The discovery of the Higgs boson by the ATLAS and CMS collaborations in 2012 was crucial for unravelling the mysteries of the Higgs field and its potential. Understanding the shape of the Higgs potential provides crucial information about the long-term stability of the Universe. Through careful study of the Higgs boson's properties, in particular its self-interaction, ATLAS physicists are gaining new insights into the Higgs potential.

The Higgs boson is one of the results of the “spontaneous symmetry breaking” of the Higgs field. Depending on how this symmetry breaking occurs, additional Higgs bosons could exist. These extra bosons may interact with each other and with the Standard-Model Higgs boson. Their existence would mean that the shape of the Higgs potential is different than originally assumed, and could explain the matter-antimatter imbalance of the Universe.

The ATLAS Collaboration has just published a search for two new Higgs bosons, X and S, that would interact with the Standard-Model Higgs boson (H). The signal is characterised by the resonant production of the heavy X boson, which decays into the lighter S and H bosons. The S boson is assumed to decay to b-quarks, whereas the H boson decay to photons is used. The invariant masses of these decay products can therefore be used to reconstruct the masses of the respective bosons. This final state exploits the clean di-photon mass resonance to separate all signals from background. It also has the highest sensitivity to light X and S bosons.

As physicists don’t know the masses of these possible new Higgs bosons, they needed to conduct a wide search for possible X and S masses (m_X and m_S). This made their search especially challenging, as researchers had to scan a large mass range with fine enough granularity as to avoid any sensitivity gaps, but not too fine as to unnecessarily increase the “look-elsewhere effect”. This effect takes into account the larger probability that an excess is produced by a statistical fluctuation by looking at not one but many points.

To overcome this challenge, physicists trained a parametrized neural network (PNN) to differentiate signal from background for a range of X and S masses. As shown in Figure 1, by specifying the targeted X and S masses, the PNN is able to separate a signal with those characteristics from background processes. This technique not only allowed researchers to probe a range of X and S masses with fine granularity, ensuring no signal was missed, but also enabled a clear understanding of the masses of the new bosons if found.

A total of 359 (m_X, m_S) pairs are probed, and an excess is found for a signal where mX is 575 GeV and mS is 200 GeV, with a local significance of 3.5 standard deviations (in yellow), as depicted in Figure 2. The global significance, which takes into account the look-elsewhere effect of all 359 points, reduces the excess to 2.0 standard deviations. This excess is consistent with the Standard Model, and researchers set upper limits on possible masses of the X and S bosons.

Looking towards the future, the models investigated in this analysis remain promising avenues for uncovering new physics beyond the Standard Model. The data collected during LHC Run 3 and the future operation of the High-Luminosity LHC will shed more light on what this paper has started to reveal.

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