Proton’s radius 4% smaller than previously thought
Scientists lobbed a bombshell into the world of sub-atomic theory on Wednesday by reporting that a primary building block of the visible Universe, the proton, is smaller than previously thought.
More precisely, revised measurements shave four per cent off the particle’s radius, according to a study in Nature that is highlighted on the journal’s cover.
That may not seem like much, especially given the proton’s infinitesimally tiny size.
But if borne out in further experiments, the findings could challenge fundamental precepts of quantum electrodynamics, the theory of how quantum light and matter interact, say its authors.
A team of 32 international scientists led by Randolf Pohl of the Max Planck Institute in Garching, Germany, had initially set out only to confirm what was already known rather than overturn time-honoured assumptions.
For decades, particle physicists have used the hydrogen atom as a benchmark for measuring the size of protons, which are part of the core of atoms.
The advantage of hydrogen is its unrivalled simplicity: one electron circles a single proton.
But this unit of measure turns out to have been wrong by a small but critical margin, if the paper is right.
“We didn’t imagine that there would be a gap between the known measures of the proton and our own,” admitted co-author Paul Indelicato, director of the Kastler Brossel Laboratory at the Pierre and Marie Curie University in Paris.
The new experiment — at least 10 times more accurate than any performed to date — was envisioned by physicists 40 years ago, but only recent developments in technology made it feasible.
The trick was to replace the electron in the hydrogen atom with a negative muon, a particle with the same electric charge but more than 200 times heavier and unstable to boot.
The muon’s larger mass gives muonic hydrogen a smaller atomic size and allows a much larger interaction with the proton. As a result, the proton’s structure can be probed more accurately than by using hydrogen.
Jeff Flowers, a researcher at Britain’s National Physical Laboratory in Teddington, near London, said the work could take the theories of particle physics into new territory. If confirmed, it would do even more than the multi-billion-dollar giant particle smasher at CERN in Switzerland to test the so-called Standard Model, which sets down the notional list of sub-atomic particles, he said in a commentary. Either the previously accepted measures upon which hundreds of calculations have been based are wrong, or there is a problem with the theory of quantum electrodynamics itself.
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