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Search for New Physics in events with 4-top quarks with ATLAS - - PowerPoint PPT Presentation

Jun 07, 2023 245 likes •1.05k views

Introduction 4-tops Analysis Conclusion & Outlooks Search for New Physics in events with 4-top quarks with ATLAS detector at the LHC Daniela Paredes Laboratoire de Physique Corpusculaire de Clermont-Ferrand Universit e Blaise Pascal

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Introduction 4-tops Analysis Conclusion & Outlooks

Search for New Physics in events with 4-top quarks with ATLAS detector at the LHC

Daniela Paredes

Laboratoire de Physique Corpusculaire de Clermont-Ferrand Universit´ e Blaise Pascal – CNRS/IN2P3

2nd year Seminar

Director: David Calvet September 14, 2012

Daniela Paredes New Physics in events with 4 top quarks 1/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks

Summary

Goal: Find New Physics in events with 4-top quarks. New Physics Model: Low-energy effective field theory. Channel of decay: two leptons with the same electric charge. Analysis performed on the full 2011 data set (4.7fb−1) at 7 TeV.

This analysis doesn’t test a particular theory, but rather a class of theories where New Physics manifests itself at low energy as a 4 right handed top contact interaction!

Daniela Paredes New Physics in events with 4 top quarks 2/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks

Outline

1

Introduction

2

4-tops

3

Analysis

4

Conclusion & Outlooks

Daniela Paredes New Physics in events with 4 top quarks 3/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

1 Introduction LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics 2 4-tops 4-tops production in SM Top decay & Final states for 4-tops Motivation Models with New Physics involving 4-top quarks 3 Analysis Procedure Signal Channel of decay Background Determining final selection of events 4 Conclusion & Outlooks

Daniela Paredes New Physics in events with 4 top quarks 4/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Large Hadron Collider

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Large Hadron Collider

27 Km ring

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Large Hadron Collider

27 Km ring

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Large Hadron Collider

27 Km ring pp collider ⇓ 14 TeV!

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Large Hadron Collider

27 Km ring pp collider ⇓ 14 TeV! pp collider ⇓ 8 TeV! (2012)

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Large Hadron Collider

27 Km ring pp collider ⇓ 14 TeV! pp collider ⇓ 8 TeV! (2012) Analysis done at 7 TeV

Daniela Paredes New Physics in events with 4 top quarks 5/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Large Hadron Collider

Four main experiments:

LHCb Atlas Alice CMS

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Large Hadron Collider

Four main experiments:

LHCb Atlas Alice CMS Atlas

Daniela Paredes New Physics in events with 4 top quarks 6/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Atlas detector

Daniela Paredes New Physics in events with 4 top quarks 7/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Atlas detector

  • ✒

Inner detector Measures the momentum of each charged particle

Daniela Paredes New Physics in events with 4 top quarks 8/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Atlas detector

❅ ❅ ❅ ❅ ❅ ❘

  • ✠

Calorimeters Measure the energies carried by the particles EM Calorimeter Hadronic calorimeter

Daniela Paredes New Physics in events with 4 top quarks 9/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Atlas detector

  • ✠

Muon spectrometer Identifies and measures the momenta of muons

Daniela Paredes New Physics in events with 4 top quarks 10/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Atlas detector

  • ✒

❅ ❅ ❅ ❅ ❅ ❘

Magnet system Bending charged particles for momentum measurement Solenoid Magnet Toroid Magnets

Daniela Paredes New Physics in events with 4 top quarks 11/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics Daniela Paredes New Physics in events with 4 top quarks 12/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics Daniela Paredes New Physics in events with 4 top quarks 12/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics Daniela Paredes New Physics in events with 4 top quarks 13/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics Daniela Paredes New Physics in events with 4 top quarks 14/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics Daniela Paredes New Physics in events with 4 top quarks 15/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Atlas detector

Atlas detector was designed to: Confirm predictions of the Standard Model. Improve measurements of the Standard Model. Look for New Physics. It recorded 5.25 fb−1 at 7 TeV. It has recorded more than 13 fb−1 in 2012! Efficiency ≈ 94%.

