FOR half a century, finding the Higgs boson was top of particle physicists’ to-do lists. Its eventual discovery in 2012 was celebrated as the final piece of the puzzle to complete the “standard model”, our picture of reality at its most fundamental level. The Higgs became famous, a rare household name among elementary particles.
But now, almost a decade on, we still barely know the Higgs boson – and our understanding of the pantheon of particles and forces that makes the universe what it is remains manifestly imperfect. We were hoping that, alongside the Higgs, new particles and forces would reveal unexpected exotic phenomena and bring into focus an even bigger picture. Alas, the Higgs is behaving exactly as expected, undermining a notion that its unseen interactions would help us uncover new physics.
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Is the Higgs as boring as it seems? Possibly not. Closer inspection could expose its true self, and the shadows of strange siblings or exotic “pink elephant” particles, any of which would shake up our understanding of the universe. We need to “get the Higgs on the table, dissect it, prod it, see where it starts to disagree”, says , a particle physicist at the University of Cambridge.
With that in mind, many in the field are now pushing for a new particle collider to churn out Higgs bosons in industrial quantities, so we can interrogate it like never before. But will such a Higgs factory open doors to new physics? Or is the Higgs as mundane as it seems, which might itself tell us something about our ability to understand the universe?
The standard model of particle physics is our best description of all the known particles in the universe and the ways they interact, and it is impressively accurate. Since its formulation in the 1970s, it has been a guiding light for particle physicists. Faith in the notion that there must be elegant mathematical rules governing particles and forces drove the construction of more powerful and more precise particle accelerators, each designed to find particles predicted by the standard model. Time and again, we found them.
In its most simplified form, the standard model comprises an equation with four terms. The first describes three of the known forces in the universe: electromagnetism, and the strong and weak nuclear forces. The second sketches out the elementary particles and how the forces act on them. The two final terms are only just being written now. They largely tell the story of the Higgs – the particle thought to hold the clues to a better understanding of what the standard model is missing.
Peter Higgs and others proposed the Higgs boson’s existence in 1964 to help explain why fundamental particles have such a range of masses, from zero to quite large. The idea is that all of them are submerged in an invisible “Higgs field”, which drags on them to different degrees. This mechanism almost immediately acquired added significance when physicists realised that, at high energies, electromagnetism and the weak nuclear force were merged in one unified “electroweak” force. Particles of light, or photons, which carry the electromagnetic force, are massless, whereas the force carriers of the weak nuclear force, the W and Z bosons, aren’t. The Higgs mechanism explained this asymmetry.
Hence the relief when the Higgs, the last missing particle of the standard model, was finally confirmed in particle collisions at CERN’s Large Hadron Collider near Geneva, Switzerland, in 2012. , declared the front page of The New York Times.
The bigger picture
Even then, however, particle physicists knew that this couldn’t be everything. The standard model can’t account for why there is so much more matter than antimatter in the universe. It doesn’t make room for dark matter, the mysterious stuff that keeps galaxies from flying apart. And it doesn’t describe gravity.
Unifying the quantum world of particle physics with gravity, which is governed by the laws of general relativity, is the next leap towards a full picture of reality. But gravity is bafflingly weaker than all the other forces and doesn’t gel easily with the standard model. In particular, hypothesised particles of quantum gravity aggravate an existing problem: that the Higgs’ interaction with “virtual” particles popping into and out of existence in the field around it should make its own mass far heavier than its measured value. Explaining why the Higgs is so light without awkwardly rigging the equations has stumped theorists.
More broadly, the Higgs is connected to many of the most troublesome aspects of the standard model. It is the linchpin for what seems to be a ramshackle arrangement of particle masses, varying according to how strongly the Higgs couples to them. Electrons, for instance, are far lighter than their sister particles called muons, which are far lighter than their siblings called tau particles, and no one knows why. “It’s so chaotic,” says at the University of Freiburg in Germany. “The standard model has all these numbers in it that we don’t understand. There are no laws for them. It’s like the Wild West”.
Physicists hate putting numbers into theories by hand, as opposed to those numbers emerging naturally from a theory. “Fine-tuned” and “ad hoc” are insults in a field that seeks to discover the most basic order of reality. “It’s like gravity would act differently on apples, on humans and on planets,” says Heinemann. “It’s just so unsatisfactory. What is the origin of these numbers?”
The only difference between electrons, muons and taus in the standard model is the way they interact with the Higgs. The mysterious origin of particle masses suggests that some deeper structure exists, which studying the Higgs in detail may reveal. The idea is that by precisely measuring these interactions, we will see inconsistencies that the standard model can’t explain, offering clues towards a new, further-reaching theory.
We have already eavesdropped on some of these interactions. In 2018, the LHC revealed particle processes in which the Higgs is produced along with a top quark and its antimatter equivalent, a top antiquark. The top quark is the most massive fundamental particle, heavier than even the Higgs, which means any deviations from the standard model should show up most prominently here. “It’s a great way to hit the Higgs hard and see if it does what we expect,” says Freya Blekman at the Free University of Brussels in Belgium. Unfortunately, the top quark measurements revealed nothing untoward. The same was true last year, when we caught a glimpse of the Higgs decaying into lower-mass muons for the first time.
So far, the Higgs boson has shown itself to be resolutely vanilla. That is deeply frustrating. And yet the measurements at the LHC leave plenty of wiggle room to think that the Higgs is hiding something beneath its boring facade. Indeed, there is no shortage of ideas about what the Higgs really is and what it really does. “There are all kinds of tweaks and bells and whistles you can put on it,” says at University College London.
Particles we have previously considered to be fundamental and unsplittable have peeled open like the layers of an onion. Atoms broke apart into protons, neutrons and electrons. Then protons and neutrons broke open to reveal quarks.
