Searches for Heavy Resonances: New Scalars and BSM Higgs Decays with ATLAS Run-2 Data

The Standard Model (SM) has been remarkably successful, yet several key questions remain open, including the nature of dark matter, the matter–antimatter asymmetry, and the origin of neutrino masses. Many extensions of the SM predict additional heavy particles, such as neutral scalars (𝐻, 𝑆), pseudoscalars (𝐴), and charged Higgs bosons (𝐻 ± , 𝐻 ±±), as realised for example in Two-Higgs-Doublet Models, Supersymmetry, or hidden-sector Higgs models. We summarise ATLAS Run-2 searches for heavy resonances in these scenarios. Using 140 fb−1 of 𝑝 𝑝 collisions at √ 𝑠 = 13 TeV, we present representative results in four channels: (i) 𝐻 ± → 𝑊±ℎ → ℓ𝜈𝑏𝑏¯, (ii) 𝐻 ± → 𝜏𝜈, (iii) exotic decays 𝑆 → 𝑍𝑑 𝑍𝑑 → 4ℓ, and (iv) model-independent dijet resonance searches using weakly supervised anomaly detection. No significant excess is observed. Limits are set in benchmark frameworks (2HDM, hMSSM/MSSM, HAHM) and on generic resonance cross sections, with notable sensitivity gains driven by advanced machine-learning techniques.