杏吧原创

IT鈥橲 an old joke. A woman returning home finds a neighbour searching for his keys beneath a street lamp. 鈥淚s that where you dropped them?鈥� she asks. 鈥淣o,鈥� he replies, 鈥渂ut it鈥檚 where the light is.鈥�

Change a few details and you could be describing the current state of particle physics. Except nobody鈥檚 laughing.

For over a decade, the most expensive experiment on Earth has been flinging protons at one another at a hair鈥檚 breadth shy of the speed of light. These trillions of collisions at the Large Hadron Collider (LHC) 鈥� part of the CERN particle physics laboratory near Geneva, Switzerland 鈥� have helped confirm our existing picture of reality. But many fundamental problems that the LHC was built to answer remain infuriatingly open.

For a rising generation of physicists, the threat of stagnation means it is time to rethink our search. Instead of building ever more powerful experiments, they have a radical proposal. They believe experiments like the LHC may have already found the signature of exciting new physics 鈥� we just didn鈥檛 know they were there.

Combing through millions of gigabytes of data for those missed clues is complicated and time-consuming, and will require powerful new algorithms. That is why some want a shortcut. They are zeroing in on the blind spots that plagued our previous searches, hot on the trail of a promising new idea lurking in the shadows. Their proposal of a nest of particles whose interactions kept them hidden could hold the key to the most perplexing problems in physics. Alternatively, it could amount to nothing. Either way, it is time we ventured into the darkness.

As street lamps go, the LHC shines a remarkably powerful beam. Its proton collisions rattle off at a rate of 30 million per second, generating a continuous stream of data that is impossible to store in its entirety. Instead, the debris is instantly filtered to isolate interesting events, with some 1000 per second chosen for further analysis and the rest discarded. This still leaves more than 80 million events being recorded each day, and even the filtered results remain so complex that they need selective analysis, says Nathaniel Craig, a theoretical physicist at the University of California, Santa Barbara. 鈥淵ou can only explore a finite number of possibilities.鈥�

That means relying on hunches to prioritise where to search. Sometimes these theoretical preferences can pay off. Take the case of the Higgs boson, the particle dreamed up in the early 1960s to explain how fundamental particles acquire mass. Theorists predicted that when two protons collide with sufficiently high energy, this elusive particle could burst into existence and then decay into two telltale photons. When the LHC was first switched on in 2009, much of the analysis was designed to pick that distinctive death rattle out of the deafening background noise.

鈥淵ou know what a particle should look like,鈥� says Noam Tal Hod at the Weizmann Institute of Science in Israel, 鈥渂ut the analysis is very, very complex.鈥� On 4 July 2012, all that hard work paid off. The LHC data revealed that a suspiciously high number of photon pairs were coming into existence with an energy of 125 gigaelectronvolts, corresponding to a likely value for the mass of the Higgs. Such a spike in the data, colloquially known as a bump, was exactly the signal physicists had been hunting for.

Cracks in the picture

The Higgs was the final piece of a puzzle called the standard model of particle physics, which paints a comprehensive picture of the known particles and forces in the universe. Its completion marked the end of a decades-long hunt, but not everyone was celebrating. All sorts of fundamental mysteries remained unsolved, suggesting that something beyond the standard model had yet to be found.

Many of these remaining problems boil down to one. Crudely phrased, some things are exceptionally small while related things are exceptionally big. This is known as the hierarchy problem, and once you spot it, you start seeing it everywhere.

Take the four fundamental forces of nature. The weakest two are gravity and the weak nuclear force, which only operates on the tiniest of scales and is responsible for certain types of radioactive decay. The weak force is weak, but compared with it, gravity is some 25 orders of magnitude weaker 鈥� a bizarre state of affairs that, as yet, has no good explanation.

鈥淣ature doesn鈥檛 care about our aesthetics. It doesn鈥檛 need to be beautiful鈥�

The asymmetry reappears elsewhere. Dark energy, the mysterious force that is causing the universe鈥檚 expansion to accelerate, is 120 orders of magnitude weaker than we would expect. Dark matter, which is the dominant form of matter in the universe, interacts very weakly with regular matter. Neutrinos, the lightest particles in the standard model, are thousands of times lighter than anything else.

These disparities are profoundly vexing to physicists, who prefer to see related parameters in a theory take broadly consistent values. This preference for 鈥渘补迟耻谤补濒苍别蝉蝉鈥� drives much theoretical speculation 鈥� some would say to a fault. 鈥淣ature doesn鈥檛 care about our aesthetics,鈥� says Craig, and reality does not need to be beautiful.

Still, in the lead-up to the LHC鈥檚 first collisions, natural solutions were in demand. Because the Higgs was supposed to endow all fundamental particles with mass, some calculations predicted that each of those particles would, in turn, increase the mass of the Higgs. That resulted in a particle 17 orders of magnitude more massive than physicists thought was likely, a hierarchy problem that desperately needed to be fixed. The most popular solution was to imagine that each particle had a heavier 鈥渟upersymmetric鈥� twin, whose interactions with the Higgs would perfectly cancel out all that excess mass. But bumps corresponding to supersymmetric particles never showed up at the LHC, at least not in the form we sought.

Ten years on, nothing has changed. We were fixated on supersymmetry for too long, says Isabel Garcia Garcia at the University of California, Santa Barbara, searching under the convenient street light to the detriment of the field. But the story of the LHC is far from over. The collider has recorded only 3 per cent of the data we expect it to collect in its lifetime, and an upgrade to higher energies in 2020 will further raise its chances of seeing something surprising.

