“The new claim now is boson with a mass of 16.7 MeV,” Naviliat-Cuncic said. (The third was a preliminary version of the Physical Review Letters paper.) The first two bumps have disappeared in the latest data, collected with an improved experimental setup. The first paper claimed evidence of a new boson of mass 12 MeV, and the second described an anomaly corresponding to a 13.45-MeV boson. The Atomki group has produced three previous papers on their beryllium-8 experiments-conference proceedings in 2008, 20. Because they do not get as big a forward push as the decay products of photons, the X boson’s remnants diverge at a wider angle, striking the detector at about 140 degrees. (The four known forces are electromagnetism, gravity, and the strong and weak interactions.) This slow-moving X boson, which has a mass of 16.7 million electron volts (MeV), splits into an electron-positron pair. But for every million photon decays, one time, Feng and his colleagues hypothesize, beryllium-8 yields a different kind of vector boson, temporarily dubbed X, which mediates an obscure new fundamental force. Sometimes, the photon immediately decays into an electron and a positron, which are propelled forward by the photon’s momentum and strike the detector near one another. In a paper of their own posted on in April of this year and submitted for publication, the Irvine team described the Atomki scientists’ result as follows: When a beryllium-8 nucleus undergoes a particular transition, it sheds energy and typically one unit of quantum spin-almost always in the form of a particle of light, or photon, which is the mediator of the electromagnetic force. (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.” Physicists desperately seek an extension of the Standard Model to account for dark matter, neutrino masses, unification of the forces and other mysteries. 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. 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. The proposed boson has become lunch-table talk in physics departments far and wide, and plans are afoot for testing the idea. 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.
That changed in April with a much-discussed paper by Jonathan Feng, a theoretical particle physicist at the University of California, Irvine, and colleagues.
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