Daniela Paredes New Physics in events with 4 top quarks 16/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Standard Model

The Standard Model explains all the hundreds of particles and complex interactions

  • nly with :

6 quarks ×3 colors. 6 leptons. 12 Force carrier particles. Higgs boson. The SM explains the fundamental forces as resulting from matter particles exchanging other particles (force carrier particles). ... but it does not tell the whole story!

Daniela Paredes New Physics in events with 4 top quarks 17/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Standard Model

Gravity is not incorporated. Generations matter → Why are there three generations of particles? Antimatter → Why is there more matter than antimatter in the universe? What about the dark matter and dark energy? EWSB → Which is its origin? Is the new boson discovered the SM Higgs particle? l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l 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l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

Daniela Paredes New Physics in events with 4 top quarks 18/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics

Top quark as the most sensitive particle to New Physics

In many theories the top quarks plays a special role. Large mass ⇓ Strong coupling to the Higgs ⇓ Suggests significant interactions with the New Physics. Its large mass can be an indication that it is special in some way!

Daniela Paredes New Physics in events with 4 top quarks 19/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks 4-tops production in SM Top decay & Final states for 4-tops Motivation Models with New Physics involving 4-top quarks

1 Introduction LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics 2 4-tops 4-tops production in SM Top decay & Final states for 4-tops Motivation Models with New Physics involving 4-top quarks 3 Analysis Procedure Signal Channel of decay Background Determining final selection of events 4 Conclusion & Outlooks

Daniela Paredes New Physics in events with 4 top quarks 20/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks 4-tops production in SM Top decay & Final states for 4-tops Motivation Models with New Physics involving 4-top quarks

4-tops production in SM

gg → t¯ tt¯ t (85% at the LHC at 7 TeV)

g g ¯ t t ¯ t ¯ t g ¯ t t

76 Feynman diagrams! q¯ q → t¯ tt¯ t (15%)

q ¯ q g g g ¯ t t ¯ t t

28 Feynman diagrams!

Daniela Paredes New Physics in events with 4 top quarks 21/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks 4-tops production in SM Top decay & Final states for 4-tops Motivation Models with New Physics involving 4-top quarks

Top decay

Top decay: t → Wb around 100% times.

W + t b νℓ, q′ ℓ+, ¯ q

Leptonic Hadronic W + → ℓ+ν W + → q¯ q′ W − → ℓ−¯ ν W − → q¯ q′ W decay How many final states can we obtain from 4-top quarks?

Daniela Paredes New Physics in events with 4 top quarks 22/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks 4-tops production in SM Top decay & Final states for 4-tops Motivation Models with New Physics involving 4-top quarks

Final states

There are 35 final states from 4-top quarks depending on the W decay (h, e, µ, τ), which are constituted of 5 different classes of channels: Full hadronic : 8j + 4b Most hadronic: 1ℓ + 6j + 4b + MET Semi leptonic: 2ℓ + 4j + 4b + MET Most leptonic: 3ℓ + 2j + 4b + MET Full leptonic: 4ℓ + 4b + MET

Daniela Paredes New Physics in events with 4 top quarks 23/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks 4-tops production in SM Top decay & Final states for 4-tops Motivation Models with New Physics involving 4-top quarks

Motivation

The SM prediction for 4-tops at the LHC is very small: σSM ≈ 0.5 fb at 7 TeV Some models with New Physics predict an enhancement of the t¯ tt¯ t production rate at the LHC compared to the SM: Top composite ≈ 103 compared to the SM!

Cross sections for multi-top production in the Standard Model with mH = 130GeV (arXiv:1001.0221v3 [hep-ph]) Daniela Paredes New Physics in events with 4 top quarks 24/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks 4-tops production in SM Top decay & Final states for 4-tops Motivation Models with New Physics involving 4-top quarks

Models with New Physics involving 4-top quarks

Some models can be tested by studying events with 4-top quarks: Composite top → Randall-Sundrum. Universal Extra Dimensions model. SUSY signal. ...

Predicts a 5th fundamental force.

