For 20 years, physicists in Japan have monitored a 13-story-tall tank of pure water cloistered deep inside an abandoned zinc mine, hoping to see protons in the water spontaneously fall apart. In the meantime, a Nobel Prize has been won for a different discovery in the cathedral-esque water tank pertaining to particles called neutrinos. But the team looking for proton decays — events that would confirm that three of the four forces of nature split off from a single, fundamental force at the beginning of time — is still waiting.
“So far, we never see this proton decay evidence,” said Makoto Miura of the University of Tokyo, who leads the Super-Kamiokande experiment’s proton decay search team.
Different “grand unified theories” or “GUTs” tying together the strong, weak and electromagnetic forces make a range of predictions about how long protons take to decay. Super-K’s latest analysis finds that the subatomic particles must live, on average, at least 16 billion trillion trillion years, an increase from the minimum proton lifetime of 13 billion trillion trillion years that the team calculated in 2012.The findings, released in October and under review for publication in Physical Review D, rule out a greater range of the predicted proton lifetimes and leave the beloved, 1970s-era grand unification hypothesis as an unproven dream. “By far the most likely way we would ever verify this idea is proton decay,” said Stephen Barr, a physicist at the University of Delaware.
Without proton decay, the evidence that the forces that govern elementary particles today are actually splinters of a single “grand unified” force is purely circumstantial: The three forces seem to converge to the same strengths when extrapolated to high energies, and their mathematical structures suggest inclusion in a larger whole, much as the shape of Earth’s continents hint at the ancient supercontinent Pangea.
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