Search for heavy particles decaying into top-quark pairs using lepton-plus-jets events in proton–proton collisions at $$\sqrt{s} = 13$$ s = 13   $$\text {TeV}$$ TeV with the ATLAS detector

Abstract

A search for new heavy particles that decay into top-quark pairs is performed using data collected from proton–proton collisions at a centre-of-mass energy of 13 by the ATLAS detector at the Large Hadron Collider. The integrated luminosity of the data sample is 36.1 fb. Events consistent with top-quark pair production are selected by requiring a single isolated charged lepton, missing transverse momentum and jet activity compatible with a hadronic top-quark decay. Jets identified as likely to contain b-hadrons are required to reduce the background from other Standard Model processes. The invariant mass spectrum of the candidate top-quark pairs is examined for local excesses above the background expectation. No significant deviations from the Standard Model predictions are found. Exclusion limits are set on the production cross-section times branching ratio for hypothetical bosons, Kaluza–Kein gluons and Kaluza–Klein gravitons that decay into top-quark pairs.

LHC bounds on colored scalars

“…In the event generation we have used the Particle Distribution Functions (PDF) set NNPDF23_nlo_ as_0119 of the package LHAPDF_6.1.6 [79]. The centre-of-mass energy has been fixed to √ s = 13 TeV, while the experimental data had an integrated luminosity of 36.1 fb −1 for the ATLAS data [72][73][74] and 137 fb −1 for the CMS data [75]. Furthermore, we have estimated the theoretical uncertainties varying the values of the renormalisation and factorisation scales by a factor between 1/2 and 2.…”

“…For the single production we have taken the experimental data from the ATLAS search for Z bosons and Kaluza-Klein gravitons and gluons, in Ref. [72]. The interference with the SM production amplitudes has not been considered in those searches, although it could have some effects on the signal shape for scalars and pseudoscalars decaying to top quarks [80].…”

“…Note that in order to compare our theoretical predictions with the experimental data we make use of specific analyses performed by the ATLAS and CMS collaborations. In the case of the single production, we compare our predictions with the experimental limits found for Z particles and Kaluza-Klein gluons and gravitons [72]. For the associated production, we exploit the limits obtained for the type-II two-Higgs-doublet model (2HDM) in Ref.…”

“…[70], where the single and pair production of neutral scalars was studied. The recent release of high-luminosity experimental searches for massive resonances decaying into heavy quarks [72][73][74][75] makes now possible to improve those bounds. In this work we focus on the associated production of neutral and charged scalars with heavy quarks that was not considered in previous analyses.…”

Adversarially-trained autoencoders for robust unsupervised new physics searches

“…This selection allows one to study the impact of the spin and colour quantum numbers of the resonances on the classification efficiencies † . The resonances are assumed to subsequently decay into top quarks [62][63][64][65]. Top quark samples are an ideal playground for anomaly detection, as they can be purified to a very high degree, i.e.…”