The Era of Higgs Physics

Standard model or not, the Higgs itself revolutionized the field. “There was a blossoming of new ideas, and the landscape of particle physics completely changed,” says Kado. Carena agrees: “After you discover the Higgs, the first question you have is, ‘Well, why not more of that kind?’” Many theorists have wondered about models with multiple Higgs particles, possibly hiding at higher energies, or even entire sectors connected to the Higgs.

Models with Higgs portals—connections to a coterie of extremely feebly interacting particles coupling only to the Higgs—have flourished because of their ability to explain dark matter and other outstanding issues with the standard model. Supersymmetric (SUSY) theories—which propose a new symmetry between bosons and fermions—still have a place in the theorist’s toolbox, but because of a lack of evidence, they are no longer predominant. Instead, theorists work more closely with experimental results. “There are a lot of inputs we have gotten from experimental data that have constrained the way we build models,” Carena says.


Photo of the Compact Muon Solenoid (CMS) detector. As its name suggests, it is compact—half as large as ATLAS but nearly twice its weight.

New ideas from theorists have also shaped the experimental program. This fruitful interplay is perhaps best encapsulated by the effective field theory (EFT) approach at the LHC. Using the EFT approach, theorists can guide experimentalists to make precision measurements sensitive to undiscovered particles. For example, the decay of a Higgs to a Z boson and a photon is mediated by contributions from all particles. If the decay rate deviates from predictions, that could be a sign of an unknown heavy particle.

Even if a Higgs decay matches standard model predictions, it can still provide valuable information. Some alternative Higgs models, Wardle points out, predict that the Higgs boson couples to leptons, such as the electron, differently than to quarks. As such, determining all the coupling strengths and other Higgs parameters isn’t stamp collecting—as some derisively call such measurements—but a vital effort to put limits on theories, he says.

Run 3 will begin in earnest on July 5, 2022, the day after the 10th anniversary of the Higgs discovery, promising particle physicists a new tranche of data to pore over. Wardle hopes data will allow CMS to reach five sigma and declare a discovery for the Higgs decay to two muons. LHCb (one of the more-specialized LHC experiments) could play a role as well by probing the Higgs coupling to charm quarks. The search for beyond-the-standard-model Higgs properties is “now one of the main pillars of the CMS search program,” says Jan Steggemann, a colleader of Higgs physics for the CMS Collaboration. Carena is particularly interested in the Higgs’ self-coupling, which the High Luminosity LHC—an upgrade planned for 2029—will be able to measure via two-Higgs production.

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