by Harry Cliff
Eventually, the media frenzy became so great that CERN assembled a panel of experts to assess the various doomsday scenarios. They produced a truly excellent document titled Review of the Safety of LHC Collisions, which is probably the most exciting risk assessment ever written and worth a read just for lines like “The possible concern about high-energy particle collisions is that they might stimulate the production of small ‘bubbles’…which would then expand and destroy not just the Earth, but potentially the entire Universe.”
Exciting stuff. Fortunately, the panel concluded that since the universe has been steadily bombarding the Earth with cosmic rays far higher in energy than we can achieve at the LHC, if such doomsday events were possible they would already have happened and the Earth and every other heavenly body would have been destroyed long ago. The panel appears to have been right as the world still seems to exist, so far at least. Anyway, I guess if the world does end, no one will have time to sue.
The reason that tiny black holes aren’t thought to be a threat is encapsulated in Stephen Hawking’s famous prediction that they should evaporate by emitting Hawking radiation. For big, star-sized black holes that lurk in deep space, this process is absurdly slow, but tiny black holes of the size that might be made at the LHC would disintegrate almost instantly into a spray of particles, which could then be spotted by the giant ATLAS and CMS detectors.
Existential terror aside, supersymmetry and extra dimensions remain the two most popular ways to explain the strength of the Higgs field, though they are by no means the only ones. Regardless of which phenomenon is ultimately responsible for keeping our metaphorical kite aloft, you would more or less always expect to find something new close in energy to the Higgs boson itself. So when the LHC started colliding in 2010, hopes were high that we would soon glimpse new ingredients of our universe along with the Higgs itself.
The first year of the LHC’s running was basically a warm-up lap, as the engineers driving the collider in the CERN Control Centre learned how to operate their shiny new machine. When collisions restarted in the spring of 2011 after a winter break, the collider raced out of the starting blocks, accumulating more data in the space of a few days than in the whole of the previous year. Now the game was well and truly on.
As Christmas 2011 approached, telltale hints of the Higgs boson were already emerging from the data recorded by ATLAS and CMS. However, despite hopes that sparticles might reveal themselves at the same time, all the searches were drawing blanks. Still it was early days for the LHC, nothing to get too concerned about just yet.
Fast-forward to July 2012; CERN jubilantly announced the discovery of the Higgs boson to a world that briefly became fascinated by particle physics. However, as champagne bottles were uncorked in the offices of CERN, anxiety was already growing about the absence of any of the other predicted new particles. My colleague Sarah Williams, newly arrived at CERN as a PhD student and sleep deprived after weeks of intense work, had just unblinded a search for supersymmetric versions of the leptons, or sleptons (I know, right?). Despite huge excitement among her more senior colleagues that they were about to see something new, when they looked at the data there wasn’t even a whiff of a sparticle.
In fact, every search for supersymmetry, micro black holes, and other exotica had come up empty-handed. Perhaps even more troubling was the mass of the newly discovered Higgs boson. The simplest supersymmetric theories generally predicted a Higgs with a mass close to the Z boson, around 90 GeV; however, the particle seen by ATLAS and CMS was uncomfortably heavy, up at 125 GeV. Although this could be accommodated with a bit of theoretical gerrymandering, it started to put the theory under strain.
Toward the end of 2012, my own experiment, LHCb, announced more bad news for supersymmetry fans. We had found evidence for an extremely rare decay of the beauty quark that was predicted to get a big boost in certain versions of supersymmetry. However, the decay had been seen with a rate that was in more or less perfect agreement with the standard model. When the BBC broke the story it ruffled a few feathers, particularly thanks to a quote from my colleague Chris Parkes at the University of Manchester, who declared that the new result had put supersymmetry “into hospital.” My boss and head of the Cambridge LHCb group, Val Gibson, joined the fray saying that the result was “putting our supersymmetry theory colleagues in a spin.” After all, experimental physicists like nothing better than proving their smarty-pants theory colleagues wrong. At CERN, the eminent theoretician John Ellis, who had spent more than thirty years working on supersymmetry, fired back dismissively that the result “was actually expected in (some) supersymmetric models. I certainly won’t lose any sleep over the result.”
How can a bunch of respected professors of physics have such different interpretations of a result? Well, an important point to understand about supersymmetry is that it is not a single theory—it is a principle that can be used to build a huge number of different theories with wildly different predictions. The upshot of this is that supersymmetry is fiendishly difficult to kill. If your favorite supersymmetric model doesn’t show up at the LHC, it’s almost always possible to adjust some of its parameters or add some extra bells and whistles to explain why you haven’t seen it yet. However, as you tinker and augment to explain away its failures, you begin to corrupt supersymmetry’s very purpose. After all, it was invented to avoid fine-tuning in the standard model, so fine-tuning supersymmetry itself feels like a betrayal of its founding principle.