Contribution from a 4-top operator to 4-top production Daniela Paredes New Physics in events with 4 top quarks 25/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks 4-tops production in SM Top decay & Final states for 4-tops Motivation Models with New Physics involving 4-top quarks

Models with New Physics involving 4-top quarks

Some models can be tested by studying events with 4-top quarks: Composite top. Randall-Sundrum → Universal Extra Dimensions model. SUSY signal. ...

Predicts a Universe with 5 dimensions. gKK → t¯ t

Daniela Paredes New Physics in events with 4 top quarks 26/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks 4-tops production in SM Top decay & Final states for 4-tops Motivation Models with New Physics involving 4-top quarks

Models with New Physics involving 4-top quarks

Some models can be tested by studying events with 4-top quarks: Composite top. Randall-Sundrum. Universal Extra Dimensions model → SUSY signal. ...

Predicts a Universe with 6 dimensions. Pair production of heavy photons Aµ: AµAµ → t¯ tt¯ t It provides a candidate for dark matter!

arXiv:1107.4616v2 [hep-ph] Daniela Paredes New Physics in events with 4 top quarks 27/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks 4-tops production in SM Top decay & Final states for 4-tops Motivation Models with New Physics involving 4-top quarks

Models with New Physics involving 4-top quarks

Some models can be tested by studying events with 4-top quarks: Composite top. Randall-Sundrum. Universal Extra Dimensions model. SUSY signal → ...

Predicts a supersimetric partner for each SM particle. Pair production of gluinos: ˜ g → t¯ tχ0

1

It provides a candidate for dark matter!

arXiv:1101.1963v1 [hep-ph] Daniela Paredes New Physics in events with 4 top quarks 28/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

1 Introduction LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics 2 4-tops 4-tops production in SM Top decay & Final states for 4-tops Motivation Models with New Physics involving 4-top quarks 3 Analysis Procedure Signal Channel of decay Background Determining final selection of events 4 Conclusion & Outlooks

Daniela Paredes New Physics in events with 4 top quarks 29/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Analysis: Procedure

1

Generate events for the New Physics signal.

2

Select channel of decay.

3

Estimate background.

4

Determine the final selection of events.

5

Determine systematic uncertainties.

6

Validate the background.

7

Results. The analysis is performed on the full 2011 data set (4.7 fb−1) at 7 TeV.

Daniela Paredes New Physics in events with 4 top quarks 30/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Analysis: Procedure

1

Generate events for the New Physics signal.

2

Select channel of decay.

3

Estimate background.

4

Determine the final selection of events.

5

Determine systematic uncertainties.

6

Validate the background.

7

Results.

Daniela Paredes New Physics in events with 4 top quarks 31/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

4-tops signal

Model given by C. Degrande et al. “Non-resonant New Physics in Top Pair Production at Hadron Colliders”, arXiv:1010.6304. General and model-independent approach: Low-energy effective field theory. All possible operators with hypotheses: All SM symmetries conserved. Only top-philic new physics. No change in electroweak couplings of top (γ/Z). No change in top decay.

It introduces a new 4-tops contact interaction! Daniela Paredes New Physics in events with 4 top quarks 32/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

4-tops signal

Event generation with MadGraph 5 at 7 TeV. 4-tops contact interaction introduced by a new colorless vector particle ρ. New coupling between tR and ρ, with gρ. mρ = 100 TeV. gρ = 100 √ 8π Cross-section computed at LO, σ = 12.6 fb. Cross section is taken to be a free parameter that we place a limit on.

This analysis doesn’t test a particular theory, but rather a class of theories where New Physics manifests itself at low energy as a 4 right handed top contact interaction! Daniela Paredes New Physics in events with 4 top quarks 33/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Analysis: Procedure

1

Generate events for the New Physics signal.

2

Select channel of decay.

3

Estimate background.

4

Determine the final selection of events.

5

Determine systematic uncertainties.

6

Validate the background.

7

Results.

