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Search for four-top-quark production in final states with one charged lepton

and multiple jets using √s = 13 TeV data collected with the ATLAS detector at the LHC

Leonid Serkin

INFN Gruppo Collegato di Udine and ICTP Trieste, Italy

1. Motivation

• LHC: unique window to search for BSM signatures • Signal signatureRepresentative tree-level SM Feynman diagrams

2 Analysis strategy

3. Systematic uncertainties

4. Results

• Classification of events into 18 independent topologies

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References

[4] ATLAS Collaboration: ATLAS-CONF-2016-020 (2016).

• No significant excess of events observed [4]

5. Limits

[1] C. Degrande et al., Non-resonant new physics in top pair production at hadron colliders, JHEP 1103 (2011) 125.

[2] G. Cacciapaglia et al., Four tops on the real projective plane at LHC, JHEP 1110 (2011) 042.

final state topology given by the

decays of each W-boson (t → Wb)

use largest branching fraction final

state with a single electron or muon

up to 10 jets and up to 4 b-tagged jets

focus on the heaviest known elementary particle: top quark

search for four-top-quark production with SM kinematics

test BSM models: four-top-quark production via Contact Interaction [1]

and from universal extra-dimensions scenarios (2UED/RPP) [2]

target resolved topologies in single lepton decay channel

exploit high jet multiplicity and large total sum of jet pT

• Background modelling main background: top-quark pair production in association with additional

jets, categorised into tt + light jets, tt + cc and tt + bb events, the latter

improved via NLO reweighting [3]

Small contributions from tt + V/H, W/Z + jets, single top and diboson

[3] F. Cascioli et al., NLO matching for ttbb production with massive b-quarks, Phys. Lett. B 734 (2014) 210.

Simultaneous binned profile likelihood fit in 6 control and 3 signal regions

leads to significant reduction in overall background uncertainty

9 regions validate the assumption that the tt + jets modelling extracted from

control regions can be extrapolated to signal regions

N ≡ normalisation, SN ≡ shape and normalisation

• Using 3.2 fb-1 of pp data at √s = 13 TeV

SM tttt production: obs (exp) 95% CL upper limit on the SM production

cross-section of 190 fb (143 fb)

SM tttt cross-section ≈ 9.2 fb

four-top-quarks production via contact

interaction: upper limit of 148 fb (115 fb)

tttt via 2UED/RPP:

limit on mKK of

1.28 TeV (1.30 TeV)

Event display of the most energetic event in signal region

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Impact of the nuisance parameters on signal strength

S/B and S/√B values for each of the regions Fractional contributions of the various backgrounds in each region

After fitting of the nuisance parameters to data under the background-only hypothesis

Post-fit

Pre-fit