Higgs-adjacent particle discovered in “tabletop” experiment

We’ve grown accustomed to the discovery of new particles every couple of years. They either slot nicely into the Standard Model – the holy Bible of particle physics – or they give physicists endless migraines by refusing to adhere to our theories, such as the recent mass measurement of the W boson.

But when we think of such new particles, we also tend to think of the vast machines that are required to find them. The accelerators, colliders, synchrotrons and whizzy doohickies can be kilometres long.

Now, a team led by physicists at Boston College in the US claims to have found a new particle – or previously undetectable “quantum excitation” – in a room-temperature, “tabletop” experiment.

In research published in Nature, the team reports finding the magnetic offspring of the mass-defining Higgs boson called the axial Higgs mode. Quantum excitation is the fancy name given by particle physicists to particles. Particles are thought of as quantum excitations of the various fields that permeate the universe. The Higgs boson, for example, is the quantum excitation of the Higgs field, which defines mass. And the photon is the quantum excitation of the electromagnetic field.

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Unlike the Higgs boson, famously discovered a decade ago, the axial Higgs mode has a magnetic moment. That means the new particle produces a magnetic field. It also means the new particle requires a more complicated theory to explain its properties.

The kinds of theories that might explain the axial Higgs mode are also invoked to explain dark matter, the theorised invisible stuff five times more plentiful in the universe than ordinary matter.

“It’s not every day you find a new particle sitting on your tabletop,” says Kenneth Burch, a lead author of the paper and a Boston College physics professor.

The team included a broad range of scientific experts from around the world and a range of fields. “This shows the power of interdisciplinary efforts in revealing and controlling new phenomena,” says Burch. “It’s not every day you get optics, chemistry, physical theory, materials science and physics together in one work.”

Focusing on RTe3, or rare-earth tritelluride – a well-understood quantum material – the team developed a room temperature “tabletop” experimental format to search for new quantum excitations. RTe3 has properties that mimic the theory that produces the axial Higgs mode. Such “tabletop” experiments are small-scale, usually cheap, and sensitive experiments that are dwarfed by the large-scale setups such as the Large Hadron Collider at CERN.

But Higgs particles don’t play nice. They are very good at eluding experimental probes. The kinds of experiments usually required to reveal their subtle quantum properties include enormous magnets, high-powered lasers, and temperatures approaching absolute zero.

Overcoming these challenges, says Burch, was a matter of using scattered light and a quantum “simulator” to mimic the properties required for the study. Enter RTe3, which has a “charge density wave” – a state in which electrons self-organise periodically. Because of the special properties of the material, the team was able to probe Higgs particles with additional axial components, meaning they contain “angular momentum” – that is, momentum associated with spinning and orbiting particles.

The team shined a laser on the material to scatter the photons. As the light hit the medium, it changed colour and polarisation. The colour change was a result of the light generating the Higgs particle in the material, and the polarisation occurred due to the particle’s symmetries. Changing the polarisation created the particle with different properties such as absent magnetism.

“As such, we were able to reveal the hidden magnetic component and prove the discovery of the first axial Higgs mode,” says Burch.

“The detection of the axial Higgs was predicted in high-energy particle physics to explain dark matter. However, it has never been observed. Its appearance in a condensed matter system was completely surprising and heralds the discovery of a new broken symmetry state that had not been predicted.

“Unlike the extreme conditions typically required to observe new particles, this was done at room temperature in a tabletop experiment where we achieve quantum control of the mode by just changing the polarisation of light.”

Burch believes the simple setup provides opportunities for using similar experiments in other areas. “Many of these experiments were performed by an undergraduate in my lab,” he says. “The approach can be straightforwardly applied to the quantum properties of numerous collective phenomena, including modes in superconductors, magnets, ferroelectrics and charge density waves. Furthermore, we bring the study of quantum interference in materials with correlated and/or topological phases to room temperature, overcoming the difficulty of extreme experimental conditions.”

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