Simulated symmetries

One hope is that low-energy analogues could reveal new ways of detecting the Higgs in high-energy collisions. But more importantly, they might help in discovering new physics. The Higgs turns out to comply very neatly with all of the standard model’s predictions – and this is not necessarily a good thing. There is much that the standard model does not explain, such as the nature of the dark matter that seems to make up most of  the mass in the universe, or the dark energy thought to account for the universe’s apparently accelerating expansion – and indeed why there is much more matter than antimatter in the universe. Supersymmetry (SUSY) is a framework that makes a better fist of some of these things, making it a favoured next-generation theory. One of its central predictions is that there should be more than one Higgs particle. Last year Grigori Volovik of Aalto University in Finland and Mikhail Zubkov of the Institute for Theoretical and Experimental Physics in Moscow, Russia, think they might have found some clue as to where those extra particles might be – in superfluid helium-3. The purity of this rare form of helium makes it highly Long-sought magnetic monopoles are popping up in all sorts of analogues – but do they exist in nature?

prized for the study of delicate quantum processes. The discovered Higgs weighs in at around 125 gigaelectronvolts (GeV). Studying the spectrum of excitations in the superfluid helium suggests Higgs particles should also exist at energies of 210 GeV and 325 GeV. These possibilities are not excluded by results collected so far at the LHC, although it is too early to say anything definite, says Zubkov (Physical Review D, vol 87, p 075016). Ashvin Vishwanath, a theorist at the University of California, Berkeley, thinks he can take things further. Supersymmetry rests on the idea that the standard model’s messy division of particles into fermions (which make up matter) and bosons (which transmit forces) can be replaced by a more symmetric representation in which every fermion has a boson partner and vice versa. “We had hoped to see signatures of SUSY at the LHC, but we haven’t, so that motivated us to look for other ways to realise it,” says Vishwanath. Those other ways are analogues. Strange things happen within materials around “phase transitions”, at which their atoms rearrange themselves and the material changes state. For instance, as a piece of molten iron cools from liquid to solid, the quantum-mechanical spins of its electrons all align, making the material magnetic. At closer to absolute zero, quantum fluctuations drive phase transitions that create other, exotic types of symmetry, such as the speed of electrons and the speed of sound-wave packets, known as phonons, travelling through a metal converging until they are identical. Since electrons are fermions and phonons are bosons, Vishwanath and his colleagues think this emerging symmetry could suggest the mechanisms by which supersymmetry emerges, potentially leading to lab experiments that could explore the finer points of the theory. They are currently discussing what type of analogue might best realise this kind of physics in the lab. That’s all well and good, but finding something in an analogue is no guarantee  that it exists in the real world, cautions Kibble. In the early 2000s, he was part of a pioneering initiative called COSLAB that aimed to foster the use of analogues to explore, among other things, his ideas about cosmic strings. These are defects in space-time that many theories predict would have appeared as the early universe expanded and cooled, or perhaps not. “They may well not exist,” says Kibble. A more current case in point is the magnetic monopole. In nature, anything magnetic always seems to have two poles. But as the physicist Paul Dirac showed in the 1930s, if independently moving magnetic poles were not created in the big bang, we are hardpressed to explain the explain the existence  of single, freely moving electric charges today. Monopoles are now popping up all over the place in analogues – first  in low-temperature crystals known as spin ice, and earlier this year in a superchilled cloud of rubidium atoms (Nature, vol 505, p 657). But do they exist  freely in nature? David Hall of Amherst College in Massachusetts, who did the rubidium experiments, is hedging his bets. “They will exist or not independent of our experiment,” he says. “But it is reassuring that the Dirac monopole structure can exist in nature.