# Presentation: 684093

## New Results on the Top Quark from DØ

- Pushpa Bhat
- Fermilab
*for *the DÆ Collaboration
- Joint Experimental  Theoretical Physics Seminar
- Feb. 21, 1997

## Outline

- Introduction
-     Production Cross Section
- Dilepton channels
***e*****n** Channel
- Lepton +jets channels
- Measurement of the Top Quark Mass (Lepton +jets Channel)
- Kinematic fit
- Multivariate Methods
- Log likelihood
- Neural Networks
- Summary

## “All-jets” channel

- “All-jets” channel
- Mass Analysis from dilepton +jets channel
- New results should be coming out soon

## Top Production and Decay

***Top Production and Decay***

- At the Tevatron, top quarks are pair-produced via
- Assuming standard model couplings,
- BR(t ® Wb) = 100%
- Categorize Channels by W  Decay Mode
- W® ln,   lepton
- W® qq¢,  “jets”
***dilepton,  lepton +jets,  all jets ***

## The DØ Detector

## Slide 6

## Dilepton Channels

***Dilepton Channels***

- Signal Characteristics
- Two isolated, high pT leptons
- Significant ET
- 2 Jets from b quarks
- Branching ratio: ~5% (em + ee + mm)
- Backgrounds
- Z ® l+l-
- WW,  Z ® t+t-
- QCD: bb, cc ® l+l-
- Jets mis-identified as electrons

- b jet

- l

- n

- n

**Notes:**
19
19
20

## Dilepton Channels

***Dilepton Channels***

- After all cuts except HT

**Background**

- HT (GeV)

**Notes:**
24
24
25

## Dilepton Analysis

***Dilepton Analysis***

- Other cuts for Z and Cosmic ray background removal

**Notes:**
20
20
21

## en  Channel

***e******n******  Channel***

- Signal
- Isolated high pT electron
- Large missing ET , High
- Two or more jets
- Backgrounds:
- W+jets
- QCD fakes
- WW, WZ
- Selection made orthogonal to all other channels

- l

**Notes:**
52
52
53

## en Analysis

-  Selection:

-  Results for òL*dt* = 108 pb-1

- Background

- Expected top (events)

- mt = 170 GeV/c2	     1.66 ± 0.36

- 4 events observed

- 1.16 ± 0.34 events

**Notes:**
57
57
58

## Lepton+jets Channels

- Signal characteristics
- One isolated high pT lepton
- Missing ET
- Several jets
- Branching ratio: ~15% per lepton flavor
- Dominant backgrounds
- W+jets production
- Fake leptons from QCD multijet events
- Two Strategies
- Event Shape and kinematic cuts (topological)
- b ® m tagging

## W+jets background estimated using

- jet multiplicity scaling*

- * Berends *et al.*, NP B357 (1991) 32

## Lepton+jets Topological Analysis

- Event shape / kinematic variables:
- Total leptonic ET: ETL = ETl + ET
- Hadronic Activity: HT  =  S ETjet
**Large H****T**** signifies the decay of massive objects**
- Aplanarity (**A**):
-       Mab = Spapb/Sp2    momentum tensor for the W* *and jets
- 		**A**** **= 3/2 x (smallest eigenvalue)
**A =1/2 for spherical events, 0 for planar events**
**  ****events are expected to be more spherical than those from radiative QCD processes**

-   Selection Cuts:

**Notes:**
33
33
34

## RAndom Grid Search... to Riches

- Take **each point** of
- the signal class as
**a cut-point**

***RAndom Grid Search******... to Riches***

- Number of cut-points
- equals the number of
- Monte Carlo signal
- events available.

**D****Æ**

## Optimization with Grid Search

- HT = 180 GeV
- A = 0.065

## Contours of Constant fractional error in Cross Section

## A vs HT

## Topological Analysis Resultse/m +jets Channels

- Data		19 events

- Background (events)

- W+jets		     7.3 ± 1.7
- QCD (fake lepton)    1.4 ± 0.5

- * Use top production cross sections from E. Laenen, et al.
- Phys. Lett. **321B, **254 (1994).