Daniela Paredes New Physics in events with 4 top quarks 34/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Channel of decay

Easiest channel to select is with two leptons: hhℓ±

e/µℓ± e/µ → BR = 4.15%

Why? Production of events with two leptons of the same electric charge have a very low background in the Standard Model. ⇒ Potentially large contributions from new theories! Most probable decay comes from the 1 lepton + jets! → But... 1 lepton can be produced easily by SM. Two leptons + jets is more promising. → It’s less probable in SM. Branching Ratios

Daniela Paredes New Physics in events with 4 top quarks 35/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Channel of decay

Channel topology: 2 charged leptons (electrons and muons). 8 jets, including 4 b-jets. Missing Transverse Momentum ET .

t t t

  • W

+

W

t

+

W

  • W

b b

1

q

2

q

+

e

e

ν

+

e

e

ν

3

q

4

q t t b b Daniela Paredes New Physics in events with 4 top quarks 36/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Analysis: Procedure

1

Generate events for the New Physics signal.

2

Select channel of decay.

3

Estimation background.

4

Determine the final selection of events.

5

Determine systematic uncertainties.

6

Validate the background.

7

Results.

Daniela Paredes New Physics in events with 4 top quarks 37/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Background

Sources of background : Several processes can mimic a final state with 4-top quarks. True same-sign dilepton paris: physics processes which give same sign dilepton events. False same-sign dilepton pairs: physics processes which don’t give same-sign dilepton events, but are reconstructed as such. True same-sign dilepton paris ⇒ estimated from Monte Carlo samples: WZ + jets (σ = 1.41 pb). ZZ + jets (σ = 0.86 pb). W±W±jj (σ = 0.22 pb). t¯ t + Z(j) (σ = 0.15 pb). t¯ t + W(j) (σ = 0.10 pb). t¯ tWW (σ = 0.001 pb).

Daniela Paredes New Physics in events with 4 top quarks 38/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Background

Sources of background : Several processes can mimic a final state with 4 top quarks. True same-sign dilepton paris: physics processes which give same sign dilepton events. False same-sign dilepton pairs: physics processes which don’t give same-sign dilepton events, but are reconstructed as such. False same-sign dilepton pairs ⇒ estimated from data-driven techniques : Mis-id → electron charge misidentification (for muons is negligible). Fakes → mis-reconstructed leptons. SM processes as t¯ t, single top, WW+jets, will contribute to this background and therefore are not included as Monte Carlo samples.

Daniela Paredes New Physics in events with 4 top quarks 39/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Electron mis-id estimation

The sign of the electric charge of one of the two leptons in the selected same-sign pair has been mis-reconstructed: True opposite-sign lepton pair reconstructed as a same-sign pair! They could come from: Incorrect measurement of the sign of the track curvature → dominant effect for high transverse momentum. Hard beemsstrahlung producing trident electrons: e± → e±γ∗ → e±e+e− (1) Energy cluster assigned to the wrong track! Muons are only affected by the sign of the track curvature → negligible!

Daniela Paredes New Physics in events with 4 top quarks 40/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Electron mis-id estimation

Estimated by measuring the charge misidentification rate ǫ reconstructing a Z peak using 2 same-sign electrons in data. ǫ is computed as a function of |η| bins for three different methods:

1

Tag and Probe method.

2

Direct extraction method.

3

Likelihood method.

Differences on ǫ comes from the kinematic selection Daniela Paredes New Physics in events with 4 top quarks 41/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Electron mis-id estimation

The final same-sign distribution is obtained from Me+e− weighted with ω(i, j). ω(i, j) = ǫi + ǫj (1 − ǫi)(1 − ǫj) (2) ǫi is the charge flip rate in the η bin i. Method validated by Egamma Working Group. Likelihood method is used to extract the event. The other two methods are used to compute the systematics.

Likelihood method gives the best fit! Daniela Paredes New Physics in events with 4 top quarks 42/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Tight lepton fake estimation

At least one of the two leptons in the selected same-sign pair is not a real isolated lepton but has been reconstructed as such! They could come from: Semi-leptonic decay of a b or c hadron → falsely identified as an isolated lepton. π0 or photons → mis-reconstructed leptons. The matrix method is used to determine the magnitude of the mis-reconstructed leptons in the signal region.

