An interference search for new scalar and pseudoscalar states decaying to a top-anti-top quark pair with the ATLAS experiment

A search for heavy pseudoscalar (A) and scalar (H) Higgs bosons decaying into a top-quark pair ($t\bar{t}$) is presented. The search has been conducted using 140~fb$^{-1}$ of proton--proton collision data collected by the ATLAS experiment at the Large Hadron Collider (LHC), with a centre-of-mass energy of $\sqrt{s}=13$~TeV.The analysis accounts for interference effects between the signal process and Standard Model (SM) $t\bar{t}$ production. Final states with exactly one or two electrons or muons are considered. Specific methods for statistical inference in the presence of signal-background interference are presented in the context of the search.\\No significant deviation from the SM prediction is observed.The results of the search are interpreted within the framework of a two-Higgs-doublet model (2HDM) of type II in the alignment limit, in which the pseudoscalar and scalar Higgs bosons are mass-degenerate ($m_A = m_H$), as well as within the hMSSM parameterisation of the minimal supersymmetric extension of the Standard Model.Ratios of the vacuum expectation values of the two Higgs fields, $\tan\beta$, smaller than 3.49 (3.16) are excluded at the 95\% confidence level for $m_A = m_H = 400$\,GeV in the 2HDM (hMSSM).In the 2HDM interpretation, masses up to 1240~GeV are excluded at 95\% confidence level for the lowest tested $\tan\beta$ value of 0.4. In the hMSSM interpretation, masses up to 950~GeV are excluded for $\tan\beta=1.0$. Additionally, exclusion limits on the (pseudo)scalar-top-quark coupling as a function of the (pseudo)scalar mass are provided in the context of a generic model in which the couplings are varied independently of the decay width of the new particle.The LHC and its experiments are scheduled for an extensive upgrade, with the goal of accumulating an integrated luminosity of $3\text{ ab}^{-1}$ at the end of the High-Luminosity LHC programme. An important part of the ATLAS upgrade is the replacement of the current Inner Detector (ID) with a new, all silicon, Inner Tracker (ITk).Dense hadronic environments, encountered in the core of high-\pt jets, are particularly challenging for the reconstruction of charged-particle trajectories (tracks), as they are characterised by a high local density of ionising particles. The energy deposits (clusters) left by these particles in the silicon sensors are likely to merge and form clusters with contributions from multiple particles. This negatively affects the track reconstruction efficiency, and the precision in the evaluation of track properties. A study of the expected performance of clustering and tracking in dense hadronic environments with the ITk is presented. The tracking performance is found to be improved with respect to ID tracking, thanks to the fine granularity of the ITk. The study also highlights the potential gain from dedicated algorithms for merged-cluster identification for the mitigation of the negative effects of cluster merging within the ITk.