Physicists discover new elusive particle through tabletop experiment

An interdisciplinary team led by Boston College physicists has discovered a new particle — or a previously undetectable quantum excitation — known as the axial Higgs mode, a magnetic relative of the mass-defining Higgs Boson particle, the team reports in the journal Nature. Credit: Nature

Materials that contain the axial Higgs mode could serve as quantum sensors to evaluate other quantum systems and help answer lingering questions in particle physics.

According to the Standard Model of Particle Physics, scientists’ current best theory for describing the most basic building blocks of the universe, particles called quarks (which make up protons and neutrons) and leptons (which include electrons) make up all known matter. . Force-carrying particles, which belong to a broader group of bosons, influence quarks and leptons.

Despite the Standard Model’s success in explaining the universe, it has its limitations. Dark matter and dark energy are two examples, and it’s possible that new particles, yet to be discovered, could eventually solve these puzzles.

Today, an interdisciplinary team of scientists led by Boston College physicists announced that they have discovered a new particle — or previously undetectable quantum excitation — known as the axial Higgs mode, a magnetic relative of the mass-defining Higgs Boson particle. The team published their report today (June 8, 2022) in the online edition of the magazine. Nature.

The detection of the much sought after Higgs boson a decade ago has become central to understanding mass. Unlike its parent, the axial Higgs mode has a magnetic moment, and this requires a more complex form of theory to explain its properties, said Boston College physics professor Kenneth Burch, co-lead author of the report “Axial Higgs Mode Detected by Quantum path interference in RTe3.”

Theories that predicted the existence of such a mode were invoked to explain “dark matter,” the nearly invisible material that makes up much of the universe but reveals itself only through gravity, Burch said.

The Higgs boson is the fundamental particle associated with the Higgs field, a field that gives mass to other fundamental particles such as electrons and quarks. The mass of a particle determines how much it resists changing its speed or position when it encounters a force.

While the Higgs Boson was revealed by experiments at a massive particle collider, the team focused on RTe3or rare earth tritelluride, a well-studied quantum material that can be examined at room temperature in an experimental “table” format.

“It’s not every day you find a new particle on your desk,” Burch said.

RT3 has properties that mimic the theory that produces the Higgs axial mode, Burch said. But the central challenge in finding Higgs particles in general is their loose coupling to experimental probes, such as beams of light, he said. Likewise, revealing the subtle quantum properties of particles often requires quite complex experimental setups, including huge magnets and high-powered lasers, while cooling samples to extremely cold temperatures.

The team reports that it overcame these challenges through unique use of light scattering and proper choice of quantum simulator, essentially a material that mimics the desired properties for study.

Specifically, the researchers focused on a compound known to have a “charge density wave,” that is, a state in which electrons self-organize with a periodic density in space, Burch said.

The fundamental theory of this wave mimics the components of the standard model of particle physics, he added. However, in this case, the charge density wave is quite special, it arises far above ambient temperature and involves the modulation of both charge density and atomic orbits. This allows the Higgs boson associated with this charge density wave to have additional components, i.e. it can be axial, meaning it contains angular momentum.

To reveal the subtle nature of this mode, Burch explained that the team used light scattering, where a laser shines on the material and can change color and polarization. The color change results from light creating the Higgs boson in the material, while the polarization is sensitive to the symmetry components of the particle.

Furthermore, through proper choice of incident and output polarization, the particle can be created with different components – such as a missing magnetism or an upward pointing component. Exploring a fundamental aspect of quantum mechanics, they used the fact that, for a configuration, these components cancel out. However for a different setup they add.

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

“Axial Higgs detection was predicted in high-energy particle physics to explain dark matter,” Burch said. “However, this has never been observed. Its appearance in a condensed matter system was completely surprising and heralds the discovery of a new state of broken symmetry that had not been predicted. Unlike the extreme conditions normally required to observe new particles, this was done at room temperature in a tabletop experiment where we achieved quantum control of the mode just by changing the polarization of light.”

Burch said the seemingly accessible and straightforward experimental techniques deployed by the team could be applied to study in other areas.

“Many of these experiments were performed by a graduate student in my lab,” Burch said. “The approach can be applied directly to the quantum properties of various 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 at room temperature, overcoming the difficulty of extreme experimental conditions.

In addition to Burch, Boston College co-authors of the report included graduate student Grant McNamara, recent doctoral graduate Yiping Wang, and postdoctoral researcher Md Mofazzel Hosen. Wang won the American Physical Society’s Best Dissertation in Magnetism award, in part for his work on the project, Burch said.

Burch said it was crucial to tap into the wide range of expertise among researchers at BC, Harvard University,

Reference: “Axial Higgs Mode Detected by Quantum Pathway Interference in RTe3” by Yiping Wang, Ioannis Petrides, Grant McNamara, Md Mofazzel Hosen, Shiming Lei, Yueh-Chun Wu, James L. Hart, Hongyan Lv, Jun Yan, Di Xiao, Judy J. Cha, Prineha Narang, Leslie M. Schoop and Kenneth S. Burch, June 8, 2022, Nature.
DOI: 10.1038/s41586-022-04746-6

Funding: US Department of Energy

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