Daniela Paredes New Physics in events with 4 top quarks 43/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Overlap: Fake-Mis-id

90% of the mis-id background comes from trident electrons: e± → e±γ∗ → e±e+e− (3) They also tend to be identified as fakes! → The overlap (≈ 23%) is measured, and this amount is used to rescale the final mis-id estimate. In this moment we are using the Fakes from LPNHE and the Mis-id estimated by Saclay group.

Daniela Paredes New Physics in events with 4 top quarks 44/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Analysis: Procedure

1

Generate events for the New Physics signal.

2

Select channel of decay.

3

Estimation background.

4

Determine the final selection of events.

5

Determine systematic uncertainties.

6

Validate the background.

7

Results.

Daniela Paredes New Physics in events with 4 top quarks 45/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Events selection

Trigger → Single isolated lepton. At least 2 leptons with the same sign: Leading lepton PT > 25 GeV. If multiple leptons: choose pair with highest PT (µ: PT > 20 GeV, e: PT > 25 GeV ). Separate in three samples: ee sample. µµ sample. eµ sample. Z veto → ee and µµ events must satisfy |Mll − 91| > 10 GeV, and Mll > 15 GeV. At least 2 jets (PT > 20 GeV). ET > 40 GeV. HT > 350 GeV (HT = Pe

T + Pµ T + Pjets T

) At least 1 b jet.

Daniela Paredes New Physics in events with 4 top quarks 46/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Discriminant variables: Ex. eµ channel

[GeV]

T

H 200 400 600 800 1000 1200 1400 1600 1800 2000 Entries 1 2 3 4 5 4-tops (rescaled) Diboson +W/Z t t Fakes Mis-id

channel µ e

Daniela Paredes New Physics in events with 4 top quarks 47/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Discriminant variables: Ex. eµ channel

[GeV]

T

E 50 100 150 200 250 300 350 400 Entries 1 2 3 4 5 6 7 8 4-tops (rescaled) Diboson +W/Z t t Fakes Mis-id

channel µ e

Daniela Paredes New Physics in events with 4 top quarks 48/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Discriminant variables: Ex. eµ channel

b-jets

N 1 2 3 4 5 6 7 8 9 Entries 2 4 6 8 10 4-tops (rescaled) Diboson +W/Z t t Fakes Mis-id

channel µ e

Daniela Paredes New Physics in events with 4 top quarks 49/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Discriminant variables: Ex. eµ channel

jets

N 2 4 6 8 10 12 14 Entries 1 2 3 4 5 6 4-tops (rescaled) Diboson +W/Z t t Fakes Mis-id

channel µ e

Daniela Paredes New Physics in events with 4 top quarks 50/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Cut optimization

Based on the discriminant variables → The following parameters were variated: HT ∈ [350, 650] per step of 50 GeV. Number of all jets ∈ [2, 5]. Number of b jets ∈ [1,3]. ET > 40, 60 GeV. Optimization done including full systematics (described later)!

GOAL: Try to get the best expected limit!

Daniela Paredes New Physics in events with 4 top quarks 51/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Cut Optimization: ET > 40 GeV

[GeV]

T

Cut on H 350 400 450 500 550 600 650 Expected 95% C.L. uppper limit [pb] 0.09 0.1 0.11 0.12 0.13 0.14 0.15 0.16 0.17

1 ≥

b-jets

2, N ≥

jets

N 2 ≥

b-jets

2, N ≥

jets

N 3 ≥

b-jets

2, N ≥

jets

N 1 ≥

b-jets

3, N ≥

jets

N 2 ≥

b-jets

3, N ≥

jets

N 3 ≥

b-jets

3, N ≥

jets

N 1 ≥

b-jets

4, N ≥

jets

N 2 ≥

b-jets

4, N ≥

jets

N 3 ≥

b-jets

4, N ≥

jets

N

Choice: HT > 550 GeV, Nj ≥ 2 and Nb−jets ≥ 1

Daniela Paredes New Physics in events with 4 top quarks 52/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Analysis: Procedure

1

Generate events for the New Physics signal.

2

Select channel of decay.

3

Estimation background.

4

Determine the final selection of events.

5

Determine systematic uncertainties.

