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Scientists find precise proton size, smaller than expected

Researchers at York University in Toronto, Canada, have made a precise measurement of the size of a proton, marking a step closer to solving a puzzle in fundamental physics on Friday, September 6, 2019.


The study, published in the journal Science, reported a new measurement for the size of the proton at 0.833 femtometers, just under one trillionth of a millimeter.

The size of the proton became problematic in 2010 when a team of physicists found the proton-radius value to be four per cent smaller than expected.

The new measurement is approximately five per cent smaller than the previously accepted radius value from before 2010, confirming the 2010 finding that the proton is smaller than previously believed.

The 2010 study was the first to use muonic hydrogen to determine the proton size, compared to prior experiments that used regular hydrogen.

The muonic hydrogen is an exotic atom in which the electron is replaced by a muon, the electron’s heavier cousin.

The researchers from York University spent eight years to understand why the proton radius took on a different value when measured with muons, rather than electrons.

They conducted a high-precision measurement using a new technique with a fast beam of hydrogen atoms created by passing protons through a molecular hydrogen gas target.

The method allowed them to make an electron-based measurement of the proton radius that is directly analogous to the muon-based measurement. Their result agrees with the smaller value found in the 2010 study. The quest to resolve the proton-radius puzzle has far-reaching consequences for the understanding of the laws of physics, such as how light and matter interact in the quantum world.

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