# Presentation: 847111

## Top quark production at the TevatronElizaveta Shabalina University of Illinois (Chicago)for CDF and D0 collaborations

**Outline**
**  **dilepton channels in D0 and CDF
-   topological analysis in lepton+jets channel in D0
-   lepton+jets analysis with *b*-tagging in D0
-   lepton+jets analysis with *b*-tagging in CDF**   **

- new

## Top quark production and decays

-  in proton-antiproton collisions
-    at Tevatron energies, top
-    quarks are primarily **produced **
**   ****in pairs**
** ****Run I measurement based **
**   ****on 100 events:**
**    ****CDF:    **
**    ****D0:  **
-  Run II top cross section (1.96
-    TeV) ~ 30% higher than Run I
-    (1.8 TeV)
-  Br (t→Wb) = 100%
-  final states are determined by
-     the decay mode of W

- Both W’s decay via Wl (l=e or )
**dilepton** channels – studied by CDF and D0 in Run II
- One W decays via Wl (l=e or )
**lepton+jets** channels
-  topological analysis – D0
-  analysis using lifetime *b*-tagging – D0 and CDF

***All jets***

**

*e+jet*

***+jet*

*  **14.8%*

*1.2%    1.2%*

*2.5%*

*44.4%*

*21.1%*

*           *

*           *

*  **14.8%*

## Top cross section: dilepton channels

- Event selection:
- 2 high PT isolated charged leptons (e,)
- Neutrinos: large missing ET
- At least 2 jets
- Large transverse energy
- Backgrounds
- Physics: WW/WZ, Z→determined from Monte Carlo, Drell Yan (Z/*→ ee,)       data
- Instrumental: fake leptons in W+jets and QCD, fake missing ET
-             measured on data
- Features:
- Smaller yield (~5% e,μ)
- Cleaner signal (2 high PT leptons)
- Smaller systematics (fewer jets)

## CDF and D0: dilepton channels

- CDF:
- 79 pb-1

|  | ee | μμ | eμ |
| --- | --- | --- | --- |
| Background | 1.00 ± 0.48 | 0.60 ± 0.30 | 0.07 ± 0.01 |
| ttllbb | 0.25 ± 0.02 | 0.3 ± 0.04 | 0.50 ±0.01 |
| SM expectation | 1.25 ± 0.48 | 0.90 ± 0.30 | 0.57 ± 0.01 |
| Data | 4 | 1 | 1 |

- 33pb-1

- 42pb-1

- 48.2pb-1

- CDF

- D0:

- +- + 2 jets

**μ****1**

**μ****2**

**Jet****1**

**Jet****2**

## Top cross section: lepton+jets channels

**b**

****

**p**

- t(→W±b) t(→W±b)

- e±,±

- qq

*jet*

*jet*

*jet*

*jet*

-   Event preselection:
-   1 high PT isolated charge
-     lepton (e,)
-   Neutrinos: large missing ET
-   Large jet multiplicity
-   dilepton veto
-   Backgrounds:
-   W+jets and fake leptons in QCD
-   Features:
-   Larger yield (~30%)
-   Higher background
-   Further selections and techniques
-   make use of event topology (D0)
-   tag *b* jets with Soft Lepton Tag (D0)
-   tag *b* jets with displaced VTX (CDF & D0)

## D0: lepton+jets channels: topological analysis 

- “Matrix” method

-  Preselect a sample enriched in
-   W events
-  Evaluate QCD multijet background
-   from data for each jet multiplicity
-   bin using “matrix” method
-           e+jets: due to fake jets (o and )
-           +jets: due to heavy flavor decays
-   Estimate real W+4 jets contribution
-     with scaling law
-  Additional topological cuts:
-  ≥ 4 jets
-  HT>180 GeV (e)
-  Aplanarity>0.06
-  HT(jets,pT(W))>220GeV (μ)

|  | NW | NQCD | All BG | Exp Signal | Nobs |
| --- | --- | --- | --- | --- | --- |
| e+jets | 1.30.5 | 1.40.4 | 2.70.6 | 1.8 | 4 |
| +jets | 2.10.9 | 0.60.4 | 2.71.1 | 2.4 | 4 |

**N****loose**** = N****W**** + N****QCD****             **
**N****tight**** = ********sig**** ******** N****W**** + ********qcd ******** N****QCD**

## D0: lepton+jets channels: soft lepton tag

- same preselection as topological analysis
- ≥ 3 jets
- softer topological cuts:
- HT>110 GeV
- Aplanarity>0.04
- soft muon within jet (b→, b→c→μ)

|  | All BG | Exp Sig | Nobs |
| --- | --- | --- | --- |
| e+jets | 0.20.1 | 0.5 | 2 |
| +jets | 0.70.4 | 0.8 | 0 |