6

Validate the background.

7

Results.

Daniela Paredes New Physics in events with 4 top quarks 53/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Systematics uncertainties

Monte Carlo samples: MC cross-section: t¯ t + W(j) → 30%, t¯ t + Z(j) → 50%, WZ/ZZ → 34.3%, WWjj→ 50%, t¯ t+WW → +35%/-24%. Jets, e and µ energy resolution. Jets, e and µ energy scale. Jets, e and µ efficiency. Jet b-tag efficiency. Luminosity: 3.7%. Data-driven background: MisID → uncertainties computed as the difference between the 3 methods (≈ 12%). Fakes → ee: 50%, µµ: 30%, eµ: 40% (recommended by the Top Group).

Daniela Paredes New Physics in events with 4 top quarks 54/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Analysis: Procedure

1

Generate events for the New Physics signal.

2

Select channel of decay.

3

Estimation background.

4

Determine the final selection of events.

5

Determine systematic uncertainties.

6

Validate the background.

7

Results.

Daniela Paredes New Physics in events with 4 top quarks 55/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Background validation: Control Region

At least one same-sign pair of leptons. Nb−jets ≥ 1. Z veto. HT ∈ [100, 500] GeV. Njets ≥ 2.

Daniela Paredes New Physics in events with 4 top quarks 56/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Background validation: Control Region

Channel Samples ee eµ µµ False same-sign dilepton pairs Mis-id 5.2 ± 0.3 ± 0.6 7.9 ± 0.3 ± 1.0 — Fakes 10.0 ± 5.3 ± 5.0 34.0 ± 5.2 ± 13.6 17.4 ± 1.8 ± 5.2 Diboson

  • WZ/ZZ+jets

0.69 ± 0.23 ± 0.12 2.15 ± 0.36 ± 0.37 2.17 ± 0.40 ± 0.44

  • W ±W ±+2 jets

0.06 ± 0.03 ± 0.03 0.27 ± 0.06 ± 0.14 0.15 ± 0.04 ± 0.07 t¯ t + W /Z

  • t¯

tW (+jet) 0.77 ± 0.04 ± 0.17 3.34 ± 0.09 ± 0.73 2.06 ± 0.07 ± 0.45

  • t¯

tZ(+jet) 0.32 ± 0.02 ± 0.12 1.33 ± 0.05 ± 0.48 0.88 ± 0.04 ± 0.32

  • t¯

tW ±W ∓ 0.008 ± 0.001 ± 0.002 0.033 ± 0.001 ± 0.010 0.024 ± 0.001 ± 0.007 Total 17.0 ± 5.3 ± 5.0 49.0 ± 5.2 ± 13.7 22.7 ± 1.8 ± 5.2 Observed 16 34 18 Signal contamination

  • 4 tops (σ = 12.6 fb)

0.012 ± 0.003 0.046 ± 0.005 0.027 ± 0.004 Observed number of events and expected number of background events with statistical (first) and systematic (second) uncertainties for the control region selection. Daniela Paredes New Physics in events with 4 top quarks 57/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Comparison with data: eµ channel

Final selection with no cut on HT

Daniela Paredes New Physics in events with 4 top quarks 58/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Comparison with data: eµ channel

Final selection with no cut on E miss

T

Daniela Paredes New Physics in events with 4 top quarks 59/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Analysis: Procedure

1

Generate events for the New Physics signal.

2

Select channel of decay.

3

Estimation background.

4

Determine the final selection of events.

5

Determine systematic uncertainties.

6

Validate the background.

7

Results.