But the LHC鈥檚 failure to break any new ground has emboldened a new generation to question the hunches that motivated previous searches. 鈥淭his optimism is most widespread amongst the youth,鈥� says Matthew McCullough, a theoretical physicist at CERN. 鈥淲e鈥檝e shaken off the cobwebs of the theories handed down by our PhD advisers.鈥�

Buried treasure

Instead of waiting for a new collider to be built to their specifications, they want to exploit an unparalleled resource at their disposal: more than 300 million gigabytes of archived data. So far, analysis of these collisions has focused almost exclusively on comparing it with the narrow predictions of the standard model or specific extensions, like supersymmetry. But there are other ways to look at the data.

In an ideal world, physicists could free themselves from preconceptions and then try to compare every aspect of every collision with perfect simulations based on the standard model. Any discrepancy, no matter how slight, would sound an alarm.

Such complete, unfiltered comparisons exceed our current computational abilities. What鈥檚 more, many of the predictions made by the standard model are not precise values, but approximations produced by simplifying complex calculations. That means the alarm could ring just because our maths was slightly off, leading to a potentially limitless sequence of false positives.

A practical way forward may come from the growing field of machine learning. While CERN has used it to some degree for decades, the field is rapidly expanding as computers become exponentially more powerful and algorithms evolve.

Maurizio Pierini, an experimentalist at CERN, envisions new automated searches that would flag unexpected events. The anomalies would probably number in the hundreds per month 鈥� a minuscule amount of data by comparison with the primary collection. Once humans had reviewed them, the algorithm would be taught to ignore the ones deemed to be false positives, limiting future detections of those types, while learning to seek more of the ones that had physicists genuinely baffled.

鈥淢achine learning is both promising and necessary,鈥� says Craig, but any full-blown attempt to sift through the data already collected is likely to take time. Rather than hunting for missing keys on every square inch of the street, some suggest we find a better light to search under.

Whichever theoretical hunch we follow next, it needs to be chosen with care. With particle physics only just breaking free of its decades-long obsession with supersymmetry, the last thing we need is to replace that set of blinkers with another. But one new candidate seems too tempting not to investigate. For increasingly many physicists, it ticks all the boxes, making sense of the mysteries that plague the standard model while leaving a signature that could be hiding in plain sight in the LHC data.

The work has its origins in research done in 2015 by two independent groups of physicists. They were trying to solve one of the hierarchy problems: why gravity is so much weaker than the other fundamental forces of nature. The way to bridge this gap, they decided, was to summon the particle equivalent of a mechanical clock.

Thanks to the gears nestled within every old-fashioned timepiece, each tick of the second hand yields a corresponding movement in the hour hand, allowing two very different timescales to be effortlessly connected. Replace the gears with particles, each capable of interacting with its nearest neighbours, and a bridge can be created between two areas of physics operating on vastly different scales.

鈥淚n and of itself, it鈥檚 very pretty,鈥� says McCullough. 鈥淭here鈥檚 an inherent beauty in the way the original clockwork arrived at an exponentially weak force.鈥� But McCullough realised the idea could do more. With his colleague Gian Giudice, head of CERN鈥檚 theory department, he extended the clockwork analogy across the universe, with infinite chains of gears connecting the puny force of gravity to the other fundamental forces.

鈥淎 bridge can be created between two vastly different areas of physics鈥�

Christophe Grojean, a theorist at DESY, an accelerator laboratory in Germany, is one of many impressed by the work鈥檚 power. 鈥淐lockwork could reveal a new hidden face of matter,鈥� he says.

Since then, the clockwork mechanism has taken off, with a spate of papers suggesting ways to apply it to many of the most frustrating problems in physics, from the identity of dark matter to the masses of neutrinos and beyond. Each iteration is a unique contraption tailor-made for the problem at hand. 鈥淭here鈥檚 a bit of a Rube Goldberg element to it,鈥� says Craig.

Clockwork isn鈥檛 the only mathematically sound and compelling way to answer the questions left unanswered by the standard model. What sets it apart, however, are its experimental predictions: if a clockwork mechanism exists, then the LHC would have failed to notice it.

The Higgs was easy to spot because it offered a clear bump in the data. Clockwork, however, predicts a series of new, very closely connected particles that appear 鈥渓ike a hair comb of bumps鈥�, says McCullough. That is precisely the kind of pattern physicists usually filter out as background noise.

Closer inspection

Teasing out those signatures calls for a radical shift in perspective: rather than picking out the tallest peaks in a distant mountain range, we need to pay more attention to the landscape as a whole. Such a shift would be truly valuable, says Garcia, potentially unearthing new results beyond the clockwork mechanism itself.

Performing the necessary searches would be easy, says Pierini. Indeed, Tal Hod has been sifting through LHC simulations 鈥� more tractable and closer to hand than the real data 鈥� testing whether distinctive clockwork-like signatures could have escaped our attention all this time. He says this is the first time anyone has hunted for something that seems to fluctuate, although such proposals have been floating around since 2012. And he has already found something in line with the clockwork idea, raising hopes that fuller analyses could have results to show sometime this year.

Put to the test, of course, the idea may not survive. But even a failing hypothesis can be inspiring 鈥� just ask the theorists who grew up in the long shadow of supersymmetry.

鈥淣obody knows where the new physics is going to be discovered,鈥� says McCullough. After all, we may be blind to more than just wiggles in our current search. Other theories posit long-lived particles that we cannot see directly and that do not decay into things we can see until long after they have passed through the LHC detectors. Complementary, small-scale experiments are under way, intended to expand the search for new physics in some of these alternative ways that the bigger colliders cannot. But many eyes ultimately remain on the LHC.

鈥淭he LHC is a giant battleship,鈥� says Craig. 鈥淓very time you go into battle, many of these small ships may have the first skirmish, but the battleship wins the war.鈥