The same could be true of the Higgs, with smaller constituents hidden inside it. For example, “twin Higgs” or “little Higgs” models add intricate new symmetries into the standard model as imaginative solutions to the problem of why the Higgs has such a strangely small mass. By looking for slight deviations in how the Higgs is expected to decay into other particles, we may find that another, more complex Higgs lies at the core of reality.
Hidden in the Higgs’ interactions is also the prospect of new particles. The Higgs is the only elementary particle whose quantum-mechanical “spin” is zero. This makes it uniquely promiscuous. If you flip most elementary particles on their head, they will behave differently because of their spin, but a spinless particle is the same no matter how you twist and turn it. This means the Higgs connects very easily to other particles, including those waiting to be discovered.
Pink elephants
If you measure how the Higgs decays into all known particles, but find that some energy has gone missing, it would suggest the existence of novel particles that current detectors aren’t able to see. As many as one in four Higgs bosons could decay into such “pink elephants”, as Heinemann calls them. Any such elephants would be prime candidates for dark matter.
At high enough energies, theories predict the Higgs boson can even decay into itself. Not only is this a previously unknown type of interaction, but how the Higgs does this determines our cosmic story. This “self-coupling” tells us about how the Higgs field came into being shortly after the big bang. Aside from giving mass to particles – and so enabling planets, stars and galaxies to form – knowing how this shift happened could tell us why there is so much more matter than antimatter in the universe.
The trouble is that, so far, measurements from the LHC have been unable to rule out or pinpoint these various possibilities for what the Higgs is really up to. The LHC does “dirty physics”, says Allanach, smashing together protons in high-energy, messy collisions to explore what’s out there. Amid this chaos, it is hard to get a handle on the finer details of the Higgs. Most of the Higgs’ couplings to other particles have so far only been measured to about 10 or 20 per cent precision, depending on the particle. “It’s very easy to say something agrees with the data when the uncertainties are large,” says Blekman.
All of which explains why Blekman and others are now lobbying for a new particle collider that would produce Higgs bosons in their droves. It would produce millions of the particles without much “noise” to obscure our view of what they get up to, allowing us to measure their couplings to other particles much more precisely. Moreover, an upgraded Higgs factory that bashes together heavier, and so more energetic, protons instead of electrons would allow us to measure the Higgs self-coupling..
Last June, CERN’s 23 member states agreed that their was to pursue the construction of a Higgs factory that collides electrons and positrons, the electron’s antiparticle. “Everybody agrees that we need something that makes a lot of Higgs bosons,” says Blekman.
Yet for all the confidence that a Higgs factory is the right way to expose the particle’s secrets, some physicists acknowledge the prospect that the Higgs may not be keeping anything from us after all – so a factory might find nothing. “It would be equally amazing, although difficult to deal with,” says
Until recently, the standard model was the blueprint giving us assurance that there was something out there to discover. Now, with that puzzle complete and few clues as to what comes next, we have been left scrambling in the dark.
Finding the Higgs boson and nothing else at the LHC was dubbed the “nightmare scenario” by theorists at CERN. Many physicists thought they would also see “superpartner” particles predicted by supersymmetry theory, which aims to fill gaps in the standard model. By adding new particles to the mix, theorists could explain the puzzlingly light mass of the Higgs boson. While interactions with already known particles drag the mass of the Higgs upwards, these superpartners drag it back down to the value measured by the LHC. Not only did supersymmetry offer an elegant way to unify the four forces of nature, but its superpartners also gave an identity to dark matter.
“There is plenty of wiggle room to think the Higgs has a few secrets”
With no hints of other particles at the LHC, the most plausible supersymmetry theories have crumbled. The only way to resolve the small measured Higgs mass is to plug in by hand a starting value for the “bare mass” Higgs, meaning the mass before you take into account all the interactions with virtual particles around it, that just so happens to cancel out those interactions. “It’s too suspiciously fine-tuned to be a coincidence,” says Butterworth.
Supersymmetry is rooted in an idea called naturalness, in which the laws governing the universe are elegant and explicable, as opposed to makeshift and arbitrary. Throughout history, when numbers have popped up that seemed fine-tuned, physicists have suspected that something was missing from their theory – and usually they were right. That’s why the continued absence of new particles at the LHC is a “sobering moment”, says at the University of California, Santa Barbara. “There is now a great reluctance to use aesthetic criteria,” he says.
With naturalness under question, it is hard to know whether new particles beyond the standard model exist at energies that particle colliders could ever reach. “One of the things we’ve learnt is that the standard model could be valid all the way up to very high energy scales,” says , a theorist at Durham University, UK. “It’s a depressing prospect.” Ultimately, nature may not be as elegant as physicists hope, and some parts may be unknowable – no matter how powerful or precise your particle collider.
Allanach remains hopeful. He has shifted his approach from top-down theories that begin with grand aesthetic principles to what he calls “bottom-up” thinking. It starts from small cracks in the standard model – such as particles that decay too quickly or are more magnetic than you might expect – and builds theories piece by piece. If adding a new particle explains the data better, then it is worth considering, regardless of how aesthetically appealing it is.
A Higgs factory will allow us to examine these small cracks, says Allanach. While not as exciting as discovering new particles, measuring the Higgs precisely is “not to be sniffed at”, he says. It offers a bedrock of vital data for new ideas to leap from.
“In my heart, I feel there will be a paradigm like the standard model which will come out of everything, and we will be able to understand it. Of course I do,” says Allanach. “But we need a change of approach. I do worry that we’ve got too locked into doing what the theorists tell us and lost sight of the fact that we’re actually exploring unknown territory.”
