Probing four-top-quark production with the ATLAS detector

ATLAS: Probing four-top-quark production with the ATLAS detector

Anıl Sonay

XIV CPAN DAYS, November 23-25, 2022, Bilbao

Introduction

Anil Sonay 2 / 13 CPAN 2022

❖ Four top production: pp→tt̄tt̄ , an extremely rare and theoretically challenging process

❖ σ^NLO (tttt) = 12.0+/-2.4 fb at NLO QCD+EW at 13 TeV arXiv:1711.02116 [hep-ph]

❖ Sensitive to the top-Higgs Yukawa coupling

❖ Extremely high energy scale production makes it naturally sensitive to many BSM models, e.g.

➢ Gluino pair production in SUSY

➢ Heavy pseudoscalar/scalar boson in 2HDM

➢ Four-fermion contact interaction within EFT

❖ Signatures:

➢ 2ℓSS/3ℓ: 2ℓSS (7%) / 3ℓ (5%)

■ Low branching fraction, but small background (ttW, ttZ, non-prompt leptons, charge

misidentification) Eur. Phys. J. C (2020) 80:1085

➢ 1ℓ/2ℓOS: 1ℓ (42%) / 2ℓOS (14%)

■ Dominant branching fraction, but large irreducible background from tt+jets JHEP 11 (2021) 118

Top Quark Production in ATLAS

ATL-PHYS-PUB-2021-014 Top pair

production

4 top

❖ The main background is top pair production

❖ Four top quark production highly suppressed by the main background

➢ Five orders of magnitude

❖ MVA strategy used for the signal background

discrimination

2ℓSS / 3ℓ

Anil Sonay 4 / 13 CPAN 2022

❖ Process characterised by leptonic signatures creating a cleaner signal

➢ Main challenge is handling of dominant tt̄W+jets background

■ Mismodelled at high jet multiplicities

■ Dedicated control region (CR) for proper tt̄W+jets estimation

➢ Further CRs for Fake/Non-prompt lepton background estimations

➢ Data-driven charge misassignment background estimation

➢ One inclusive signal region (SR) with a BDT distribution

First Evidence in ATLAS

❖ First evidence of tt̄tt̄ production in SSML at ATLAS, observed (expected) significance 4.3(2.4)σ

1ℓ/2ℓOS

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❖ Event signature characterized by one or two OS leptons and high jet multiplicity and b-jet multiplicities

❖ Main background: tt̄+jets (incl heavy-flavor) production, which is challenging to model accurately by the MC simulation

❖ Signal extracted via profile likelihood fit across all event categories including for a complex model of systematic uncertainties

➢ Simultaneous fit performed on the scalar sum of the pT at control region and the BDT on the SR

1ℓ/2ℓOS Results

❖ Good agreement obtained between data and post-fit prediction

❖ Dominant uncertainties on signal strength from tt̄tt̄ theoretical cross-section and modelling, and from tt̄+>=1b modelling.

❖ The extracted signal strength is:

Combined Results

Anil Sonay 8 / 13 CPAN 2022

❖ Measurements in 1ℓ/2ℓOS and 2ℓSS/3ℓ channels found to be in good agreement

➢ Combine for improved sensitivity

❖ The combined four-top-quark production cross section is measured to be:

❖ with a corresponding observed (expected) signal significance of 4.7 (2.6) standard deviations over background-only hypothesis

❖ It is consistent within 2.0 standard deviations with the Standard Model expectation:

Beyond the Standard Model – Heavy Higgs Search

❖ Search for a new heavy scalar or pseudo-scalar Higgs boson (𝐻/𝐴) in association with a pair of top quarks

❖ Targeting 2HDM type-I/II tt̄H/A → tt̄tt̄ signal

➢ interpretation on low tanβ region (where A/H → tt̄

dominates)

➢ alignment limit sin(β − α) → 1 for which the lighter Higgs boson is SM like

➢ Mass range 400 GeV to 1 TeV

❖ Same-sign dilepton and multilepton (2ℓSS / 3ℓ) final states is explored

➢ Exploration in the 1ℓ/2ℓOS final state is ongoing

❖ A SM four-top like analysis performed due to similar signature using full Run 2 dataset (139 fb^-1)

❖ Two Boosted Decision Tree (BDT) models using XGBoost

Background Modeling

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❖ Template fit method to estimate fake backgrounds and t t̄W QCD with normalization factors from the fit to real data in 5 dedicated CRs using shape from MC

❖ tt̄W validation region to check tt̄W modeling:

N₊−N₋ to remove charge symmetric processes with

≥ 4 jet and ≥ 2 b-jet

Validation region Control region

Signal Discrimination

❖ SM BDT used to extract tt̄tt̄ -like events and to define the BSM SR

❖ BSM mass-parameterized BDT (BSM pBDT): discriminate BSM tt̄tt̄ from the SM backgrounds (important input: H_T and SM BDT)

➢ BSM pBDT trained in the baseline SR but reweighing on the background fractions to mimic BSM SR yields in the training

➢ Baseline SR split into two fitted regions with two BDTs sequentially

■ BSM SR: Additional BSM pBDT as Data / Bkg. discrimination

■ CR low BDT: Use SM BDT to control the background modeling

Results

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❖ No excess of events above the SM expectation is observed

❖ Four times stronger expected upper limits at 95% CL on cross-section times branching ratio than previous ATLAS 36 fb^-1 search (JHEP12(2018)039) with improved multivariate technique and larger dataset

❖ The exclusion on tanbeta ranges from 1.6 to 0.6, depending on the assumed mass, when both H and A are considered

Summary and Prospects of the Four-Top-Quark Production

❖ The SM four-top-quark production has been measured to be 25 fb with a significance of 4.7 standard deviations

❖ The potential BSM scenarios under investigation

➢ The heavy higgs search via four-top-quark production with the 2ℓSS / 3ℓ final state preprint available recently

■ No significant excess on the SMtop cross-section

➢ The study for 1ℓ/2ℓOS final state ongoing

❖ ATLAS Extrapolation relies on knowledge of ATLAS Run2 measurements of tt̄tt̄ in 2ℓSS / 3ℓ (and 1ℓ/2ℓOS) decay channels

➢ Two different syst. assumptions:

■ Conservative

ATL-PHYS-PUB-2022-004

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