Daniela Paredes New Physics in events with 4 top quarks 60/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Number of events after selection

Channel Samples ee eµ µµ False same-sign dilepton pairs Mis-id 0.13 ± 0.04 ± 0.02 0.23 ± 0.04 ± 0.03 — Fakes 0.52 ± 1.12 ± 0.26 0.82 ± 1.05 ± 0.33 0.13 ± 0.13 ± 0.04 Diboson

  • WZ/ZZ+jets

0.19 ± 0.20 ± 0.07 0.34 ± 0.21 ± 0.13 0.28 ± 0.22 ± 0.10

  • W ±W ±+2 jets

0.06 ± 0.03 ± 0.03 0.07 ± 0.03 ± 0.03 0.03 ± 0.02 ± 0.03 t¯ t + W /Z

  • t¯

tW (+jet) 0.23 ± 0.02 ± 0.07 0.79 ± 0.04 ± 0.24 0.57 ± 0.04 ± 0.18

  • t¯

tZ(+jet) 0.17 ± 0.02 ± 0.09 0.61 ± 0.03 ± 0.31 0.33 ± 0.02 ± 0.17

  • t¯

tW ±W ∓ 0.008 ± 0.001 ± 0.002 0.023 ± 0.001 ± 0.007 0.016 ± 0.001 ± 0.005 Total Expected 1.31 ± 1.14 ± 0.29 2.88 ± 1.07 ± 0.53 1.36 ± 0.26 ± 0.27 Observed 2 2 Observed number of events and expected number of background events with statistical (first) and systematic (second) uncertainties after selection. Channel ee eµ µµ 0.138 ± 0.010 0.483 ± 0.019 0.343 ± 0.015 Expected number of events after selection for signal (σ = 12.6 fb). Daniela Paredes New Physics in events with 4 top quarks 61/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Number of events after selection

Channel Samples ee eµ µµ False same-sign dilepton pairs Mis-id 0.13 ± 0.04 ± 0.02 0.23 ± 0.04 ± 0.03 — Fakes 0.52 ± 1.12 ± 0.26 0.82 ± 1.05 ± 0.33 0.13 ± 0.13 ± 0.04 Diboson

  • WZ/ZZ+jets

0.19 ± 0.20 ± 0.07 0.34 ± 0.21 ± 0.13 0.28 ± 0.22 ± 0.10

  • W ±W ±+2 jets

0.06 ± 0.03 ± 0.03 0.07 ± 0.03 ± 0.03 0.03 ± 0.02 ± 0.03 t¯ t + W /Z

  • t¯

tW (+jet) 0.23 ± 0.02 ± 0.07 0.79 ± 0.04 ± 0.24 0.57 ± 0.04 ± 0.18

  • t¯

tZ(+jet) 0.17 ± 0.02 ± 0.09 0.61 ± 0.03 ± 0.31 0.33 ± 0.02 ± 0.17

  • t¯

tW ±W ∓ 0.008 ± 0.001 ± 0.002 0.023 ± 0.001 ± 0.007 0.016 ± 0.001 ± 0.005 Total Expected 1.31 ± 1.14 ± 0.29 2.88 ± 1.07 ± 0.53 1.36 ± 0.26 ± 0.27 Observed 2 2 Observed number of events and expected number of background events with statistical (first) and systematic (second) uncertainties after selection. Channel ee eµ µµ 0.138 ± 0.010 0.483 ± 0.019 0.343 ± 0.015 Expected number of events after selection for signal (σ = 12.6 fb).

Expected events: 5.6 ± 1.7 Observed events: 4

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Number of events after selection

Channel Samples ee eµ µµ False same-sign dilepton pairs Mis-id 0.13 ± 0.04 ± 0.02 0.23 ± 0.04 ± 0.03 — Fakes 0.52 ± 1.12 ± 0.26 0.82 ± 1.05 ± 0.33 0.13 ± 0.13 ± 0.04 Diboson

  • WZ/ZZ+jets

0.19 ± 0.20 ± 0.07 0.34 ± 0.21 ± 0.13 0.28 ± 0.22 ± 0.10

  • W ±W ±+2 jets

0.06 ± 0.03 ± 0.03 0.07 ± 0.03 ± 0.03 0.03 ± 0.02 ± 0.03 t¯ t + W /Z

  • t¯

tW (+jet) 0.23 ± 0.02 ± 0.07 0.79 ± 0.04 ± 0.24 0.57 ± 0.04 ± 0.18

  • t¯

tZ(+jet) 0.17 ± 0.02 ± 0.09 0.61 ± 0.03 ± 0.31 0.33 ± 0.02 ± 0.17

  • t¯

tW ±W ∓ 0.008 ± 0.001 ± 0.002 0.023 ± 0.001 ± 0.007 0.016 ± 0.001 ± 0.005 Total Expected 1.31 ± 1.14 ± 0.29 2.88 ± 1.07 ± 0.53 1.36 ± 0.26 ± 0.27 Observed 2 2 Observed number of events and expected number of background events with statistical (first) and systematic (second) uncertainties after selection. Channel ee eµ µµ 0.138 ± 0.010 0.483 ± 0.019 0.343 ± 0.015 Expected number of events after selection for signal (σ = 12.6 fb).

