IT’S not unusual to feel worse for wear after a big celebration, but I would bet you’ve never had a hangover that lasted a decade. Ten years ago, almost to the day, researchers gathered at the CERN particle physics laboratory near Geneva, Switzerland, for the announcement of the discovery of the Higgs boson, the particle that underpins why fundamental particles have mass. It was a huge triumph, both for the mathematical theory that predicted its existence and for the Large Hadron Collider (LHC), which revealed the Higgs by smashing protons together with record-breaking violence.
A fair bit of champagne was drunk at CERN that day. But while the post-celebration lethargy soon lifted, a more lasting malaise has lingered over particle physics ever since. At the time there were good reasons to think the LHC would discover a whole host of new particles that would address some of the biggest mysteries in the universe. But as search after search came up empty, anxiety set in. “We have a bit of a hangover from the Higgs discovery,” says at University College London, who works on the LHC’s ATLAS experiment. “But we’re getting over it.”
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Now, as the LHC fires up again, there is renewed optimism. We are “off the edge of the map” with no theoretical stars to guide us, says Butterworth, and that is no bad thing. Indeed, the mood among experimentalists like me is one of growing excitement – not only because we already have hints of new physics, but also because there is much data still to come. “So far, we’ve barely scratched the surface,” says Butterworth.
The standard model of particle physics does a stunning job of describing the particles and forces that make up our universe, but we know that it can’t be the final picture because, even with the addition of the Higgs, it is manifestly incomplete. Ten years ago, one of the most popular ideas for what might lie beyond the standard model was supersymmetry, known in the field as SUSY, which predicts a zoo of heavy counterparts, or superparticles, for every known particle.
The appeals of supersymmetry were manifold. It was based on a beautiful idea, a mathematical symmetry relating force-carrying particles like photons to matter particles like electrons and quarks. Many SUSY models also contained a stable, electrically neutral particle that was a perfect candidate to explain the mysterious dark matter that dominates the cosmos, keeping galaxies together when they would otherwise fall apart. Better still, with SUSY, the three quantum forces of nature – the electromagnetic, weak and strong forces – appeared to unify at high energies rather precisely.
But perhaps the primary motivation for supersymmetry was that it could stabilise the Higgs boson itself. The Higgs provided firm evidence for the existence of an invisible quantum field that fills the universe: the Higgs field. This acts as a sort of invisible gloop that slows down particles like electrons and quarks, preventing them from zipping around at the speed of light and imbuing them with mass. However, physicists have long been troubled by the fact that the strength of the Higgs field, which determines how massive the particles are, seems to have only two natural values, both of which would be extremely bad news for your existence. In one case, the Higgs field would have a strength of zero, leading to a universe containing a haze of massless particles and not much else; in the other, its strength would be so high that everything in the cosmos would collapse into black holes.
To get the Goldilocks value we observe, the Higgs field has to be extraordinarily carefully fine-tuned, as if some great cosmic tinkerer had set things up just so. And that smells fishy. Supersymmetry solves this problem, with its superparticles interacting with the Higgs in such a way that they stabilise the Higgs field around the value we see without the need for fine-tuning. The trouble is that for this to work, the superparticles must have masses close to that of the Higgs boson itself – well within the reach of the LHC. So the fact that none of them have shown up so far is a problem.
, a theorist at King’s College London and a leading expert on supersymmetry, earned himself something of a reputation as a SUSY diehard as search after search drew blanks over the past decade. However, he says his focus has now broadened to other topics. “Shares in SUSY have gone down,” he concedes, “but I wouldn’t put them in the junk category.” It is possible that superparticles could still be hiding in the ocean of collision data recorded by the LHC’s ATLAS and CMS detectors. “Most rational investors would put them in the sell category, but I’m hanging on to at least a few of my supersymmetry shares,” he says.
Another possibility that many particle physicists were invested in 10 years ago was the idea that the Higgs itself would reveal something beyond the standard model, for instance by acting as a “portal” to a range of dark matter particles. In the decade since its discovery, physicists at the CMS and ATLAS experiments have put the Higgs under the microscope, measuring its mass and its interactions with increasing accuracy. But so far at least, the Higgs looks remarkably like the version predicted by the standard model, with little evidence for anything more exotic.
All of which explains the long hangover. The lack of new physics has dismayed some, particularly researchers from my (relatively) youthful generation who were enticed into the field by the prospect of a discovery bonanza at the LHC. There have been one or two false dawns over the years, most notably in 2015, when everyone got briefly excited about a tantalising bump in the data that turned out to be a statistical fluke. But broadly speaking, the past decade has been one in which the field has gradually changed tack as physicists moved away from beautiful theoretical ideas like SUSY and listened increasingly carefully to the data.
The bonfire of speculative theories has led to a clear shift away from theoretically motivated searches. After the fall in SUSY’s fortunes, no great overarching paradigm has emerged to replace it, says Ellis. “What theory do you invest in? If you look at the theory papers being published, you will see complete confusion. Many theorists are running around like headless chickens,” he says.
Butterworth is pleased by this turn of events. “I would have been a little disappointed if we’d discovered [SUSY] because it would have been just another example of experiment following theory,” he says, before adding with a puckish grin: “And there would have been so many smug theorists around!” Instead, he is excited for what the next decade and a half will bring. That makes sense because, by my calculations, we have only seen roughly 5 per cent of the data we will ultimately get from the LHC in its various iterations.
