Count all the known kinds of particles in the universe. Now double it. This is the promise of a family of theoretical models known as Supersymmetry, or SUSY for short.

The notion of theories predicting a doubling of observed particles may not be as bizarre as it seems. In fact, it has historical precedent with the story of antimatter.

“The first hints of antimatter came from Paul Dirac trying to solve problems in relativistic quantum mechanics,” says Laura Jeanty, who co-leads the Supersymmetry (SUSY) group on the ATLAS experiment at the Large Hadron Collider. “He came up with equations that essentially had four solutions instead of two, and the symmetries of the maths allow positive as well as negative values.” In 1928, Dirac concluded that if the negative values represented electrons, the positive values must represent an equivalent positively charged particle. The positron, or antielectron, was eventually discovered by Carl Anderson in 1932.

“At the time of Dirac’s theoretical work, however,” Jeanty adds, “it was a mathematical quirk that didn’t have any known physical reality.” Today, we have discovered antiparticles for all the charged particles in the “Standard Model” of particle physics – the best description we have of our universe at the quantum scale. The Standard Model, however, has important limitations, and SUSY provides a theoretical extension to it by introducing new mathematical symmetries.

Inexplicable hierarchies

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A CMS event display from 2016 containing 10 jets (orange cones) and a muon (red line), which is representative for the signatures that certain supersymmetry models would leave in the CMS detector. (Image: CMS/CERN)

As a scientific theory, the Standard Model is incredibly robust. Frustratingly so for physicists, because they are aware that this theory does not explain everything about the infinitesimal world of particles and quantum forces. Nevertheless, experimentalists have found no chinks in its armour, no deviations from its very accurate predictions, despite its limitations.

One such limitation is that the Standard Model accounts for only three of the known quantum forces in the universe: the strong, electromagnetic and weak forces; gravity is not in the mix. A symptom of this is known as the hierarchy problem, pertaining to the vast difference between the strengths of the strong, electromagnetic and weak forces on one hand and gravity on the other. Despite its name, the weak force is around 24 orders of magnitude (1024) stronger than gravity. But why does this matter?

“The hierarchy problem,” remarks Pieter Everaerts, Laura’s counterpart on the CMS experiment, “tells us that there have to be corrections to our current knowledge of physics.” The problem affects, for example, the mass of the Higgs boson that was discovered by ATLAS and CMS in 2012. According to quantum mechanics, the Higgs boson should have a mass several orders of magnitude higher than what was observed, because of its interactions with ephemeral virtual particles that pop in and out of existence.

SUSY provides an elegant theoretical solution to this problem. It does this by proposing the following: fermions – particles that make up matter – have force-carrying super-partners known as “sfermions”, while bosons – force-carriers in the Standard Model – are paired up with fermionic “bosinos”.

“With SUSY, the Higgs boson has twice as many particles to interact with,” adds Everaerts. Neatly, this allows the excess values of its expected mass coming from its interactions with ordinary particles to cancel out with the values of its interactions with supersymmetric particles. You are left with a predicted mass for the Higgs boson that is close to the observed mass of 125 GeV.

All that remains is the one small detail of finding at least one of the predicted SUSY particles.

Optimism at a new frontier

Before the LHC began colliding protons together, there was a buzz of expectation among experimental and theoretical particle physicists.

“It seems strange to say this now,” continues Everaerts, “but when the LHC began colliding protons in 2010, some were expecting us to discover six or seven SUSY particles immediately.”
Density of allowed supersymmetric models before and after the ATLAS Run-1 searches
The density of allowed supersymmetric models before and after ATLAS had searched through Run-1 data (data gathered up to February 2013)(Image: ATLAS/CERN)

There was even concern that too many SUSY particles (or “sparticles”) at a low enough mass would add to the “background”, or noise, and make it harder to study Standard Model processes at the LHC. This optimism had to quickly face reality when no sparticles manifested. Indeed, no deviations from the Standard Model have been observed in the nearly 15 million billion (15 000 000 000 000 000) proton–proton collisions that have taken place inside each of ATLAS and CMS.

