Search for a top-philic heavy resonance in association with top quarks in $pp$ collisions at $\sqrt{s} = $ 13 TeV and 13.6 TeV with the ATLAS detector

Abstract

A search for a new top-philic vector boson ($Z'$) produced in association with a top-quark or a pair of top quarks, and decaying into a top quark pair is presented. The search uses $pp$ collision data collected by the ATLAS detector at the Large Hadron Collider at centre-of-mass energies of $\sqrt{s} =$ 13 TeV and 13.6 TeV, corresponding to integrated luminosities of 140 fb$^{-1}$ and 56 fb$^{-1}$, respectively. Events containing exactly two same-sign leptons or at least three leptons (electrons or muons) are selected. A transformer-based classifier parameterised as a function of the $Z'$ mass is used to optimise the signal-to-background discrimination. The results are interpreted in a simplified model with a colour-singlet vector boson coupled to top quarks with coupling strength $c_t$ and chirality angle $θ$. No significant excess is observed above the expected Standard Model background. A $Z'$ boson with mass below 1150 GeV is excluded at 95% confidence level for a benchmark scenario with $c_t = 1$ and $θ= 0$, with an expected limit of 1250 GeV.