Ten years ago, physicists working at CERN, the international research institute for particle physics near Geneva, announced the discovery of the Higgs boson. This elusive particle, first predicted in the mid-1960s by British physicist Peter Higgs and others, had finally been coaxed into revealing itself by colliding mundane particles called protons together at incredibly high energies in CERN’s 27km-long accelerator, the Large Hadron Collider (LHC).
It was a big deal. Without the Higgs boson, there would be no atoms, no stars and planets, and no you or me. This particle is what gives some other particles (like the protons and electrons in atoms) mass. More precisely, the Higgs boson is the particle associated with an underlying, all-pervasive energy field in otherwise empty space, the Higgs field, which some other particles feel, making them “heavy” (massive) as a result.
The Higgs particle itself has a lot of mass (about 125 times that of a proton), which means that huge amounts of energy were needed to reveal it—to pull it, as it were, out of the Higgs field. The LHC is the only collider in the world that can create collisions with enough energy to realise that process. The instrument was built between 1998-2008, with the search for the Higgs as its primary goal, and the particle was identified in two separate experiments, called ATLAS and CMS, that both analysed the debris of the collisions between protons which were accelerated to circulate in the tunnel at close to the speed of light.
For the past three years, the LHC has been shut down for an upgrade, prolonged by the pandemic. But from 5th July it has been up and running again, its colliding beams now more intense, generating more collisions and more data to sift, and having slightly higher energies too. The new runs are slated to continue for four years. What more—if anything—will it find? And what if this new tranche of high-energy collisions reveals nothing we don’t already know? With the central theories in fundamental physics still leaving important explanatory gaps, and anxiety over where the next breakthrough is coming from, some have started to ask whether particle physics is at a dead end. The results from the rebooted LHC will be anxiously scrutinised to see whether they can provide an answer.
Gaps in physics
The 2012 discovery of the Higgs boson secured the 2013 Nobel Prize in Physics for Peter Higgs and Belgian physicist François Englert, one of the others to have had much the same idea at the same time. But it was not the start of a new field of research so much as the end point of an existing one. The Higgs particle was the last remaining piece of a puzzle called the Standard Model, which brings together all the known fundamental particles and forces that make up the physical world (with the exception of gravity, a notoriously difficult phenomenon to fit into the microscopic picture of physics). With the discovery of the Higgs, that framework was finished and consistent—there is no place in it for anything else.
Yet the Standard Model doesn’t explain everything we know about the world of particles and forces. One major remaining puzzle is dark matter: a hypothetical substance that seems to interact with known particles and light only via its gravitational effect. Because dark matter is seemingly immune to all other forces, it can pass ghost-like through ordinary matter. We can only infer its existence at all from its effects at astronomical scales: it is needed to explain why galaxies don’t simply spin apart, and why light seems to get bent by otherwise empty space. But we have no idea what dark matter is. There are no particles in the Standard Model to account for it, and despite decades of searching, no other candidate particles have ever been detected. Some physicists suspect “dark matter” might not be some undiscovered particle at all, but rather that some other law (such as a modification to the theory of gravity) is needed to explain its apparent effects.
The Standard Model does not, meanwhile, explain why the amount of ordinary matter in our universe seems greatly to exceed that of its opposite, antimatter. Every known particle has an antimatter sibling: they are mirror images, rather like left and right. The negatively charged electrons that are constituents of all atoms, for example, have an antimatter partner with a positive charge, called the positron. When matter and antimatter meet, they annihilate one another in an outburst of energy. Our physical theories suggest they should have been formed in equal amounts in the Big Bang—so why were they apparently not?
The Standard Model also fails to explain how three of the fundamental forces at work in the universe—electromagnetism and the strong and weak forces that operate inside the atomic nucleus—might have once been one single force very early in the universe, just instants after the Big Bang. It is widely believed that this unity of forces existed: it has already been shown to be the case for the electromagnetic and weak forces, which were once a single “electroweak” force. The leading theories that describe this unification of forces imply the existence of a property called supersymmetry, which is not included in the Standard Model. Supersymmetry predicts that every particle has a “supersymmetric” partner. The existence of such supersymmetric particles could explain why the Higgs boson is not even heavier than it is, as the Standard Model seems to imply it should be.
Supersymmmetry looks to many physicists like a very enticing idea. Yet no evidence for it has ever been found. Some hoped that the LHC might provide that, for example by spitting out some of the hypothetical supersymmetric partner particles in its collisions. But this has not happened so far.
On the one hand, these and other remaining mysteries ensure that the work of particle physicists is not done: there is plenty still to keep the field alive. On the other hand, there has been a sense of frustration, even desolation, in the community since the discovery of the Higgs, because experiments at the LHC and elsewhere have failed to offer any clues about how to make progress.
Searching for flaws
Physicists like those at CERN are thus in the curious position of wanting to find flaws in their most prized theoretical framework, the Standard Model. They have no shortage of theories for, say, how supersymmetry could arise or what dark matter might be. But without any empirical data to guide them, how can they ever know who, if anyone, is right?
“You make progress by having an interplay of experiment and theory,” says particle physicist Tara Shears of the University of Liverpool, who works at CERN. “You can be driven by a theory and make measurements to confirm or deny it. And you can make measurements that, when they disagree with a theory, need a theorist to think of explanations. You need both.”
Physicists are in the curious position of wanting to find flaws in their most prized theoretical framework
Hunger for clues about how to move beyond the Standard Model explained the excitement over an announcement in April 2021 of the results of an experiment called Muon g-2 at Fermilab, near Chicago. From 2017 to 2020, Muon g-2 looked at fine details of the properties of particles called muons, and found measurements that seemed just slightly, but robustly, inconsistent with the predictions of the Standard Model. (It takes a lot to convince particle physicists, who have strict criteria for when an experimental finding can be considered reliable and not just a fluke in the data.) Could this be the crack they were waiting for?
