One Higgs boson found – could there be more?

24 March 2022 | By

As decades of experiments have demonstrated, the Standard Model provides an impressively accurate description of fundamental particles and their interactions. One great example is the Higgs boson. The Standard Model predicts the existence of only one neutral scalar (with spin 0) Higgs boson, which perfectly describes the particle discovered in 2012 at the LHC.

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

Nevertheless, there are no fundamental principles dictating that only one elementary scalar particle can exist. Many theories suggest that the 2012 Higgs boson is but the first to be observed from a larger Higgs family. This extended Higgs sector could contain charged and even doubly-charged Higgs bosons. The discovery of any of these particles would be clear evidence of physics beyond the Standard Model.

In a new analysis, ATLAS physicists searched for the presence of a singly-charged Higgs boson. According to the theory being probed, this new particle can interact solely with W and Z bosons. Its characteristic production and decay is shown the diagram shown in Figure 1. The parameter sin(θH), which arises from a particular extension of the Standard Model called the Georgi-Machacek model, serves as a measure of the strength of a new, hypothetical Higgs field relative to the Standard Model.

Since charged leptons (electrons and muons) are well measured by the ATLAS experiment, the new search focused on instances where the W and Z bosons decay leptonically: the W boson to a lepton and neutrino, and the Z boson to two leptons. This would leave an experimental signal with two forward particle “jets”, three charged leptons (two of which have a mass compatible with the Z boson), and missing transverse energy (from the neutrino). Though other Standard-Model processes also decay in this way, the angular and energy distributions of the measured particles would be different.

The Higgs boson discovered in 2012 by the ATLAS Experiment could be but the first to be observed from a larger Higgs family, containing charged or even doubly-charged Higgs bosons.

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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 select events most likely originating from a charged Higgs-boson decay. They then compared experimental data to the expected Standard-Model background and to hypothetical signals from a charged Higgs boson. The Georgi-Machacek model doesn’t predict the mass of any of the extra Higgs bosons but, if a signal exists, it would appear as a concentrated excess of events at a given invariant mass. Researchers scanned the entire WZ mass distribution (see Figure 2) and no significant excess was found. Instead, they set limits on the strength of a potential signal, and hence on the parameter sin(θH), as a function of mass (see Figure 3).

But what about doubly-charged Higgs bosons? Or any of the other neutral Higgs bosons predicted by the Georgi-Machacek model? If these exist in nature, their presence will not only be seen in WZ decays, but in other final states as well. ATLAS physicists are actively looking for such excesses and Figure 3 shows the new WZ channel limits on the parameter sin(θH) alongside those from other channels. Specifically, the diboson channel decaying in the semileptonic final state, a pair of Z bosons and the multilepton channel. Currently, the limits obtained by the WZ fully-leptonic channel are the most stringent.

Aside from new Higgs bosons, ATLAS researchers also explored new-physics scenarios that predict the existence of an additional vector particle (with spin 1). Such a new heavy particle (W') might be produced through vector-boson fusion, similar to the charged Higgs boson production shown in Figure 1. Researchers carefully considered all the possible backgrounds that could mimic such a signal, and were able to set new limits on the signal strength of this heavy vector particle. This result provides the first limits on a W’ produced via vector-boson fusion.

ATLAS researchers now look forward to collecting and analysing Run 3 data. As the detector records more data, the sensitivity of the search for potential new particles will continue to grow!

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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)

About the event & Video Briefing

About the event display: Display of a candidate event passing all the vector-boson-fusion signal region selection. The WZ pair in the event decay to a pair of muons (orange lines), an electron (red line), and a neutrino, seen as missing transverse energy (purple dashed line). (Image: ATLAS Collaboration/CERN)