PARTICLE TO UNLOCK THE UNIVERSE
While we know an awful lot about the workings of the cosmos, there are some big, unanswered questions – could a single hypothetical particle solve them all?
Cosmology and particle physics are arguably two of the most mindboggling fields in all of science, seeking to explain the fundamental workings of our universe on very different scales. So far, the 21st century has seen huge advances in both these allied quests, with cosmologists confirming the existence of dark energy and making further advances in the understanding of mysterious dark matter. Meanwhile, particle physicists have pieced together the missing pieces of the ‘Standard Model’, a long-standing theory explaining the structure of the matter that makes up our visible universe.
But there are still some big questions left for scientists in both fields. For cosmologists, one of the biggest puzzles is the nature of dark matter – we may now understand many of its properties and behaviours, but we still don’t know what it is. Another question surrounds the driving force behind inflation – the sudden expansion of the newborn universe in the first microsecond after the Big Bang.
Meanwhile, particle physicists wrestle with annoying loose ends at the fringes of the Standard Model. Why do the six distinct quarks – particles that bind together to create more familiar particles such as protons and neutrons – have wildly different masses? How does the famous Higgs boson – associated with the process that gives other particles their mass – originate, and why does this particle display less than 100 million-billionth of the mass and energy predicted by theory?
Is it possible that a single new theory could resolve all these annoying questions and perhaps provide a way of finally uniting the physics of the smallest and largest scales? That’s the remarkable possibility emerging from recent theoretical breakthroughs at theoretical hubs such as the
Max Planck Institute (MPI) for Nuclear Physics in Heidelberg, Germany. And while various theories proposed to ‘patch’ the Standard Model so far have called for anything from half a dozen new particles to an entire supersymmetric mirror-universe of undiscovered stuff, the new theory relies on just a single new subatomic particle. Different researchers have proposed various names for this elusive particle, but so far the ‘axiflavon’ seems to have gained most traction.
This curious name combines those of two other hypothetical particles of longer standing
– the ‘axion’ and the ‘flavon’. Both proposed independently as long ago as the 1970s, each aimed at solving its own distinct problem in the Standard Model.
Flavons, to put it as simply as possible, are associated with the ‘flavon field’, a hypothetical force field affecting quark particles that make up the protons and neutrons in the cores of atoms. Quarks come in six different flavours, but can sometimes change from one flavour to another using the so-called weak interaction, one of nature’s four known fundamental forces; this is how atoms change their identity in certain forms of radioactive decay. In order to explain how the different quark flavours display wildly different masses, physicists suggested another layer of complexity on top of the Standard Model – a ‘flavon field’ that linked quarks with the mass-producing Higgs boson. Interactions in this new field would be carried by a new ‘messenger particle’ known simply as the flavon.
At around the same time, other physicists were puzzled over a problem with the ‘strong interaction’, an even more powerful nuclear force that binds particles together in the atomic nucleus. Known as the ‘strong CP’ problem, the issue was just why certain strong-force interactions are symmetric when the Standard Model suggests that they shouldn’t be.
According to the Standard Model the strong interaction is expected to break so-called ‘CP’ or ‘charge-parity’ symmetry – a rule that interactions will look the same if the electric charges of the particles involved are reversed, while their spatial coordinates are also flipped. The fact that the strong interaction does not break symmetry in this way led physicists Roberto Peccei and Helen Quinn at Stanford University to hypothesise another set of unseen interactions taking place at the tiniest scales involving a previously undetected force field keeping things in line. Princeton physicist Frank Wilczek extended the theory by suggesting that the force field should have an associated particle, for which he coined the name ‘axion’.
At first glance the axion and flavon seem to have little in common, but the world of nuclear physics is strange, and particles and forces that seem very different in everyday circumstances can reveal unexpected similarities at high temperatures and energies, such as those that occurred in the Big Bang itself. By colliding particles at close to the speed of light in accelerators such as the Large Hadron Collider (LHC), physicists have found