The final protons of the first run of the LHC smashed into one another just before Christmas 2012. As the engineers who had built and operated their remarkable machine looked back on the past three years with justified pride, the physics community struggled to make sense of the landscape that the LHC had revealed. Against great expectations of a rich vista full of new and exciting opportunities for exploration, the LHC had revealed a wasteland, at the center of which stood the solitary Higgs boson, an inexplicable tree alone in an arid desert.
Some physicists began to whisper of the “nightmare scenario”—the possibility that the LHC would ultimately only discover the Higgs while providing no other clues to the great problems of fundamental physics. Some young people began to reassess their career plans. Matt Kenzie, who had been involved in the Higgs discovery at CMS, made the bold decision to switch to LHCb after his PhD, believing that the writing was already on the wall for the chances of seeing new particles at ATLAS or CMS. Older heads advised caution. It was still very early days, they said. We had waited more than thirty years for supersymmetry; we could afford to wait a little longer. Ben Allanach at Cambridge captured the mood of many of his theory colleagues: “Supersymmetry is a bit late to the party, but I don’t think it’s lost yet.”
Hope was on the horizon. After two years of engineering work to remedy the faults that had forced the LHC to run at around half its maximum energy for the first few years, the collider restarted with a record-breaking collision energy of 13 TeV in May 2015. Once again, the frontier of exploration was being pushed farther into uncharted territory. Perhaps the promised riches were finally within reach.
Then, just before Christmas 2015, a bolt from the blue. ATLAS and CMS released evidence of a new bump appearing in the high-energy data recorded that year. In eerie echoes of the Higgs hints that had been revealed before Christmas 2011, they were both seeing evidence of a new particle decaying into two photons, but six times heavier than the Higgs, with a mass all the way up at 750 GeV.
The accumulated tension that had been building in the theory community for more than five years was suddenly released in a torrent of speculative proposals explaining the new bump. Within a few weeks more than five hundred papers had been uploaded to the online preprint repository,*4 including one from Ben and his colleagues. Many speculated that the new discovery might be one of the long-awaited superparticles, the herald of a new quantum army that would soon march into view.
In August the following summer, physicists gathered in Chicago for the biggest particle physics event of the year, the International Conference on High Energy Physics. ATLAS and CMS were both scheduled to give eagerly anticipated updates on the 750 GeV bump using the additional data that had been recorded that year. However, the night before the talks, CMS jumped the gun by accidentally posting their paper online. The result landed like a punch to the gut. As more collisions had accumulated, the bump had melted away to nothing. It appeared to have been little more than a random fluctuation in the data. More than five hundred papers had been written about a cruel statistical fluke.
Meanwhile, Sarah was working as part of a team on ATLAS searching for signs of micro black holes using the 2015 data. There were hopes that with the higher energy collisions it might now be possible to create them, but again, they failed to show.
The LHC and its giant experiments continued to perform admirably for a further three years, churning out vast quantities of high-quality data, smashing almost all the expectations set at the start of the run. By the time the machine switched off again on December 3, 2018, for another scheduled two-year shutdown, the LHC experiments had recorded more than 10 thousand trillion collisions, and yet not a single hint of a new particle beyond the Higgs had been spotted among all the subnuclear debris. The nightmare scenario seems to be coming true.
Fundamental physics now faces a crisis unlike anything seen in the last one hundred years. We know that there are major features of our universe that we do not understand: the origin of matter during the big bang, what dark matter is made from, and, most of all, how we find ourselves living in a universe that appears spookily fine-tuned for life. And yet, the machine that was built to provide us with answers, the largest ever built by humankind, has offered up only the same standard model that we know must be incomplete. This isn’t a failure of the experiment; the LHC is an engineering and technical triumph. It has simply shown us how nature is, and nature, it seems, doesn’t care much for our clever theories.
Although many still hold out hope that supersymmetry will appear at the LHC in the coming years and save us from the pseudoscientific jaws of the multiverse, others have refocused their efforts in what might be more fruitful directions. Supersymmetry—at least in the grandest and most ambitious forms that explain the strength of the Higgs field, the nature of dark matter, and the unification of the forces all in one go—appears to have failed. To have escaped detection, the predicted sparticles must now be so massive that they would no longer balance the powerful vacuum fluctuations that threaten to blast the Higgs field away from its Goldilocks value, leaving us with an uninhabitable universe. Seeing the way the wind was blowing, at the start of 2019, the Cambridge Supersymmetry Working Group quietly rebranded itself as the Phenomenology Working Group.
So where do we go from here? Is this it, the end of the road? Are there simply features of the universe that are beyond our power to explain? It may be a cliché, but every crisis is an opportunity, and this crisis presents a huge one. The LHC may not have given the answers we were hoping for yet, but it is telling us something. The challenge now is to figure out what. This is a moment to reexamine our assumptions and look at old problems from a different angle. More than anything, it is a time to put our grand ideas and preconceptions to one side and to listen carefully to what nature is saying.
In fact, it may already be speaking to us in unexpected ways. Over the past few years, a series of strange and unanticipated signals have started to emerge from the LHCb experiment, signs, at long, long last, of nature deviating from the standard model. It’s too early to be certain just yet, but maybe, just maybe, we are about to see a deeper layer of the cosmic onion.
