Search for a new scalar resonance in flavour-changing neutral-current top-quark decays $t \rightarrow qX$ ($q=u,c$), with $X \rightarrow b\bar{b}$, in proton-proton collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

TL;DR

This paper reports a search for rare top-quark decays involving a new scalar particle decaying into bottom quarks, using ATLAS data to set upper limits on the decay probabilities at 13 TeV.

Contribution

First search for flavor-changing neutral-current top decays to a scalar X with X to b-bbar, setting new upper limits on branching fractions at 13 TeV.

Findings

Upper limits on $ ext{Br}(t ightarrow uX)$ between 0.019% and 0.062%.

Upper limits on $ ext{Br}(t ightarrow cX)$ between 0.018% and 0.078%.

Analysis of 139 fb$^{-1}$ of ATLAS data at 13 TeV.

Abstract

A search for flavour-changing neutral-current decays of a top quark into an up-type quark (either up or charm) and a light scalar particle $X$ decaying into a bottom anti-bottom quark pair is presented. The search focuses on top-quark pair production where one top quark decays to $qX$, with $X \rightarrow b\bar{b}$, and the other top quark decays according to the Standard Model, with the $W$ boson decaying leptonically. The final state is thus characterised by an isolated electron or muon and at least four jets. Events are categorised according to the multiplicity of jets and jets tagged as originating from $b$-quarks, and a neural network is used to discriminate between signal and background processes. The data analysed correspond to 139 fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector at the LHC. The 95% confidence-level upper limits between 0.019% and 0.062% are derived for the branching fraction $\mathcal{B}$($t \rightarrow uX$) and between 0.018% and 0.078% for the branching fraction $\mathcal{B}$($t \rightarrow cX$), for masses of the scalar particle $X$ between 20 and 160 GeV.