Over the years, data collected by ATLAS and CMS enabled the collaborations to discard several of the simpler SUSY models, ruling out sparticles with masses up to around a teraelectronvolt (or TeV). All this showed, though, was that the most rudimentary interpretations of the theories were inadequate. “When we rule things out to a certain energy, we are aware that these are not realistic models, they are benchmarks,” says Federico Meloni of ATLAS. “And when you look at the same data through a less simplified interpretation, what we call an analysis in multidimensional parameter space,” he continues, “a limit of 2 TeV can become 500 GeV [gigaelectronvolt] or maybe we don’t have a limit at all. When looking at the big picture, it is only after ten years of operations that we are starting to be able to make interesting statements about the key issues.”

For the moment, in the absence of a discovery, SUSY remains firmly in the realm of theory alone. “Supersymmetry in the way we were thinking has been ruled out and we have to now look for it in a different way,” says Gian Giudice, head of CERN’s Theoretical Physics department. “We continue to advance techniques to search for supersymmetric particles,” Jeanty adds. “More LHC data will help us to look further into the challenging corners of phase space, where new physics could still be lurking.”

The LHC however is after more than just SUSY. Indeed, extensions of the Standard Model come in many forms, and on ATLAS and CMS the several teams performing searches for physics beyond the Standard Model are grouped together under the name “Exotics”. (The name “Miscellaneous” is quite obviously less exciting.) Some of these searches seem to come right out of science fiction…

Extra dimensions and micro black holes

Elementary particle physics concerns itself with the very small: tiny particles interacting through quantum forces at an unimaginably minuscule scale. Gravity, on the other hand, applies to the very large – think planets, stars and galaxies – and has sat apart from the quantum domain in our understanding of the universe. A quantum theory of gravity has remained the holy grail of high-energy physics for decades.

“If there is a quantum description of gravity, is there a particle that is responsible for mediating gravity?” asks Carl Gwilliam, a former coordinator of ATLAS exotic physics.

Discovery of such a particle, known as a graviton, would help settle the debate on the many theoretical models that attempt to unify gravity with the other three forces.

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A multi-jet event display observed by the CMS detector in 2015 in the search for microscopic black holes. (Image: CMS/CERN)

ATLAS and CMS are searching for gravitons directly, as for any other new particle, by looking for a bump in a smoothly falling distribution in the data. But, because the theories that predict the existence of a graviton also predict the existence of more than four dimensions of spacetime, the physicists are also looking for particles that are produced in collisions before they disappear into the extra dimensions. You cannot detect these disappearing particles directly; indeed, you cannot even ask the detectors to take snapshots of such collision events because there is nothing to “trigger” the detectors. You can, however, infer their presence by detecting an accompanying jet of particles produced from the same collision and observing simultaneously a lot of missing energy from the interaction itself. Niki Saoulidou, who co-leads the CMS Exotics team, points out that before the LHC was switched on, it was thought that these kinds of mono-jet searches, which also look for dark matter or supersymmetric particles, were too challenging for hadron-collider environments. “But we have evolved our tools, our techniques, our detector and our physics understanding so much that we now consider these as standard searches,” she says.

Another way of detecting extra dimensions made headlines before the LHC began operations: micro black holes. If produced in high-energy proton–proton collisions at the LHC, these tiny black holes would instantly evaporate, leaving behind multiple jets of particles. “The thing with black-hole searches is that they would be very spectacular!” remarks Gwilliam. “You would expect to see a black-hole event very early on in a new energy regime.” Since it began operations, the LHC has explored three new high-energy regimes: 7 TeV, 8 TeV and 13 TeV. ATLAS and CMS also searched at the lower energy of 900 GeV. “Unfortunately, these have not showed up in the data,” adds Gwilliam, “so we have set very strong limits on their existence.”

What does this mean for unifying gravity with the quantum forces? Saoulidou is philosophical: “It could be that we don’t have a quantum theory of gravity for a very good reason, which nature knows but we don’t.”

Nevertheless, those 15 million billion collisions that ATLAS and CMS have analysed make up only 5% of the total data volume the LHC will deliver over the course of its lifetime. The graviton could still be out there.

Mysterious missing pieces and uncanny coincidences

“Results from searches for exotic physics were at the forefront of work done at the start of the LHC era,” says Adish Vartak, former co-leader of the CMS Exotics team. There was a lot of potential for finding new physics, given that the LHC was operating at an energy of around four times higher than the previous highest-energy collider, the Tevatron at Fermilab.

“When the LHC started,” Vartak continues, “we wanted to see whether there was a new resonance – a new particle – at a few TeV or so, at energies that the Tevatron could not probe.”

