No new particles have been found at the Large Hadron Collider since the Higgs boson in 2012, but physicists say there’s much we can still learn from the Higgs itself.
With the LHC, most of the Higgs boson’s couplings with other Standard Model particles have been measured with roughly 20 percent precision, but a future collider, by producing many more Higgs bosons, could pin the numbers down with an accuracy of 1 percent. This would give physicists a much better sense of whether the probabilities add to one, or whether Higgs bosons are occasionally decaying into hidden particles. Extra particles coupled to the Higgs appear in many theories of physics beyond the Standard Model, including the “twin Higgs” and “relaxion” models. “Unfortunately, there are so many models and so many parameters that there is no hope of a no-lose theorem,” said the particle physicist Matt Strassler — “just a might-win opportunity.”
Perhaps the most important coupling that physicists want to nail down is called the triple Higgs coupling — essentially the strength of the Higgs boson’s interaction with itself. This number is measured by counting rare events, not yet seen at the LHC, in which a Higgs boson decays into two of itself. The Standard Model makes a prediction for the value of the triple Higgs coupling, so any measured deviations from this prediction would signify the existence of new particles not included in the Standard Model that affect the Higgs.
Measuring the triple Higgs coupling would also reveal the shape of the mathematical curve that defines the Higgs field’s different possible values, helping to determine whether the vacuum of our universe is stable or only metastable — settled in a local rather than a global minimum of the curve. If the Standard Model’s prediction for the coupling is correct, then the universe is metastable, destined to decay billions or trillions of years from now. This is nothing to worry about, but rather an important clue about the larger story of our cosmos. The ability to reveal the universe’s fate is why the triple Higgs coupling “is at the heart of the experimental program at the future colliders,” said Cédric Weiland, a particle physicist at the University of Pittsburgh who has studied this coupling.
With a Higgs factory, Weiland said, physicists could measure the triple Higgs coupling with a precision of 44 percent. The second-phase proton-proton collider could nail its value to within 5 percent.
The baseline expectation is that measurements at a future collider will simply confirm the Standard Model, which seems frustratingly unbreakable even as it gives an incomplete account of the physical universe. Some physicists balk at the prospect of investing billions of dollars in a machine that might simply add more decimal places of precision to our knowledge of an existing set of equations.
Physicists and funding agencies will actively debate the value of an LHC successor over the next few years. Whether to spend 20 years and as many billions of dollars constructing a 100-kilometer-circumference collider hinges on its discovery potential. Past colliders struck upon the puzzle pieces of the Standard Model one by one. But with that puzzle complete, there’s no guarantee that a future machine will find anything new, leaving physicists with a dilemma: to build or not to build?