Intriguing anomalies
Experimentalists are now casting their nets wider. “Ten years ago, we were very focused on two aspects: the discovery of the Higgs and whether supersymmetry exists,” says , a physicist at the University of Cambridge and the LHCb experiment. “It was very narrow. Since then, the field has got a lot broader, which is definitely a good thing.” For instance, over the past decade, my colleagues at LHCb have discovered new types of matter-antimatter asymmetry and exotic new composite particles called tetraquarks and pentaquarks. While these breakthroughs fit within the standard model, they nonetheless provide us with crucial information about the forces and particles that make up our universe.
It is even possible that we are already seeing the footprints of new physics in a series of anomalies identified by the LHCb experiment. The b in LHCb stands for beauty, one of six types of quark. My colleagues at LHCb have recently seen evidence of beauty quarks behaving in ways that the standard model can’t explain. They seem to be decaying to the three electrically charged leptons – the electron, muon and tau – at different rates, something that is forbidden in the standard model, which treats these three leptons identically, apart from the fact that they have different masses.
Other measurements have found that the particles produced in the decays fly out at angles that differ from standard model predictions. Tantalisingly, theorists have found that all these anomalies can be explained if a hitherto unknown quantum force is pulling on beauty quarks, changing how they behave.
Anomalies are also cropping up in experiments in the US. In April 2021, the muon g-2 experiment at Fermilab in Illinois reported that the magnetic properties of the muon, a heavier cousin of the electron, disagree with what theorists predict using the standard model. And then, in April 2022, physicists working on legacy data from the CDF experiment at Fermilab’s now-defunct Tevatron collider reported a measurement of the mass of the W boson, a particle associated with the weak force, that was in dramatic disagreement with what we would expect.
These last two anomalies could potentially be explained by superparticles that wouldn’t necessarily have shown up yet at the LHC, offering a glimmer of hope that SUSY could yet rise again. But they could also herald something we have never even thought of, which would arguably be more significant.
Sophie Renner, a theorist at the University of Glasgow, UK, has spent the past few years trying to decode the clues coming from the LHCb anomalies, working on a variety of solutions that generally involve introducing a new type of force particle known as a leptoquark. Leptoquarks can convert quarks directly into leptons and vice versa, something that no standard model particles can do, and could be part of a larger theory that explains why there are six quarks and six leptons in the standard model.
Could there be a connection to the other anomalies from the US? Perhaps, says Renner. “It’s certainly very intriguing that there seem to be two completely different experiments [muon g-2 and LHCb] that find something weird in muons.” She is more sceptical of the W boson mass anomaly: “With the W mass, you can make connections, but it’s not obvious, and there’s also the issue that the CDF measurement deviates from all the previous measurements. You clearly shouldn’t be able to have two measurements of the same thing that don’t agree. That’s the first question that needs to be understood.”
Ultimately, knowing whether these anomalies are the real deal will require more data and new measurements – both of which are guaranteed given the recent restart of the LHC. LHCb has just received a major upgrade and will soon be recording data at around 30 times its previous rate, which should allow us to figure out whether the anomalies are random statistical flukes, missed biases in the experiment or genuine signs of new physics. Likewise, in the coming decade, ATLAS, CMS and LHCb will all be able to make precise measurements of the mass of the W boson.
As for the Higgs itself, there is still plenty of scope for surprises, says , who was involved in the discovery of the particle as a PhD student and now leads efforts at CMS to probe it with ever-increasing precision. “Discovering the Higgs was the first step on a long road,” says Wardle. “Now we’ve moved over to using the Higgs as a tool for finding new physics.”
One example of how the Higgs could guide us to new physics is if it decays to dark matter. Such a decay would show up in ATLAS and CMS as missing momentum, the sign that something invisible had escaped the detector. In fact, Wardle says that both experiments currently have small hints of just such a decay, though it is far too early to be confident that it is more than a random wobble in the data.
Wardle is also keen to see how the Higgs boson interacts with itself. Such measurements were previously thought to be almost out of reach at the LHC, but, as Wardle put it, “physicists always get smarter as we go”, improving their analysis methods to the extent that previously unreachable targets become possible. “I never want to say never. It’s completely different what we’re able to do with the data compared to 10 years ago,” he says.
Measuring the Higgs’s interaction with itself would be a huge coup, testing a fundamental assumption of the standard model about how the Higgs comes into existence in the first place. It would tell us about processes in the first trillionth of a second after the big bang that could have been responsible for the origins of matter itself.
The truth is that to pin down whether the Higgs is the same particle predicted by the standard model or something more exotic, we need a collider that goes far beyond the LHC in both size and energy. Physicists have already begun thinking about how they could build such a machine, perhaps by the 2040s. But Butterworth argues that those in the field must not lose sight of what is closer at hand. During the next decade, the LHC will receive a major upgrade to increase its “luminosity” – in effect how many protons smash together every second. This souped-up LHC will unleash a torrent of data and make previously unimaginable measurements possible.
“I think we discounted the ,” says Butterworth. “It annoys me to hear fairly early career scientists saying, ‘there’s all this uncertainty in the field, what should we do’. I say come on! You’re talking about what experiment you might be doing in 2040! What other field has a solid future up to 2040?”
The challenge now is to deliver on upgrading the LHC so that we can test the standard model to the highest precision possible. “I’m really excited for the next 10, 15 years of what’s going to happen at the LHC,” says Butterworth. “Precision and detail may not sound super exciting, but you’re searching the undergrowth of a new territory and you may find something really unexpected.”
“We’re recovering from the hangover,” he adds. “I hope the 10-year anniversary will draw a line under some of that.”