The Muon g-2 claims offered a tantalising hint that the LHC can follow up on, says Shears. One possible explanation for the results entails a new kind of boson altogether. “The LHC can look for such a new particle directly, and also indirectly via the behaviour it should induce in particles we know about,” says Shears.
The LHC is not simply colliding protons in the hope that something new will pop out of the fragments. Physicists will be using its detectors to investigate very specific questions. Some of these are being posed to deepen existing knowledge rather than to extend it. For one thing, there is still plenty we don’t know about the Higgs boson—how strongly it interacts with relatively light particles, or with other Higgs bosons, and how long it survives before decaying into other particles. (We do already know that its lifetime is very short indeed: around 10-22 seconds.)
And there is plenty still to learn about the details of other particles in the Standard Model—not least because flaws in the model might lurk in this fine print. “We will analyse fundamental particle behaviour more deeply and fill in lots we don’t know—about the Higgs particularly, but also antimatter, and anomalies seen in the data taken up to now,” says Shears. “We want to find out where the limits of the Standard Model are, and that’s reliant on testing the properties and behaviour of different fundamental particles—all of them, not just the Higgs—increasingly precisely.”
“At some point, if the Standard Model is not the whole story,” she says, “it will break down and not predict what we see. At that point we’ll have indirect evidence of new physics.”
A bigger crash?
“As we continue to explore the energy frontier at the LHC, I honestly don’t know what we’ll find,” says particle physicist Jon Butterworth of University College London, a team member on the ATLAS experiment that discovered the Higgs boson (“I was honestly quite surprised it showed up,” he admits). If the Standard Model continues to apply at higher energies, “that will be an important new piece of information about nature,” Butterworth says—because no one really knows how far that picture can be extrapolated beyond the conditions we have explored so far. But, he admits, “I think it unlikely we’ll make a clear-cut beyond-the-Standard-Model discovery [in these LHC experiments].” In other words, he doesn’t anticipate finding strong clues that might point the way to the bigger, better theory that is surely needed.
Shears is similarly cautious. “I don’t know if we will make substantive progress [with these runs],” she says, “simply because I don't know where the new physics is.” CERN’s director Fabiola Gianotti says that her most fervent hope is that the LHC might turn up clues about dark matter. But what if the upgraded machine still doesn’t find any new physics at all? Where can particle physicists go then?
In June 2020 the governing body of CERN unanimously approvedplans to undertake a feasibility study for an entirely new collider, with a tunnel 80-100km long, that would collide electrons and their antimatter siblings, positrons, in such a way as to release unprecedented amounts of energy, hopefully forming new particles and inducing new physical phenomena. It would cost an estimated €21bn, and if the final approval and funds can be obtained (that’s a big if), construction would start around 2038. Such a collider should provide more precise measurements than the LHC but “the cost/benefit analysis has to be solid in scientific, financial and environmental terms, and that discussion is ongoing,” says Butterworth.
Meanwhile, some researchers hope that plans for a large international project, the International Linear Collider (ILC), spearheaded by physicists in Japan, might also go forward. Rather than moving in circles, the particles in the ILC (electrons and positrons) would be accelerated in a straight line along two 20km accelerators before being smashed together. “The ILC technology is now ready and proven,” says Suzie Sheehy, an accelerator physicist at the Universities of Oxford and Melbourne.
Not all physicists are thrilled by such plans. Particle physicists “will keep on building bigger particle colliders as long as they can, simply because that’s what particle physicists do, [but] building larger particle colliders has run its course,” argued German theoretical physicist Sabine Hossenfelder in Scientific American after CERN’s plans for a bigger collider were endorsed. “There is no reason why the particles that make up dark matter or dark energy should show up in the new device’s energy range. There are entirely different types of experiments that could lead to breakthroughs at far smaller costs.”
It is by no means the first time there has been griping from within the physics community at the extravagant cost of the research pursued by the relatively small minority working in particle physics. (The resentment is probably fuelled by the way this sector of physics dominates public perception of what physicists do.) Objections from other physicists helped sink the Superconducting Super Collider project in Texas in 1993, when construction was already under way. Some believe that this device, had it been completed, would have found the Higgs boson before the LHC did.
“The Standard Model has held up so well to experiments at the LHC that many thought would ‘break’ it, that it is quite right to start asking questions [about the way forward],” says Sheehy. “Is particle physics a field of diminishing returns? Do we actually need to know the fundamental laws of physics any better than we know them now? These bigger-picture questions seem to me to lie at the heart of the question of whether we should build a new collider.”
But Sheehy adds that “all the particle physicists I ask agree that the question is not ‘whether’ we will need a new collider, but ‘which collider, and when?’ And we can’t afford to wait too long to decide, because these projects need extremely specialised skills, cultivated over decades.” On the other hand, she says, we won’t necessarily need a big new collider to make at least some progress on the outstanding questions. There may be smaller, cheaper experimental techniques that can help too. “The compromise is they tend [only] to be able to ask much more specific questions,” she says, “so their discovery potential is necessarily limited.”
Deciding the best way forward depends in part on what the rebooted LHC finds, says Shears. Is her field in crisis? “I don’t think so.” For an experimental particle physicist, there is still plenty to sink your teeth into. “This is an exciting time for us. We are a small fraction of the way through the LHC programme, and with each additional tranche of data our investigations become more sensitive and our ability to distinguish the effects of new physics is greater. We literally have so much that we need to explore.”
This article was corrected to make it clear that CERN has not approved plans for building a new collider but only for conducting a feasibility study for it