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

The collaboration
CERN-EP-2022-176, 2023.

Abstract (data 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.

• #### 95% CL upper limits on $t\rightarrow uX$ BR

Tabulated data from Figure 9a in the publication

10.17182/hepdata.132907.v1/t1

Expected and observed 95% CL upper limits for $\mathcal{B}$($t \rightarrow uX$) $\times$ $\mathcal{B}$($X \rightarrow b\bar{b}$). The bands surrounding the expected...

• #### 95% CL upper limits on $t\rightarrow cX$ BR

Tabulated data from Figure 9b in the publication

10.17182/hepdata.132907.v1/t2

Expected and observed 95% CL upper limits for $\mathcal{B}$($t \rightarrow cX$) $\times$ $\mathcal{B}$($X \rightarrow b\bar{b}$). The bands surrounding the expected...

• #### 95% CL upper limits on $t\rightarrow qH$ BR

10.17182/hepdata.132907.v1/t3

Expected and observed 95% CL upper limits for $\mathcal{B}$($t \rightarrow uH$) $\times$ $\mathcal{B}$($X \rightarrow b\bar{b}$) and $\mathcal{B}$($t \rightarrow cH$) $\times$...

• #### $t\rightarrow uX$ post-fit event yields

Table 5 from the publication

10.17182/hepdata.132907.v1/t4

Event yields of the signal and SM background processes in the six analysis regions after the fit to the data...

• #### $t\rightarrow cX$ post-fit event yields

Table 5 from the publication

10.17182/hepdata.132907.v1/t5

Event yields of the signal and SM background processes in the six analysis regions after the fit to the data...

• #### Acceptance

Auxiliary Table 1 from the publication

10.17182/hepdata.132907.v1/t6

Event acceptance times efficiency in percent for every $t\rightarrow uX$ and $t\rightarrow cX$ mass signal sample.

• #### $t\rightarrow uX$ cutflow

Auxiliary Table 3 from the publication

10.17182/hepdata.132907.v1/t7

Cut flow for the scalar signal in the $t\rightarrow uX$ decay combining both quark and anti-quark samples. Shown for each...

• #### $t\rightarrow cX$ cutflow

Auxiliary Table 4 from the publication

10.17182/hepdata.132907.v1/t8

Cut flow for the scalar signal in the $t\rightarrow cX$ decay combining both quark and anti-quark samples. Shown for each...