911? It’s an emergency. The most important particle in the universe is missing. Florian Goertz knows this isn’t a case for the police, but he is still waiting impatiently for a response. This 911 isn’t a phone number, but a building on the northern edge of the world’s biggest particle accelerator.
CERN, which operates the 27-kilometre-long Large Hadron Collider (LHC) running underneath the France-Switzerland border, is best known for having confirmed the existence of the Higgs Boson in July 2012. That was a big deal. But it is nothing compared with the particle Goertz wants to search for there. The Higgs solved the problem of how other fundamental particles get their mass. Goertz’s baby would be capable of solving five outstanding problems in physics at one stroke, from the unexplained workings of subatomic particles to the mystery of the origin of the universe to the identity of the enigmatic dark matter that seems to govern its workings today. “Solving any of those problems would be considered a breakthrough worth a Nobel in physics,” says Giulia Zanderighi, a particle theorist based at the University of Oxford.
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Too good to be true? Perhaps, but Goertz isn’t alone in pursuing it. From his office at the Max Planck Institute for Particle and Astroparticle Physics in Heidelberg, Germany, he has been collaborating and competing with researchers around the world to describe its properties and predict just what it would be capable of doing. If they are right, the call from 911, where their predictions would be tested, would confirm the most important particle in the universe had been found.
To understand why this is so significant, it first helps to define what a particle is. Most of the time, we can get away with imagining particles as microscopic marbles, jostling around in empty space or tied together inside solid stuff. If we want to understand their behaviour, however, this view needs to be abandoned. Instead, it would be better to think of the universe as filled by fluctuating fields, whose disturbances on the quantum scale manifest themselves to us in the form of particles or forces.
“The Higgs Boson was a big deal, but it would be nothing by comparison”
Theorists conjure fields in and out of existence all the time in order to solve one fundamental problem or another. However, because each one produces a particle, hard experimental evidence can only be provided by a particle detector.
This was the case with the Higgs field and associated Higgs boson, originally proposed in 1964 to explain the short range of the weak nuclear force, giving fundamental particles mass in the process. It took almost 50 years before the Higgs was found. In the meantime, more theoretical leaks sprang up, only for physicists to plug them with more hypothetical particles, sometimes by the fistful.
In the theory of supersymmetry, for example, every known particle was gifted a more massive partner, a doubling that was supposed to solve three problems in particle physics at once. It was, perhaps, unduly profligate. Signs of these supersymmetric particles were meant to turn up at the LHC after it first turned on in 2009, and then again after its upgrade in 2015. None has yet been spotted.
Other theorists have been more parsimonious. In 2016, Guillermo Ballesteros at the University of Paris-Saclay in France proposed solving five mysteries by adding six extra particles, a hypothesis known as SMASH.
For Goertz and his collaborators, even this is too wasteful. Instead of dreaming up lots of new particles, they say, what if some of those that have already been theorised could be folded into one? It would certainly reduce the theorists’ embarrassment: rather than admitting they couldn’t find a whole slew of particles, they could focus on the hunt for one.
The result is a Swiss Army particle with an impressive range of tools geared to solve all manner of tricky problems. The first dates from 1977, when Roberto Peccei and Helen Quinn at Stanford University were confronting one of the most irritating problems in quantum chromodynamics, the theory that describes the interactions within protons and neutrons.
Called the strong CP problem, it arises because the strong force, which occurs inside protons or neutrons, should violate a type of symmetry known as CP symmetry in certain situations. The fact that we haven’t seen such a violation requires some explanation (see “The importance of symmetry”). Peccei and Quinn’s proposal to get around this was the equivalent of introducing a new field that counteracts the unseen symmetry violation.
In an attempt to confirm this field’s existence, Nobel laureate Frank Wilczek discovered that, as with all quantum fields that ripple vigorously enough, the new field created the potential for a new particle. Wilczek called it the axion.
A second problem in particle physics was emerging at around the same time. Colin Froggatt of the University of Glasgow, UK, and Holger Nielsen of the Niels Bohr Institute at the University of Copenhagen in Denmark were grumbling about quarks, the subatomic particles that band together to make up protons and neutrons and hence all matter. There are six quarks, but they vary hugely in mass: the top quark is around 80,000 times the mass of an up quark. “You just cannot understand it. You would expect all the masses to be roughly the same,” says Goertz. “If things are very different in nature, you want to understand why there is this large discrepancy.”
Froggatt and Nielsen proposed what they dubbed a flavon field to solve the problem. This field introduces a symmetry that is broken in different ways by different quarks. This results in the quarks having highly varying masses, a prediction that matches experimental results.
So how can two seemingly different particles combine into one? In the past couple of years, two groups of physicists have independently uncovered a cosmic coincidence: the particle produced by the flavon field has a component that looks a lot like an axion. “It turns out that if the axion is a component of the flavon field, it still solves the strong CP problem,” says Goertz. Meanwhile, the flavon simultaneously resolves the problematic quark masses.
And there are more benefits to this marriage of axion and flavon – known as the axiflavon or the flaxion, depending on who you speak to. Physicists puzzled by the large-scale structure of the universe have hypothesised since the late 1970s that it went through a period of super-fast inflation soon after the big bang. Such an expansion of space and time may have been triggered by a particle known as the inflaton, which some have speculated could actually be either the axion or the flavon. What’s more, the failure of standard searches for dark matter has focused more attention on the axion as an alternative dark matter candidate. Even in its new axiflavon guise, both uses are still on the table.
“It’s thrilling stuff, if for the moment only in the realms of conjecture”
If that wasn’t enough, the axiflavon may have an additional superpower. “We’re working on a bigger unification: the axiflavon with the Higgs,” says Goertz.
