Last year, Hungarian physicists discovered a new subatomic particle that wasn't predicted by the Standard Model -- a theoretical framework that explains how the basic building blocks of matter interact, governed by four fundamental forces. Aptly named "boson X", the new particle was initially thought to be a matter particle or a dark photon, but it was soon clear it was neither. Now, a new study performed by physicists at the University of California, Irvine, seems to add weight to a hypothesis that might change modern physics forever -- the boson x might be a force-carrier for a fifth fundamental force of nature.
Finding the unpredictable
The four fundamental forces are gravity, electromagnetism and the weak and strong nuclear forces. Each force has a corresponding boson or force carrier that gives rise or mediates the forces between other particles. For instance, the electromagnetic force is carried by photons -- perhaps the most famous particles -- while weak and strong forces are carried by W and Z bosons, and gluons, respectively. Though we've yet to find a force carrier particle for gravity, physicists predict there should be one, for now hypothetically called a graviton.
In mid-2015, physicists working at the Hungarian Academy of Sciences were looking for dark photons -- another hypothetical particle that's supposed to be connected to dark matter, the elusive matter that makes up 85 percent of the universe's mass but for all intents and purposes has remained invisible to our scrutinous eyes and instruments thus far.
Attila Krasznahorkay, one of the lead researchers from the lab in Debrecen, Hungary, was looking for dark photons that might finally uncover dark matter to the world. What they found was entirely different. After firing protons at thin targets of lithium-7, the interaction formed unstable beryllium-8 nuclei which decayed into pairs of electrons and positrons (antielectron or the antimatter counterpart of the electron). Against predictions made by the standard model, the Hungarian physicists found a minute fraction of the unstable beryllium-8 nuclei shed their excess energy in the form of a new particle which is only 30 times heavier than an electron.
“We are very confident about our experimental results,” Krasznahorkay told Nature in 2015, adding that his team has repeated the experiment many times over the last three years and have eliminated every conceivable source of error.
The new particle, called a “protophobic X boson”, where X stands for unknown, acts over extremely short distances only several times the width of an atomic nucleus.
Later, a group from UCI looked at the data and concluded the results didn't conflict with any previous experiments and, moreover, could be considered evidence for a tentative fifth force of nature. This conclusion is based on the fact that the new particle interacts only with electrons and neutrons, not electrons and protons as normal electric forces do. "There's no other boson that we've observed that has this same characteristic," said Analysis co-author Timothy Tait, professor of physics & astronomy.
"If true, it's revolutionary," said Jonathan Feng, professor of physics & astronomy. "For decades, we've known of four fundamental forces: gravitation, electromagnetism, and the strong and weak nuclear forces. If confirmed by further experiments, this discovery of a possible fifth force would completely change our understanding of the universe, with consequences for the unification of forces and dark matter."
"The experimentalists weren't able to claim that it was a new force," Feng said. "They simply saw an excess of events that indicated a new particle, but it was not clear to them whether it was a matter particle or a force-carrying particle."
Right now, like any bold claim, we can only speculate as to the true nature of this newfound boson and its relation with known physics. Because the boson is so light, only a few labs around the world have the resources to experiment and confirm or disprove it. It might take a couple of years. If true, then the standard model of physics might have to be re-written to include the new boson -- one that possibly works in the playing field of dark matter.
"It's possible that these two sectors talk to each other and interact with one another through somewhat veiled but fundamental interactions," Feng said. "This dark sector force may manifest itself as this protophobic force we're seeing as a result of the Hungarian experiment. In a broader sense, it fits in with our original research to understand the nature of dark matter."
Reference: Jonathan L. Feng, Bartosz Fornal, Iftah Galon, Susan Gardner, Jordan Smolinsky, Tim M. P. Tait, Philip Tanedo. Particle Physics Models for the 17 MeV Anomaly in Beryllium Nuclear Decays. Physical Review Letters, 2016