THE AGE OF ANOMALIES
LHCb doesn’t usually get the same attention as the big beasts, ATLAS and CMS. We didn’t discover the Higgs (to be fair, we didn’t look for it), nor do we search for sexy-sounding stuff like dark matter or micro black holes. And compared to the photogenic ATLAS and CMS detectors, which look like alien portals to another dimension, go down to the LHCb cavern and you’ll be confronted with something that looks kind of like a giant multicolored toast rack.
However, as ATLAS and CMS have been burning through speculative new theory after speculative new theory, LHCb has emerged as the best hope for finally seeing something beyond the standard model at the LHC. For the past few years, anomalies have started to appear in the data, anomalies that may be hinting at something altogether new.
To understand the difference in approach between ATLAS, CMS, and LHCb, consider two hunters standing at the edge of a dense jungle. Somewhere, out there, in the miles and miles of tangled foliage, is an elephant. Or so they have been told by a local elephant theorist. One hunter strides confidently into the undergrowth, hacking his way through vines and bracken, pushing ever deeper into the forest in search of his quarry. But the jungle is big and dark and grows thicker and more oppressive with each step, and he eventually reaches a point where he can’t go any farther without having caught sight of the elephant.
Meanwhile, his companion has wandered in only a little way, where shafts of sunlight still pierce the canopy and the going is a little easier. She moves slowly and methodically, her eyes scanning the forest floor for something out of place—a footprint, or perhaps a broken branch. After a long while, she notices a faint depression in the soft earth, as wide as a tree trunk with four marks that might be toes. A little later, she finds another, and then another, leading her deeper and deeper into the jungle. The elephant is out there. She is on its trail.
The hunter hacking his way through the jungle is like ATLAS and CMS, the two giant general purpose detectors that scour through trillions upon trillions of collisions in search of new particles hidden in the quantum undergrowth. This sort of direct search can work well if you have a clearly defined target and know what energy range/bit of the jungle to search in. It was exactly how the Higgs was found, for instance. However, if the particles you are looking for lie out of reach—perhaps they’re too heavy to be produced directly in your collisions or are especially well camouflaged among the ordinary particles—then you’ll draw a blank.
But there is another approach, a so-called indirect search. Like the hunter scanning the ground for footprints, it’s possible to detect hints of new quantum fields through their effects on ordinary standard model particles. The advantage is that you can spy evidence of a new quantum field even if the associated particle is too massive to create directly. The disadvantage is that you may not be able to figure out precisely what is causing the effect, like a hunter who can’t quite tell what species of elephant they’re tracking just from its footprints.
Broadly speaking, this second, indirect approach is the one we take at LHCb. Unlike the multipurpose ATLAS and CMS experiments, LHCb is specifically designed to study standard model particles with high precision in the hope of catching them misbehaving. As I’ve noted, the “b” in the LHCb stands for “beauty,” the heaviest cousin of the ordinary down quark that makes up atomic matter. This negatively charged quark is more usually referred to as the “bottom quark”; there was an attempt to name the two heaviest quarks “truth” and “beauty,” but the community plumped for the less poetic “top” and “bottom.” At LHCb we’d rather be known as beauty physicists than bottom physicists, and so, for us at least, it’s beauty not bottom.
The beauty quark is interesting because it’s particularly sensitive to the existence of new quantum fields, which can do things like alter how long it lives before decaying, or how often it decays into different sorts of particles. One of the best ways to try to spot these sorts of effects is to study decays of the beauty quark that are predicted to be extremely rare in the standard model.
Take, for instance, a beauty quark decaying into a strange quark, a muon, and an antimuon. There is no simple way to make this decay happen in the standard model; the decay ha
s to go via a complicated mixture of different quantum fields including the W and Z bosons and the top quark field. It’s a bit like trying to get between two London Underground stations where there’s no direct route, forcing you to change trains several times. Most people would avoid the hassle of such a convoluted journey; as a result, the number of passengers traveling between the two stations is very low. Likewise, the fact that our beauty quark decay involves so many different quantum fields makes it very rare indeed.
But what if there were a more direct route, one that, for the sake of our analogy, doesn’t use the normal underground network? For instance, maybe there’s an overland train that goes directly between the two stations in a little over twenty minutes. The same could be true for our beauty quark if a new quantum field, for instance a new force of nature, exists that provides a more direct way for it to decay. This can even happen if the particle of the new field is far too heavy to be created by the LHC. Even without having particles moving about in the field, the field is still there, and some energy can still go through it briefly without having to actually create the associated particle.*5
So if we count how often a beauty quark decays into a strange quark, a muon, and an antimuon and compare that to what the standard model predicts, then we can potentially detect the influence of unseen, undiscovered quantum fields. However, these decays are extremely rare—only one in a million beauty quarks will decay in this way—so to have any chance of spotting it, you need to make a hell of a lot of them.
