What Is The Size Of A Proton

Illustration of hydrogen atom with single proton at nucleus and single electron orbiting it — The experiments used modified hydrogen atoms to get at the size of the proton

Fritz Goro/The LIFE Picture Drove/Getty Images

It'south a subatomic mystery with large implications. Vi years later physicists announced a bafflingly as well small measurement of the size of the proton, we're still not sure what'due south going on. With the release of new data today, the mystery has, if anything, got deeper.

Protons are particles found inside the nucleus of atoms. For years, the proton's radius seemed pinned down at well-nigh 0.877 femtometres, or less than a quadrillionth of a metre.

Merely in 2010, Randolf Pohl at the Max Planck Establish of Breakthrough Optics in Garching, Frg, got a worryingly different reply using a new measurement technique.

Pohl's team contradistinct the 1 proton, 1 electron composition of a hydrogen atom by switching the electron for a heavier particle called a muon. They so zapped this contradistinct cantlet with a laser. Measuring the resulting modify in its energy levels allowed them to calculate the size of its proton nucleus. To their surprise, it came out four per cent smaller than the traditional value measured via other means.

A 2013 measurement strengthened the finding, sending physicists searching for an explanation to the "proton radius puzzle".

Pohl's experiment as well practical the new technique to deuterium, an isotope of hydrogen that has one proton and one neutron – collectively known equally a deuteron – at its nucleus. Accurately calculating the size of the deuteron took enough of time, nonetheless.

"The merely thing that'due south going to allow us to solve information technology is new data"

Today, the squad have published their measurements, revealing that like the proton, the deuteron comes up short: in this case by 0.8 per cent.

These new numbers prove that the proton radius trouble isn't going abroad, says Evangeline J. Downie at the George Washington University in Washington DC. "Information technology tells united states of america that there'south notwithstanding a puzzle," says Downie. "It'southward however very open, and the only affair that's going to allow us to solve it is new data."

Several more experiments, at Pohl's lab and others, are already under way. One will return to the same muon technique to mensurate the size of heavier diminutive nuclei, like helium. Some other plans to simultaneously measure out the handful of muons and electrons.

Pohl suspects the culprit may not be the proton itself, but an incorrect measurement of the Rydberg abiding, a number that describes the wavelengths of calorie-free emitted past an excited atom. Simply this constant is well established through other precision experiments, so something desperate would have to have gone wrong.

Another explanation proposes new particles that cause unexpected interactions between the proton and the muon, without changing its relationship with the electron.

That could mean the puzzle is taking united states of america across the standard model of particle physics. "If at some point in the future, somebody will find something across the standard model, it would be like this," says Pohl, with first one small discrepancy, and then another and another, slowly edifice to a more awe-inspiring shift.

Periodical reference: Scientific discipline, DOI: 10.1126/science.aaf2468

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