25/10/2017 1

Leonid Serkin( INFN Gruppo Collegato di Udine and ICTP Trieste )

on behalf of the ATLAS Collaboration

Top quark as a window to new physics:

recent ATLAS results

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Impressive agreement between SM predictions and experimental measurements

Pro

duction c

ross-s

ection 6 orders of

magnitude!

Introduction

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Pro

duction c

ross-s

ection

Electroweak

Top

Higgs sectors

Introduction

Impressive agreement between SM predictions and experimental measurements in the

Electroweak, Top and Higgs sectors!

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I will focus on the Top sector measurements and searches of physics beyond the SM

Pro

duction c

ross-s

ection

Top&Co

Introduction

Cross-

section

< 1pb !

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ATLAS detector and LHC data

The ATLAS detector:

• Inner Detector precision tracking system

• ECAL/HCAL measure energies and positions of showers

• Muon Spectrometer, in combination with a toroidal

field, finds position and energy of muons

The datasets collected during the years 2015-2016 at

√s = 13 TeV of proton-proton collisions and corresponds to

3.2 - 13.2 fb-1 of integrated luminosity

New physics searches: strategy

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Strategy for new physics searches:

indirectly, by looking for non-standard properties (spin, CP, couplings…)

directly, by explicit search for BSM objects

In the case of direct searches, define selection based on signal signatures/acceptance

and background kinematics

Compare observed data to Standard Model background (Monte Carlo and data-driven)

and MC signal predictions

Split into control and signal regions, extrapolation checked before unblinding

(validation)

In the case of no evidence for new physics: set limits on cross-section times branching

ratio

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Top pair + ?

Extensions of the SM predict particles usually coupled preferentially to the top quark,

the heaviest fundamental particle in the SM

Possible new phenomena may enhance the SM cross-sections through the production of

heavy objects in association with a top-quark pair

High multiplicity searches: many b-jets, H, V and top quark decays

Some of the most complex analyses in ATLAS Exotics program

Profile likelihoods (shape fits) with O(100) nuisance parameters (syst. uncertainties)

and up to O(10) backgrounds

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MC Validation and Modelling

Monte Carlo validation and modelling important for background estimation strategies:

many BSM searches live in the extreme phase spaces

Harmonisation of ATLAS generation configurations, as well as recommmendations for

systematic uncertainties

Validation of the properties of simulated events from MC event

generators in order to ensure the correctness of generator

configurations and production samples used in physics analysesPseudorapidity

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We are interested in studying the final states containing (at least) one top-quark pair

Define selection based on signal signatures/acceptance and background kinematics

As an example: top-quark pair production with a single lepton final state

New physics searches: object selection

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exactly one central (|η| < 2.5)

reconstructed and isolated

electron or muon with pT > 30 GeV

Object selection

EPJC (2016) 76, 292

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jet

Object selection

JETM-2016-010

at least four central (|η| < 2.5) jets with pT > 25

GeV from calibrated topological clusters using

anti-kt (R=0.4) algorithm

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at least two of the selected jets have to be b-tagged by a

multivariate algorithm (<1% mistag) called b-jets

Object selection

ATL-PHYS-PUB-2016-012

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The top quark is the most strongly-coupled SM particle to the Higgs field

Indirect constraints on the top-Higgs Yukawa coupling come from loop diagrams with

top quarks which contribute to ggH production and H → γγ decays

ttH search

Associated production of a Higgs boson with a pair of top quarks (ttH) allows direct

probing of top-Higgs Yukawa couplings without assumption on new physics contribution

to loop diagrams

–

Any deviation could be a hint of new physics

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Final states in ttH

ttH process can be studied in a variety of final states, depending on the Higgs decay

mode and top-quark pair decay topology: γγ, WW/ZZ/ττ (multi-lepton) and bb

–

–

Top quark decays before it hadronises,

almost exclusively through t → Wb

W-boson decay modes characterise the

signature of top-quark pair final state

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ttH process can be studied in a variety of final states, depending on the Higgs decay

mode and top-quark pair decay topology: γγ, WW/ZZ/ττ (multi-lepton) and bb

–

• Clean signature thanks to excellent mass resolution, small branching ratio

(BR~0.23%)

• Looking for narrow signal peak in the di-photon invariant mass spectrum on top of a

smothly falling background

• Search dominated by statistical uncertainties

–

• ttH, H → γγ [ATLAS-CONF-2016-067]–

ttH and 2 photons

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ttH and many leptons

ttH process can be studied in a variety of final states, depending on the Higgs decay

mode and top-quark pair decay topology: γγ, WW/ZZ/ττ (multi-lepton) and bb

–

–

• ttH, H → WW/ZZ/ττ [ATLAS-CONF-2016-058]–

• (Semi)-leptonic final states with low background, but

with additional sensitivity to ttW/ttZ background

• Cut-and-count analysis in 4 categories, depending on

the number, charge and flavour of leptons (l): 2 same

sign (ss) l+0τhad , 2ssl+1τhad, 3l and 4l

• Main systematic uncertainty: fakes and charge mis-ID

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ttH and many b-jets

ttH process can be studied in a variety of final states, depending on the Higgs decay

mode and top-quark pair decay topology: γγ, WW/ZZ/ττ (multi-lepton) and bb

–

–

• ttH, H → bb [ATLAS-CONF-2016-080]– –

• Small signal on top of large background dominated by

production of tt+heavy flavour (HF) jets

• Events categorised according to jet and b-jet multiplicities:

6 control (CR) and 3 signal regions (SR) in 1l and 2 CR and 3

SR in 2l channels

• Analysis strategy uses 2-step multivariate technique:

reconstruction BDT and discrimination NN/BDT

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Combination of ttH

Combination of the three presented analysies:

95% CL observed (expected) limit: 3.0 x SM ( 2.1 x SM ) for mH = 125 GeV

Observed (expected) significance: 2.8σ ( 1.8σ )

A small excess is observed in the multilepton final state

Can that be coming from tt+W/Z background mismodelling?

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Measuring tt+W/Z

Search for deviations in the top coupling to gauge bosons: tt + W/Z

1 pb

Measuring tt+W/Z

Search for deviations in the top coupling to gauge bosons: tt + W/Z

• Cut-and-count analysis: 2 same sign leptons, 3

leptons outside Z window, 3 leptons with 2

of them inside the Z window, and 4 leptons

• Fit in 9 signal and 2 control regions

• Observed (expected) significance:

3.9σ (3.4σ) over the B-only hypothesis for ttZ

2.2σ (1.0σ) over the B-only hypothesis for ttW

• No visible excess

number of jets MET [GeV]

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tt+many jets: search for 4tops/VLQ/HBSM

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• Target several signal scenarios with tt+many jets final states:

4-top quarks production (SM, 2UED-RPP, EFT)

Vector-like quarks (VLQ) pair production (TT→HtHt, HtZt,

HtWb, ZtZt, ZtWb)

BSM Higgs: ttH/A (→tt), bbH/A(→tt), tbH+(→tb)

• Orthogonal 1-lepton and 0-lepton channels split by number of

(b)-jets and reclustered jets, effective mass used in signal

discrimination

ATLAS-CONF-2016-104

tt+many jets: search for 4tops/VLQ/HBSM

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• Total of 8 (12) search regions

and 6 (9) validation regions in

1(0)-lepton channel

• regions with ≥ 6 (7) jets are

search regions and with

exactly 5(6) jets are

validation regions

• Different phase-spaces covered

by each channel

• Complex top-quark pair

background model (low to high

signal purity)

ATLAS-CONF-2016-104

Fit

tt+many jets: search for 4tops/VLQ/HBSM

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• Model-independent limits on 2D BR plane with

good coverage obtained from combination

• Nice complimentarity of 0-lepton and 1-lepton

analyses

• Limits on T quark mass reach: 1 – 1.2 TeV

ATLAS-CONF-2016-104

• Extended Higgs sector in 2HDM model, H/A to top-quarks pair decay is

dominant for mH/A > 2mtop

• Type-II 2HDM model:

probing masses from 400 GeV to 1 TeV

• Sensitivity in HBSM ttH(tt): excluding tan β < 0.2

• First limits on bbH(tt)

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Conclusions

Recent results from the ATLAS Collaboration on searches for new phenomena

through the production of heavy objects in association with a top-quark pair

Pushing the frontiers of complexity in the searches in order to increase the

sensitivity

Extensive experimental program in ATLAS, many new searches continue to provide

stringent constraints on masses, couplings and cross-section in several models

No new physics found yet, but looking forward for new and exciting searches!

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Conclusions

Recent results from the ATLAS Collaboration on searches for new phenomena

through the production of heavy objects in association with a top-quark pair

Pushing the frontiers of complexity in the searches in order to increase the

sensitivity

Extensive experimental program in ATLAS, many new searches continue to provide

stringent constraints on masses, couplings and cross-section in several models

No new physics found yet, but looking forward for new and exciting searches!

Muchas gracias por su atención!

Back-up

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Analysis overview: VLQ/4tops/HBSM

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ATLAS-CONF-2016-104

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100 TeV

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Syst uncert

Simultaneous profile likelihood fit to nine

regions with systematic uncertainties

implemented in the fit as additional terms in the

binned likelihood:

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2HDM

Vector-like quarks

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• Chiral 4th quark generation excluded by Higgs boson

measurements

• Vector-like quarks (VLQ):

spin ½, coloured, charged

LH and RH coupling to charged currents

• Arise in little Higgs, composite Higgs, extra dimensions,

and GUTs models, naturalness would require mQ ≤ 2 TeV

• Production:

• strong pair production (less model dependent)

• single via EW (mixing angles model dependent)

• Decay: usually assume only

coupling to 3rd generation:

VLQ → t/b + Z/H/W