It is a remarkable theoretical leap, but one that would solve another fundamental problem. When the Higgs boson was discovered by the LHC in 2012, it was lighter than many physicists were expecting. The theory says it should be a whopping 1019 gigaelectronvolts, but it actually weighed in at just 125 GeV. No one can explain the 17 orders of magnitude difference. But a suitably configured axiflavon might just be able to keep the Higgs’s mass within bounds, a weight off many particle physicists’ minds.
In total, the axiflavon might be in a position to clean up five of the most troublesome problems in physics in one go: the unexpected conservation of CP symmetry in strong force interactions, the disparate masses of the quarks, the sudden inflation of the universe, the origins of dark matter and the surprisingly light mass of the Higgs. “It does seem to me remarkable that one can address so many outstanding problems of physics ‘beyond the standard model’ within such a simple framework,” says Wilczek.
It is thrilling stuff indeed, but for the moment it remains firmly in the realms of conjecture. The key to changing that lies within Building 911, home to CERN’s NA62 experiment. Not as well-known as some of the site’s bigger experiments, NA62 is nevertheless pursuing an important goal: improving our understanding of quarks by studying the decay of quark-antiquark pairs.
The hope is that NA62 finds something unexpected in the energies of these decay products, hinting at what lies beyond the standard model of particle physics. Because of the axiflavon’s properties, however, it could also pop into existence as an additional decay product in the process. The experiment is “very related to the analysis to be performed for axiflavon search”, says Babette Dobrich, one of NA62’s experimenters.
It is enough to get Goertz cautiously excited. “It makes me optimistic that it’s not impossible that we might find such a particle,” he says.
Fertile fields
When will we know for sure? Possibly soon. It all depends on exactly how heavy the axiflavon is. “The mass of the axiflavon is unknown, thus the point at which it would show up, if it exists, is unclear,” says Dobrich. If Goertz and his colleagues succeed in folding the Higgs particle into the axiflavon, however, the mass will be far more tightly constrained and the experimenters will know almost exactly where to look for it.
There are no guarantees. But a fondness for multi-tasking particles is catching on. Researchers at the Weizmann Institute of Science in Rehovot, Israel, have proposed their own Swiss army particle in the form of the hierarchion, which can accomplish more or less the same tasks as the axiflavon by different means.
All in all, it seems this is a fertile moment in particle physics. There is every reason to think that we could be on the verge of a breakthrough. “It seems overwhelmingly likely that there are new fields out there still to be discovered,” says physicist David Tong at the University of Cambridge. “This seems plausible on grounds of humility alone, but dark matter and inflation both strongly suggest the need for something new.”
While this generation of theorists let their imaginations run riot, the work of previous generations can rein in any excesses. “You can’t just imagine a new particle and assume it will work out and solve all your problems,” says Ballesteros. “There are theoretical and experimental constraints that have to be satisfied.”
With a wealth of new ideas that turn many problems into one, it is an exciting time to be a particle physicist, says Goertz. Especially since that little hut at CERN might find evidence that ends the emergency. Let’s hope his call to 911 pays off.
The importance of symmetry
Knowing where new particles may be hiding is a tricky business. The most powerful searchlight at our disposal is symmetry: the idea that something looks and behaves the same even when some aspect of its position, direction or orientation is changed. A circle, for example, has total rotational symmetry, while a square has broken rotational symmetry – it looks the same when you turn it through multiples of 90 degrees, but not under any other rotation.
When a symmetry is broken, physicists sit up and take notice. “Symmetry-breaking doesn’t just happen – there’s always some reason behind it,” says physicist David Tong at the University of Cambridge.
This is especially true of the more complicated symmetries that arise in particle physics. Rather than rotate a particle or move it about, you might swap it for its oppositely charged antiparticle. If you see no difference in their interactions, that is a symmetry. For example, two electrons repel each other in exactly the same way as two positrons do, displaying identical physics under the reversal of charge. We call that charge symmetry.
Flipping important
Another key symmetry is time reversal. Imagine you are watching a recording of a snooker match on TV, and you see a ball glance off the cushion. You press rewind, and you now see the ball move back towards the cushion. If it comes off at a different angle to what you saw before, time-reversal symmetry is broken.
The third fundamental symmetry is parity symmetry. This time, while watching the snooker match you see the initial shot reflected in a mirror. If it comes off the cushion at a different angle in the mirror view, this breaks parity symmetry.
Sometimes you don’t have to actually spot a symmetry being broken – just where it should be broken but isn’t. In 1964, for instance, Murray Gell-Mann applied symmetry considerations to the standard model of particle physics and came up with the idea that there should exist a set of particles that, put together in certain ways, would make the protons and neutrons we find in the atomic nucleus. Gell-Mann’s mathematical hunch was right on the money: his conjectured “quarks” were found in subsequent particle searches, and Gell-Mann won a Nobel prize in physics for his efforts.
To push things further, we can combine some of these symmetries and see whether anything interesting emerges. Take charge and parity (CP) symmetry, for example. Systems that might violate one or other actually end up looking the same if you impose both transformations. In other words, you can find two different perspectives, like a real particle and its antimatter equivalent reflected in a mirror, in which things appear to behave in the same way.
These symmetries matter, largely because we like to see them broken sometimes: the laws, particles and forces of physics all have their roots in symmetry-breaking. They create what David Gross of the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara, calls the “texture of the world”. These considerations have led Florian Goertz at the Max Planck Institute for Particle and Astroparticle Physics in Heidelberg to propose the existence of a new particle that is single-handedly capable of cleaning up five of the stickiest problems in physics. “Complete symmetry is boring,” says Goertz. “If symmetry is slightly broken, interesting things can happen.”
This article appeared in print under the headline “The key to it all”
