to associated Higgs production
- Exotic Models (FCNC, Top Flavor, 4th Gen)

## Single Top Status

- Cross sections:                                                       *s-channel                  t-channel           s+t*
**NLO calculation:                    0.88pb (****±****8%)        1.98pb (****±****11%)**
- Run I 95% CL limits, DØ:          < 17pb                    < 22pb                                 CDF:         < 18pb                    < 13pb           < 14pb
- Run II CDF 95% CL limits:        **< 14pb                    < 10pb           < 18pb**
- Other Standard Model production mode (*Wt*) negligible

- q

- q'

- W

- t

- b

- s-channel

-  t-channel

- u

- d

- b

- t

- W

## Signature & Backgrounds

- Signal for *s* and *t* channel mostly similar

- Lepton + Missing ET + Jets
- t-channel extra b tends to be forward
- Similar to top pair production, but with less jets

- Harder Signal To Find

- Backgrounds

- W/Z + jets Production
- Fake Leptons
- Top Pair Production
- WW, WZ, Z, etc.

- Anything with a lepton + jets + ET signature

- (t-channel)

- Much worse than for pair production because of lower jet multiplicity

## Object kinematics
Jet pT  for different jets
Tagged, untagged,...
Event kinematics
H (total energy)
HT (transverse energy)
M (invariant mass)
 MT  (transverse mass)
Summing over various objects in the event
Angular variables
Jet-jet separation
Jet pseudorapidity (t-channel)
Top quark spin

- Object kinematics
- Jet *p**T*  for different jets
- Tagged, untagged,...
- Event kinematics
*H (total energy)*
*H**T **(transverse energy)*
*M (invariant mass)*
* **M**T**  (transverse mass)*
- Summing over various objects in the event
- Angular variables
- Jet-jet separation
- Jet pseudorapidity (t-channel)
- Top quark spin

- Discriminating Variables

## Separating Signal from Backgrounds

- Four analysis methods
- Three (Cut, NN, DT)  use the same structure:
- Optimize separately for s-channel and t-channel
- Optimize separately for electron and muon channel (same variables)
- Focus on dominant backgrounds: W+jets, tt
- W+jets – train on tb-Wbb and tqb-Wbb
-  tt – train on tb –  tt *l *+ jets  and  tqb –  tt *l *+ jets
- Based on same set of discriminating variables
- 8 separate sets of cuts/networks/trees

- Cut-Based

- Neural Networks

- Decision Trees

- Likelihood
- Discriminant

## 1. Cut-Based Analysis

- Cuts on sensitive variables to isolate single top
- Separate optimizations for     s-channel and t-channel
- Loose cuts on energy-related variables: *p**T **(jet1**tagged**)** H(alljets – jet1**tagged**)**H(alljets – jet1**best**)**H**T *(alljets)*M(top**tagged**)**M(alljets)**M(alljets – jet1**tagged**)****ŝ** *

- Factor 2 improvement!

## 2. Neural Network Analysis

**Output Node:** linear combination of hidden nodes

**Hidden Nodes:**  Sigmoid dependent on the input variables

**Input Nodes:**  One for each variable xi

- full dataset

- electron

- muon

- =1 b-tag

- 2 b-tags

- =1 b-tag

- 2 b-tags

- 2d histograms, Wbb vs tt filter

- constructnetworks

## Result

- No evidence for single top signal
- Set 95% CL upper cross section limit
- Using Bayesian approach
- Combine all analysis channels (e, , =1 tag, 2 tags)
- Take systematics andcorrelations into account

** ********t**** < ****12.4 ****/ 11.3 pb **

******s**** <  ****9.8 ****/ 10.6 pb **

**Expected/Observed limit:**

- Expected limit: set Nobs to background yield

- Systematic uncertainty:

## Neural Network Output

- e+
- 1 tag

- e+
- 1 tag

- e+
- 1 tag

- e+
- 1 tag

## Result

- No evidence for single top signal
- Set 95% CL upper cross section limit
- Using Bayesian approach and binned likelihood
- Built from 2-d histogram of Wbb NN vs tt NN
- Including bin-by-bin systematics and correlations

** ********t**** < ****5.8 ****/ 5.0 pb **

******s**** < ****4.5 ****/ 6.4 pb **

**Expected/Observed limit:**

## 3. Decision Tree Analysis

- Sensitivity comparable to Neural Network analysis

** ********t**** < 6.4**** ****/ 8.1 pb **

******s**** < ****4.5 ****/ 8.3 pb **

**Expected/Observed limit:**

- Replace Neural Networks by Decision Trees
- single tree, ~100 nodes
- Remaining analysis steps identical
- Same inputs
- Same filter configuration
- Binned likelihood analysis

- HT>212

- Pass

- Fail

- pt<31.6

- Mt<352

- purity

## 4. Likelihood Discriminant Analysis

- New Analysis based on 370pb-1 dataset
- Different btagging algorithm and selection
- Likelihood Discriminant:
- Input Variables:

## Result

- ...

** ********t**** < 4.3**** ****/ 4.4 pb **

******s**** < ****3.3 ****/ 5.0 pb **

**Expected/Observed limit:**

- Best Limit !!!!

- Comparison of LH with NN analysis

## Single Top Summary

- Cross sections:                                                       *s-channel                  t-channel           s+t*
**NLO calculation:                    0.88pb (****±****8%)        1.98pb (****±****11%)**
- Run I 95% CL limits, DØ:          < 17pb                    < 22pb                                 CDF:         < 18pb                    < 13pb           < 14pb
- Run II CDF 95% CL limits:        **< 14pb                    < 10pb           < 18pb**
**RunII D**Ø** 95% Cl Limits:**
**(230 pb****-1****) **
**Cut Based                              < 10.6pb                 < 11.3pb          **
**Decision Tree                        < 8.3pb                    < 8.1pb           **
**Neural Network*                    < 6.4pb                    < 5.0pb **
**(370 pb****-1****) ****(new analysis)**
**Likelihood Discriminant****       ****< 5.0pb                    < 4.4pb **

- * = Accepted for publication, hep-ex/0505063

## Sensitivity to non-SM Single Top

- using only electron channel data

- using only muon channel data

## Conclusion

- Measurements of various top quark properties are underway and will improve with larger data sets
- The single top cross section limits and sensitivity of the analyses are getting to a level where we can expect to observe single top quark production soon!.
- 			Stay Tuned.