- òLdt = 110 pb-1

## Lepton+jets b-tag Analysis

***Lepton+jets b-tag Analysis***

*e*

- + jets/

- m

- m

- E

- (

*e*

- |

*e*

- p

- (

- m

- |

- h

- (

- m

- )| < 1.7

- T

- E

- T

- > 20

- GeV

- E

- T

- 2

- P

- T

- > 4

- GeV/c   and

- D

- R(

- m

- ,jet) < 0.5

- 3 3 jets with E

- T

- > 20

- GeV and |

- h

- | < 2

- T

## Jet Multiplicity Distribution for Lepton+jets+b-tag Events

## b-Tag Analysis Resultse/m +jets+b-tag Channels

- Data		11 events

- Background (events)

- W+jets		     1.4 ± 0.4
- QCD (fake lepton)    0.8 ± 0.3
- Z ® mm		     0.2 ± 0.1

- Total		      2.4 ± 0.5

- * Top production cross sections from E. Laenen, et al.
- Phys. Lett. **321B, **254 (1994).

**ò****Ldt = 107 pb****-1**

## Summary of Analysis 8 Channels

***Summary of Analysis ******8 Channels***

- ~125 pb-1 (full 1992 - 1996 statistics)

- *Top cross section from Laenen, *et al*, Phys. Lett. **321B**, 254 (1994)
-   with mt =170 GeV/c2

- Number of Events

**Notes:**
48
48
49

##      Cross Section by Channel

- (mt = 170 GeV/c2)

##    Cross Section

- 39  events observed  (8 channels) over background  of 13.6 ± 2.1

- òL *dt*  » 125 pb-1

## Top Quark Mass Measurement(Lepton + jets Channel)

- 	Optimal use of information
- 	Use both “fitted top quark mass” and “signal probability” for each event
- 	To demonstrate the robustness of the results, use two complementary methods

## Mass Analysis in a Nutshell

- Events with four or more jets used
- 2C Kinematic fit to
- 			    *m**fit** , **c**2*
- Compute signal probability for each
- 	event         *D*
- 2D Likelihood fit in (D, mfit)-plane to signal + background models	                              		 ***L***(mt)
- Quadratic fit to -ln ***L**** *to extract mt and sm
- Signal and background models
- HERWIG MC (v 5.7) for
- VECBOS MC (v 3.0) for W +jets background
- Data for fake lepton background

## Event Selection

- 91 events selected
- 7 events b-tagged

## Kinematic Fitting

- Constrained fit to top hypothesis
- Use 4 highest ET  jets
- 12 (6) Permutations for untagged (tagged) events
- Measured Variables (17)
- Lepton (E,h,f)
- 4 jets (E,h,f)
- Unmeasured Variables (1)
- pz(n)
- Constraints (3)
- m(ln) = mW
- m(qq) = mW
- m(lnb) = m(qqb)
- 	Þ 2C fit		 Þ mfit , c2
- Require c2 < 10, select fit with smallest c2
- Þ 77 events in data, 5 tagged

## Jet Energy Corrections

- Apply standard DÆ corrections
- Correct to parton level
- Derived from tt Monte Carlo
- Separate corrections for light quark jets, untagged b-jets, tagged b-jets
- Applied after choosing a particular jet permutation for the fit
- Further h-dependent corrections obtained from a study of ET balance in g+jets in data and MC
- Examine scale difference between data and MC as a function of ET(g)
- Þ  Energy Scale Uncertainty
- 		= ±(2.5% *ET +0.5) GeV

## Jet Energy Corrections

## Multivariate Discriminants

- Variables:
*x*3 measures the centrality of the event
*x*4 measures the extent to which jets are clustered together
- We combine the four variables into a	multivariate discriminant (single variable)     D ® 0 - 1

## Multivariate Discriminants

- Likelihood Discriminant
- 		Parametrize
- 		Form                           for each event
- 		*w**i**’*s are adjusted to minimize	correlation            	of L with mfit
- Neural Network Discriminant
- 		Sensitive to multivariate densities
- 		Train on samples of top and background
- 	DNN approximates signal probability directly

