Nature News reports Has a Hungarian physics lab found a fifth force of nature? "A laboratory experiment in Hungary has spotted an anomaly in radioactive decay that could be the signature of a previously unknown fifth fundamental force of nature, physicists say – if the finding holds up...Then, on 25 April, a group of US theoretical physicists brought the finding to wider attention by publishing its own analysis of the result on arXiv2. The theorists showed that the data didn’t conflict with any previous experiments – and concluded that it could be evidence for a fifth fundamental force."
The Hungarian team fired protons at thin targets of lithium-7, which created unstable beryllium-8 nuclei that then decayed and spat out pairs of electrons and positrons. According to the standard model, physicists should see that the number of observed pairs drops as the angle separating the trajectory of the electron and positron increases. But the team reported that at about 140º, the number of such emissions jumps — creating a ‘bump’ when the number of pairs are plotted against the angle — before dropping off again at higher angles.
Krasznahorkay says that the bump is strong evidence that a minute fraction of the unstable beryllium-8 nuclei shed their excess energy in the form of a new particle, which then decays into an electron–positron pair. He and his colleagues calculate the particle’s mass to be about 17 megaelectronvolts (MeV).
“We are very confident about our experimental results,” says Krasznahorkay. He says that the team has repeated its test several times in the past three years, and that it has eliminated every conceivable source of error. Assuming it has done so, then the odds of seeing such an extreme anomaly if there were nothing unusual going on are about 1 in 200 billion, the team says.
Feng and colleagues say that the 17-MeV particle is not a dark photon. After analysing the anomaly and looking for properties consistent with previous experimental results, they concluded that the particle could instead be a “protophobic X boson”. Such a particle would carry an extremely short-range force that acts over distances only several times the width of an atomic nucleus. And where a dark photon (like a conventional photon) would couple to electrons and protons, the new boson would couple to electrons and neutrons. Feng says that his group is currently investigating other kinds of particles that could explain the anomaly. But the protophobic boson is “the most straightforward possibility”, he says.