From WIRED: “The Mysterious X Boson Could Upend the Standard Model. If It Actually Exists”

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Natalie Wolchover

This experimental apparatus at the Institute for Nuclear Research at the Hungarian Academy of Sciences in Debrecen was used to detect possible evidence of a new particle.Courtesy of Attila Krasznahorkay

Last year, a team of nuclear physicists in Hungary observed an anomaly in the decays of excited beryllium-8 atoms—an unexpected preference for spitting out pairs of particles with a particular angle of separation. The bump in the physicists’ data was unmistakable, with odds of less than one in 100 billion of arising by chance. Reporting the anomaly in Physical Review Letters in January, the researchers argued that it could signify the existence of a new fundamental particle. But at first, few took notice.

That changed in April with a much-discussed paper by Jonathan Feng, a theoretical particle physicist at the University of California, Irvine, and colleagues. After spending months translating the nuclear physics finding into the language of particle physics and ensuring that no particle physics experiments contradicted it, the Irvine team determined that the beryllium-8 anomaly is “beautifully” explained by the presence of a previously unknown “vector boson”—a type of particle that would wield a little-felt fifth force of nature.

The proposed boson has become lunch-table talk in physics departments far and wide, and plans are afoot for testing the idea. If the particle is confirmed, it would be a definite “ticket to Stockholm” that “would completely upend our understanding of the universe,” said Jesse Thaler, a theoretical particle physicist at the Massachusetts Institute of Technology. Unlike the Higgs boson—the particle discovered in 2012 that was the last missing piece of the Standard Model of particle physics—this unforeseen boson and accompanying force would lead the way to a more complete theory of nature.

The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth

Physicists desperately seek an extension of the Standard Model to account for dark matter, neutrino masses, unification of the forces and other mysteries. (In a forthcoming paper, the Irvine team will propose a Standard Model extension that includes the new boson.) Stressing that he has a high bar for experimental anomalies after seeing many bumps come and go in the past, Feng said, “I’m more excited than I’ve been about things for a long time.”

Remarkably, whereas the world’s biggest supercollider was needed to produce the heavy Higgs boson, the hypothetical Hungarian boson is so lightweight, with a mass only 34 times that of the electron, that it could have turned up in experiments decades ago. If it really exists, how did it go unnoticed for so long? Most experts will remain skeptical until further evidence of the particle rolls in—even Feng. “It’s a huge claim to say a fifth force has been discovered, and I recognize that,” he said. “Obviously, you need to check it.”

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