## Variables & DLB

## Comparison of Data & Predictions

## The Discriminants

## NN Discriminant(DNN vs mfit )

- Signal (170 GeV/c2)

- Background

## DNN vs mfit 

## Likelihood Fitting

- Bin the data, signal and background samples in (D, mfit) plane
- 	D : Top Probability		mfit : 80 - 280 GeV/c2
- 	LB Analysis
- 	2 bins in Probability, 20 bins in mass
- 		LB Cuts: DLB > 0.43,  HT2> 90 GeV
- 			       or  b-tag
-        Signal and background bins
- 	NN Analysis
- 	Maximum use of the discriminant
- 	10 bins in DNN, 20 bins in mass
- 		  uniform population of S + B in 10 DNN bins.
- Fit the data to signal + background models

## Likelihood Fitting (contd.)

- Data partitioned into ***M*** bins and modeled by sum of signal and background of source strengths ***p******s****** ***and*** p******b ***and true bin counts asi and abi
*	*	Data: ni	i=1, ... M
- 		Model: 	di= psasi+ pbabi
- Assign Poisson probability to the counts in each bin for all samples
- 	*L*(ni| mt,ns,nb,asi,abi) =
- Integrating over true counts asi, abi Þ *L*(mt,ns,nb)
- 	ns,nb: No. of signal, bkg. events in the data
- 	ni,nsi,nbi: data, signal, bkg. events in bin i
- Find maximum likelihood estimates of ns,nb
- 		Þ

## LB fit 			mt = 174.0 ± 5.6 GeV/c2

- LB fit 			mt = 174.0 ± 5.6 GeV/c2
- NN fit 			mt = 171.3 ± 6.0 GeV/c2
- Combined Result	mt = 173.3 ± 5.6 GeV/c2

## Ensemble Studies (LB)

## Ensemble Studies (NN)

## Ensemble Studies 

## Systematic Errors

- Jet Energy Scale		    4.0
- Generator
- ISAJET/HERWIG		    3.3
- VECBOS Models		    2.6
- Multiple Interactions	    1.2
- MC Statistics		    0.8
- LB/NN Diff.    		    1.4
- Likelihood Fit		    1.0
- Total				    6.2

**  **
** ****Source of uncertainty**

-  **Error in**
**  ****GeV/c****2**

## LB and NN Fit Summary

- Systematic error on mt	Energy scale ± 4.0
- 	(GeV/c2)	              generator       ± 4.1
- 				other              ± 2.2

- Correlation between LB and NN fits = (88 ± 4)%
- Combined mt (GeV/c2)	173.3 ± 5.6 (stat.) ± 6.2 (syst.)

## mt = 173.3 ± 5.6(stat.) ± 6.2 (syst.) GeV/c2

- mt = 173.3 ± 5.6(stat.) ± 6.2 (syst.) GeV/c2

##    Cross Section

- 39  events observed  (8 channels) over background  of 13.6 ± 2.1

- òL *dt*  » 125 pb-1

**s****tt**** ****(m****t**** = 173.3 GeV/c****2****)**** = 5.53 ± 1.67 pb**

## Summary

**s****tt**** = 5.53 ± 1.67 pb**

**m****t ****= 173.3 ± 5.6(stat.) ± 6.2 (syst.) GeV/c****2**

## BACKUP SLIDES

## Jet Energy Corrections(Cross check from Z ®ee+jets)

## Slide 52

## Slide 53

##   Top Production and Decay

- P (+1)

- b (-1/3)

- t (+2/3)

- b (+1/3)

- t (-2/3)

- P (-1)

- At the Tevatron,
***  ******qq ******® ******tt (******» ******90%)***
***  ******gg ******® ******tt (******» ******10%)***

- Assuming standard model couplings,
***     ******BR(t ******® ******Wb) = 100%***

## Tagging b-Quarks with Muons

- 44% of events have a soft muon

- OR

## Sources of Systematic Uncertainties

- Jet Energy Scale
- Trigger Efficiencies
- Lepton ID Efficiencies
- Background Modeling
- Jet multiplicity Scaling
- Tag Rate
- Z-Fitter
- Luminosity
- tt Generator (HERWIG/ISAJET)