Expected events: 5.6 ± 1.7 Observed events: 4

No excess of events has been observed!

Daniela Paredes New Physics in events with 4 top quarks 61/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks Procedure Signal Channel of decay Background Determining final selection of events

Expected and Observed Limit

Channel Expected [pb] Observed [pb] ee 0.471 0.470 µµ 0.150 0.120 eµ 0.147 0.122 Combination 0.090 0.061

Table: Expected 95% CL. upper limit for all channels and combination.

Upper limit on the 4-tops production cross section:

σt¯

tt¯ t < 0.061 pb

Daniela Paredes New Physics in events with 4 top quarks 62/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks

1 Introduction LHC Atlas detector Standard Model Top quark as the most sensitive particle to New Physics 2 4-tops 4-tops production in SM Top decay & Final states for 4-tops Motivation Models with New Physics involving 4-top quarks 3 Analysis Procedure Signal Channel of decay Background Determining final selection of events 4 Conclusion & Outlooks

Daniela Paredes New Physics in events with 4 top quarks 63/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks

Conclusion & Outlooks

There are 4 observed events for an expected background of 5.6 ± 1.7 on the full 2011 data set (4.71 fb−1) at 7 TeV. → No excess of events have been observed! It has been set an upper limit on the 4-tops production cross section: σt¯

tt¯ t < 0.061 pb.

Results approved to be public yesterday! → To be presented at TOP2012. Next steps... Start to look at 2012 data. Do studies for Mis-id background estimation. Improve selection.

Daniela Paredes New Physics in events with 4 top quarks 64/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks

BACKUP

Daniela Paredes New Physics in events with 4 top quarks 65/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks

Tight lepton fakes estimation

At least one of the two leptons in the selected same-sign pair is not a real isolated lepton but has been reconstructed as such! → They could come from jets of photons. The matrix method is used to determine the magnitude of the mis-reconstructed leptons in the signal region. Two sets of leptons selection criteria are defined: Loose and Tight . The probabilities r and f that a real or fake “Loose” lepton pass the “Tight” criteria is measured using purified control regions.

Daniela Paredes New Physics in events with 4 top quarks 66/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks

The composition of the signal samples is extracted by inverting the following matrix: relating the “true” composition of the sample in terms of real and fake leptons to Tight and Loose leptons. The final fake estimation is Nfakes

TT

= r1f2Nll

RF + f1r2Nll FR + f1f2Nll FF .

Events that tend to have a charge misidentified electron (trident electrons) tend to also be identified as fakes in the matrix method: → The overlap between the charge misidentification and fakes (≈ 23%) is measured, and this amount is used to rescale the final mis-id estimate. In this moment we are using the Fakes from LPNHE and the Mis-id estimated by Saclay group.

Daniela Paredes New Physics in events with 4 top quarks 67/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks

Comparison with data: ee channel

Daniela Paredes New Physics in events with 4 top quarks 68/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks

Comparison with data: µµ channel

Daniela Paredes New Physics in events with 4 top quarks 69/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks

McLimit

Limit computed using the tool McLimit from Clement Helsens: Using test statistic defined as: LLR = −2 ln Ls+b

Lb

50000 pseudoexperiments were generated. Correlations of the systematic uncertainties taken into account. 95% CL expected limits computed using CLs.

Daniela Paredes New Physics in events with 4 top quarks 70/ 70

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Introduction 4-tops Analysis Conclusion & Outlooks

Limit Combination

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Cross section [pb] 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Daniela Paredes New Physics in events with 4 top quarks 71/ 70