It is not only the spectacular that particle physicists are after. Many of the searches conducted by the Exotics groups of ATLAS and CMS look for answers to particularly puzzling questions. For example, the data have so far shown that quarks are elementary particles; that is, they are themselves not made up of any particles. But we don’t know for sure if that is the case. Finding quarks in excited states at high energies would demonstrate that they have inner substructure. Another puzzle is that the two families of fermions – leptons and quarks – curiously come in three generations each. There is no particular reason for this, unless they are somehow related to one another. ATLAS and CMS are therefore looking for leptoquarks, particles predicted to be hybrids of both kinds of fermions.

Physicists are also looking for previously unobserved quantum forces, which would manifest in the form of heavier versions of the electroweak-force-carrying W and Z bosons, called W′ (“W-prime”) and Z′ (“Z-prime”). In the case of neutrinos, the reason for their extremely light but non-zero mass could be explained by discovering heavier exotic neutrinos, which balance the lighter regular neutrinos through a “see-saw” mechanism.

Other searches are for heavier Higgs bosons, charged Higgs bosons and even composite (non-elementary) Higgs bosons. Yet more focus on hypothesised magnetic monopoles (a single north or south pole) that, rather than bending in the high magnetic fields of ATLAS and CMS, would get accelerated through them.

Of course, experimentalists are also looking for any new particles and phenomena, even ones not explicitly predicted by theory. Giudice, a theorist, adds: “Experimentalists can make progress without a theorist telling them, ‘Oh, this comes from this model.’ Before the LHC, much of the analysis was done in terms of models. Now they try to present the data without relying on a specific model but rather on a broader language.”

The 750-GeV bump-that-was-not

Graph of axXiv submissions after December 2015
Graph showing the sharp rise in arXiv paper submissions after December 2015, as theorists attempted to explain the 750-GeV bump in the data (Image: André David)

This model-independent approach caused much excitement in 2015. Over the course of the first year of the LHC’s second run, both ATLAS and CMS began to notice something peculiar in their data. There appeared to be a slight excess of events in the two-photon channel at a mass of 750 GeV/c² in both their data sets. Initially the excess was of very low statistical significance, far from the 5-sigma threshold for claiming a discovery. Nevertheless it intrigued experimentalists and theorists alike. “As data began to be collected,” Vartak recounts, “there was a lot of hope that a new kind of physics – one that had evaded us previously – would show up.”

In December, at the annual end-of-year seminar from the LHC experiments, excitement reached fever pitch. ATLAS and CMS presented data showing the significance of the excess was around 3 sigma. In the following three months, around 300 papers were submitted to arXiv by theorists seeking to explain the inexplicable. By the time all of the data from Run 2 (2015–2018) were studied, the excess had evaporated. There is a reason physicists wait until the 5-sigma threshold is breached: smaller excesses are not unusual and are usually statistical fluctuations and low-significance flukes.

For the Exotics teams, though, it was the closest they came to something from beyond the Standard Model.

Changes in strategy and the road ahead

The lessons learnt so far are helping shape the search strategies of the future. For one, the triggers that select the collision data worth storing for further analysis are being recalibrated to handle so-called “long-lived particles”, which might transform into lighter particles outside of the typical timeframe when the triggers take their snapshots of collisions. Efforts are also underway to reanalyse the data recorded so far using novel techniques.

The challenges provide more than adequate motivation and inspiration. “I’ve always had a lot of fun with my research,” Everaerts says with a smile. “Collaborating with people from different backgrounds to work on a common goal: I find that amazing!”

So what is the legacy of the LHC after its first decade of operation? Giudice is emphatic: “The LHC has changed radically the way we view the world of particle physics today.” It might not have shown what theorists hoped it would, but it has helped make important strides in both theory and experiment. “When I start with an idea and then it turns out to be wrong, it’s not a question of failure; it is the scientific method,” Giudice continues. “You make a hypothesis, you check it with experiment, if it is right you keep on going but if it is wrong, you explore a different hypothesis.”

“As experimentalists,” Meloni adds, “when we search beyond the Standard Model, our job is to look for everything. We know that we look for something, we look for it everywhere, and chances are it’s not going to be there. Still, our job is to understand our measurement, our search, and get to the result. And then the results are up for interpretation.”

After all, looking and not finding is not the same as not looking.