For particle physicists eager to explore new frontiers, spotting the Higgs boson has become a bittersweet triumph. Detected in 2012 at the world's biggest atom smasher, the Large Hadron Collider (LHC), the long-sought particle filled the last gap in the standard model of fundamental particles and forces. But since then, the standard model has stood up to every test, yielding no hints of new physics. Now, the Higgs itself may offer a way out of the impasse. Experimenters at the LHC, located at CERN, the European particle physics laboratory near Geneva, Switzerland, plan to hunt for collisions that produce not just one Higgs boson, but two. Finding more of these rare double-Higgs events than expected could point to particles or forces beyond the standard model and might even help explain the imbalance of matter and antimatter in the universe.
"This is the next big thing," says Sally Dawson, a theorist at Brookhaven National Laboratory in Upton, New York, and an organizer of a workshop last week at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, where more than 100 physicists gathered to hone the conceptual tools needed for the long search.
The Higgs boson plays a special role in the standard model, which describes how a dozen types of particles interact through three forces: electromagnetism and the weak and strong nuclear forces. (The theory does not include gravity, a major failing.) The forces in the model arise from certain mathematical symmetries. But that math works only so long as the particles do not start out with mass. So mass must somehow emerge through interactions among the otherwise massless particles themselves.
That's where the Higgs comes in. Physicists assume that space contains a Higgs field—a bit like an electric field—generated by Higgs bosons lurking in the vacuum. Particles interact with the field to gain energy and, through Albert Einstein's famous equation, E=mc2, mass.
This Higgs mechanism received resounding support 6 years ago, when experimentalists working with the two biggest particle detectors fed by the LHC, A Toroidal LHC Apparatus (ATLAS) and the Compact Muon Solenoid (CMS), blasted into existence a fleeting particle weighing 133 times as much as a proton. It decays in the ways that the Higgs is supposed to—for example, into a pair of photons. But physicists aren't sure whether they've observed the standard model Higgs boson or something subtly different.
Double-Higgs events promise a way to tell for sure, by revealing how strongly the Higgs field interacts with itself. An electric field vanishes if there's no charge around, but the Higgs field must always linger in the vacuum—or else it wouldn't be able to impart mass to other particles. The standard model presumes this happens with a Higgs field that interacts with itself and minimizes its energy not by vanishing, but by taking a nonzero strength.
Mathematically, there are many ways to cook up such a scheme, and the standard model employs the simplest one, controlled by just one parameter. That parameter, in turn, predicts the rate at which Higgs pairs should emerge in particle collisions—giving physicists a way to test the standard model.
The challenge is finding the extremely rare decays. The standard model predicts that for every 10,000 proton-proton collisions at the LHC that produce a single Higgs boson, roughly six will produce a pair. Those double-Higgs events should generate messy showers of other particles, making them even harder to identify. The LHC has already probably produced about 1000 double-Higgs events, but ATLAS and the CMS cannot yet come close to sifting a signal from the background.
However, LHC experimenters say they're optimistic about their chances, especially because their techniques for spotting Higgs bosons are improving. Last month, they announced proof they had detected a particularly messy decay mode in which a Higgs spawns a pair of massive particles called bottom quarks, as it should in nearly 60% of all decays. That bodes well for double-Higgs searches because they rely on at least one Higgs in the pair decaying in this most probable way.
The LHC experiments may need years to see a signal. Later this year, the LHC will idle for 2 years for upgrades. In 2026 it will undergo another 2-year hiatus to boost its collision rate. The so-called High-Luminosity LHC would then run until 2034. On paper, only the full run will yield enough data to validate the standard model prediction. However, some physicists think they can beat that timetable as their Higgs-spotting algorithms continue to improve. "Even before the High-Luminosity LHC, I think we could get close to the standard model prediction," says Caterina Vernieri, a CMS member at Fermilab.
Of course, all LHC experimenters hope the rate for double-Higgs events will exceed the standard model prediction. It cannot be sky high or it would run into indirect constraints from the observed Higgs decays, says Eleni Vryonidou, a theorist at CERN. Still, the double-Higgs rate could be as much as six times as big as the standard model prediction, she estimates.
Such an enhancement would indicate a strongly self-interacting Higgs field. It could also signal fleeting new particles that tend to decay into Higgses, such as heavier Higgs partners predicted by many standard model extensions. And it might have implications far beyond particle physics, says Marcela Carena, a theorist at Fermilab. Physicists don't know why the infant universe ended up with more matter than antimatter. But Carena says the sudden emergence of a strongly self-interacting Higgs field might have locked in the imbalance.
Even if the rate of double-Higgs events doesn't defy standard model predictions, the quest to count them will pay off handsomely, says Katharine Leney, an ATLAS experimentalist at University College London. "What's driving a lot of people, including myself, is being able to say once and for all, by God, it is or isn't the standard model Higgs."