# Search for new phenomena in a lepton plus high jet multiplicity final state with the ATLAS experiment using $\sqrt{s}$ = 13 Tev proton-proton collision data

## TL;DR

This paper reports a search for new physics in events with high jet multiplicity and an isolated lepton using 13 TeV proton-proton collision data from ATLAS, setting limits on supersymmetry models and Standard Model processes.

## Contribution

It introduces a novel search strategy for high jet multiplicity final states and sets new exclusion limits on R-parity-violating supersymmetry models and Standard Model four-top production.

## Key findings

- No significant excess over the Standard Model was observed.
- Exclusion limits reach up to 2.1 TeV for gluino mass and 1.2 TeV for top-squark mass.
- An upper limit of 60 fb is set on Standard Model four-top production cross-section.

## Abstract

A search for new phenomena in final states characterized by high jet multiplicity, an isolated lepton (electron or muon) and either zero or at least three $b$-tagged jets is presented. The search uses 36.1 fb$^{-1}$ of $\sqrt{s}$ = 13 TeV proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider in 2015 and 2016. The dominant sources of background are estimated using parameterized extrapolations, based on observables at medium jet multiplicity, to predict the $b$-tagged jet multiplicity distribution at the higher jet multiplicities used in the search. No significant excess over the Standard Model expectation is observed and 95% confidence-level limits are extracted constraining four simplified models of $R$-parity-violating supersymmetry that feature either gluino or top-squark pair production. The exclusion limits reach as high as 2.1 TeV in gluino mass and 1.2 TeV in top-squark mass in the models considered. In addition, an upper limit is set on the cross-section for Standard Model $t\bar{t}t\bar{t}$ production of 60 fb (6.5 $\times$ the Standard Model prediction) at 95% confidence level. Finally, model-independent limits are set on the contribution from new phenomena to the signal-region yields.