- lepton+jets channels only

- all combined

##  Top cross section: lifetime b-tagging

- Look for displaced vertices (≥ 2 tracks),
- jet is tagged as *b* jet
- If signed decay length
- significance >3 (CDF); >5 (D0)

**Selection:**
-  same preselection as topological analysis
-  ≥ 3 jets
-  1 jet tagged with lifetime *b*-tagging algorithm

- Secondary Vertex Tag
- (SVT – D0; SVX- CDF)

- Counting Signed Impact
- Parameter tag (CSIP – D0)

- S = IP/(IP)
- Jet is positively tagged if it has
- at least two tracks with S>3 or
- at least three tracks with S>2

## B-tagging analysis: method

-      cross section is determined from the excess of the observed number of tagged events w.r.t. the predicted background for njets≥3

- after tagging

- before tagging

- lepton+ MET + njets

- QCD

- W+ njets

- N bckg tag

- from data

## B-tagging efficiency and mistag rate

|  | SVT | CSIP |
| --- | --- | --- |
| btag | (31.9 ± 1.6)% | (36.3 ± 1.9)% |

- Use a sample enriched in      production: muon-in-jet sample (at least one of the jets contains a muon from *b- *or c- semileptonic decay)

- Method 1: single tags vs no tags

- Positive tag of a light flavor jet is a mistag
- Measure from the rate of negatively tagged jets (inversed tagging cut) in QCD data sample
- Correct for heavy flavor and long lived particles which are not fully removed

- CSIP

- SVT

- from MC
- 1.11 ± 0.08  SVT 1.09 ± 0.09 CSIP

- SVT

## Signal and background summary I

| lepton + jets | 1 jet | 2 jet | 3 jet | ≥ 4 jet |
| --- | --- | --- | --- | --- |
| W + jets | 22.3 ± 4.7 | 18.7 ± 3.4 | 4.4 ± 0.9 | 1.4 ± 0.4 |
| QCD | 8.2 ± 1.4 | 7.6 ± 1.2 | 3.9 ± 0.9 | 1.1 ± 0.4 |
| Total bkgr | 30.6 ± 5.0 | 26.4 ± 3.5 | 8.3 ± 1.3 | 2.5 ± 0.7 |
| Expected  |  | 0.7±0.1 | 2.8 ± 0.2 | 4.0 ± 0.6 |
| Bkgr + | 30.6 ± 5.0 | 27.1 ± 3.6 | 11.1 ± 1.4 | 6.5 ± 1.0 |
| Before tag | 3681 | 1351 | 298 | 65 |
| tagged | 34 | 27 | 13 | 6 |

- W+jets background
-  flavor composition: from
-   ALPGEN MC
-  event tagging probabilities
-   from MC with efficiency and
-   mistag rate from data
- QCD background
-  from generic QCD data sample
-   and “matrix” method

| lepton+≥4jets | CSIP | SVT |
| --- | --- | --- |
|  | (45.7±4.9)% | (41.8±4.7)% |

-                      tagging probability

-      D0 Run II preliminary

- CSIP

- D0 Run II preliminary

- CSIP

- CSIP

## Signal and background summary II

-      D0 Run II preliminary

- SVT

- e + jets

- μ + jets

- SVT

- SVT

## D0: lepton+jets channels with b-tagging

- CSIP
- SVT

| # of tags | W+1 jet | W+2 jet | W+3 jet | W+≥ 4 jet |
| --- | --- | --- | --- | --- |
| CSIP | 34 | 27 | 13 | 6 |
| SVT | 28 | 20 | 9 | 9 |

- 44.75 pb-1

- CSIP

-      D0 Run II preliminary

-      D0 Run II preliminary

- CSIP

- SVT

## Tagged event: e+jets channel

- Event is tagged by both algorithms (run 169923 event 16396718
- Njets = 4
- pT(e) = 27 GeV
- pT(jet) = 51, 36, 30, 53 GeV
- Missing ET = 58 GeV
- HT = 207 GeV
- Aplanarity = 0.11

- Primary vertex:
- Ntrack = 17
- z = 4.6 cm

## CDF: lepton+jets channels with b-tagging

- 57.5 pb-1

## Conclusions

- Both CDF and D0 re-established signals in Run II in the majority of top production channels
- D0 has presented the first physics result based on life time b-tagging which makes use of the new D0 Silicon Microstrip Tracker.
- The Run II measurements of the top pair production cross section demonstrate significant progress in the optimization and understanding of the detector performance by both experiments
- _We are ready for the Top Physics with large datasets_