The discovery of the Higgs boson in 2012 by the ATLAS and CMS Collaborations at the Large Hadron Collider (LHC) came with pride and prejudice. Pride that a new dynamical principle, envisioned by theorists 60 years ago, found its incarnation in Nature. Prejudice that the Higgs sector may in fact be more complex than predicted by the Standard Model (SM). Many theories (e.g., Supersymmetry) suggest that the Higgs boson is but the first to be observed from a larger Higgs family, whose discovery would signal the breakdown of the SM and trigger a new revolution in particle physics.
Over the years many searches for extra Higgs bosons have been performed, each time coming out empty handed. However, there is a chance these searches were misguided by theoretical prejudice, such as e.g., that a heavy neutral Higgs boson preferentially couples to a top-antitop-quark pair. If instead it coupled to two particles of different flavour, e.g., a top-anticharm-quark pair, the phenomenology would be totally different, opening new opportunities for discovery [1]. On July 2022, the ATLAS Collaboration reported the results of the first-ever search for such a neutral Higgs boson [2], probing its main production and decay modes, giving two, three, or four top quarks in the final state (Fig. 1). The search targeted events with two same-charge leptons (electrons or muons), three leptons, and at least four leptons. An excess with a significance of up to 2.8 standard deviations (s.d.) was found in events with two positively charged leptons (Fig. 2), compatible with the production of a heavy neutral Higgs boson with a sizable coupling to top and up quarks.
Since 2015, researchers at IFAE, under A. Juste’s leadership, are playing a major role in the program of searches for an extended Higgs sector in ATLAS. The team has not only performed the search discussed here, but 1.5 years ago completed a search for a flavour violating charged Higgs boson, which found a 3 s.d. excess [3]. Additional data from the LHC Run 3 (2022-2025), along with further analysis improvements, will allow drawing definite conclusions on whether the heavier siblings of the Higgs boson have finally been found.
