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The Standard Model, as decades of tests have shown, gives an astoundingly precise explanation of basic particles and their interactions. The Higgs boson is a prime example. The Standard Model predicts exactly one neutral scalar (spin 0) Higgs boson, which accurately characterizes the particle found at the LHC in 2012.

Nonetheless, there are no basic rules stating that there can only be one elementary scalar particle. Many theories contend that the 2012 Higgs boson is only the first of a larger Higgs family to be discovered. Charged and even doubly-charged Higgs bosons might exist in this expanded Higgs sector. Any of these particles’ discovery would be conclusive proof of physics outside the Standard Model.

ATLAS researchers sought for the existence of a singly-charged Higgs boson in a recent study. According to the hypothesis being investigated, this new particle can only interact with W and Z bosons. Figure 1 depicts the flowchart of its distinctive creation and decline. The parameter sin(H), which derives from a specific extension of the Standard Model known as the Georgi-Machacek model, serves as a measure of the strength of a new, hypothetical Higgs field in comparison to the Standard Model.

Figure 1: Diagram of the singly-charged Higgs boson production and decay. (Image: ATLAS Collaboration/CERN)

Because charged leptons (electrons and muons) are well observed by the ATLAS experiment, the new search concentrated on cases in which the W and Z bosons decay leptonically: the W boson to a lepton and neutrino, and the Z boson to two leptons. This would result in an experimental signal including two forward particle “jets,” three charged leptons (two of which have masses consistent with the Z boson), and no transverse energy (from the neutrino). Although additional Standard-Model processes decay in similar manner, the angular and energy distributions of the detected particles would vary.

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Figure 2: The measured invariant mass of the reconstructed WZ pair (black points), the Standard Model backgrounds (coloured bars), and two hypothetical signals with masses of 375 GeV (red dash) and 600 GeV (blue dash), displayed for illustration. (Image: ATLAS Collaboration/CERN)

ATLAS researchers used artificial intelligence techniques to identify events that were most likely caused by a charged Higgs-boson decay. The experimental data was then compared to the predicted Standard-Model background as well as potential signs from a charged Higgs boson. The Georgi-Machacek model does not predict the mass of any of the additional Higgs bosons, but if one exists, it would emerge as a concentrated excess of events with a certain invariant mass. The researchers examined the full WZ mass distribution (see Figure 2) and discovered no substantial excess. Instead, they restrict the intensity of a potential signal, and therefore the parameter sin(H) as a function of mass (see Figure 3).

Figure 3: Limits on the strength of a potential signal, and hence on the Georgi-Machacek model parameter sin(θH), as a function of mass. (Image: ATLAS Collaboration/CERN)

The Higgs boson discovered by the ATLAS Experiment in 2012 may be the first of a larger Higgs family containing charged or even doubly-charged Higgs bosons.

But what about Higgs bosons that are double charged? Or any of the other Higgs boson neutrals predicted by the Georgi-Machacek model? If they exist in nature, their existence will be detected not just in WZ decays, but also in other final states. ATLAS scientists are actively seeking for such excesses, as seen in Figure 3 by the new WZ channel limits on the parameter sin(H), which are shown alongside those from other channels. The diboson channel decaying in the semileptonic final state, a pair of Z bosons, and the multilepton channel, to name a few. The restrictions reached by the WZ fully-leptonic channel are now the most restrictive.

Aside discovering new Higgs bosons, ATLAS researchers investigated novel-physics scenarios that indicate the creation of a new vector particle (with spin 1). Similar to the charged Higgs boson synthesis seen in Figure 1, such a new heavy particle (W’) might be formed by vector-boson fusion. Researchers carefully analyzed all of the different backgrounds that may replicate such a signal and were able to establish new limitations on the signal intensity of this heavy vector particle. This study establishes the first bounds for a W’ formed via vector-boson fusion.

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ATLAS researchers are now looking forward to gathering and analyzing data from Run 3. The sensitivity of the search for possible new particles will increase as the detector collects more data!

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