A group of scientists who, in 2010, found a discrepancy in what was long believed to be the measurements of the size of a proton, have invalidated yet another measurement; that of a nuclear particle called the deuteron.
Protons are particles found inside the nucleus of atoms. According to measurements set by the international Committee on Data for Science and Technology (Codata) many years ago, the proton’s radius was recorded at about 0.877 femtometres. That’s just under a quadrillionth of a metre, in case you needed a reference point.
However, scientist Randolf Pohl, along with his colleagues from the Max Planck Institute of Quantum Optics in Garching, Germany, found a large discrepancy from the previously accepted value in 2010 and again in 2013 after using a new measurement technique.
Pohl and his team’s new technique involved analysing the energy-level shifts of muons orbiting hydrogen nuclei. Muons are unstable subatomic particles of the same class as an electron, but with a mass around 200 times greater.
By switching a hydrogen atom’s electron for the heavier muon particle, the researchers altered the one proton, one electron composition of the atom. After shooting the altered atom with a laser beam, the team measured the resulting change in its energy levels, which allowed them to calculate the size of its proton nucleus.
To their astonishment, Pohl and his fellow researchers’ proton measurements demonstrated a radius 4 per cent smaller than the Codata’s value. This gave rise to the “proton radius puzzle”, and the radius of the proton has remained a point of debate ever since.
But before scientists could fully work out why they have obtained this curiously low radius, they went and uncovered yet another discrepancy in Codata’s spectroscopy records.
Appearing in a study in Science, the scientists said they have now found that Codata’s radius measurement of a deuterium atom nucleus, known as a deuteron, could also be wrong after using the same new measurement technique.
A deuteron is the collective term for deuterium, an isotope of hydrogen atom that has one proton and one neutron at its nucleus.
By combining the measurements with theory, the researchers came up with a deuteron charge radius of 2.125 femtometres, which is several standard deviations smaller than the radius measurement set by Codata – 0.8 per cent smaller, to be precise.
“In addition, both the proton and deuteron sizes are in tension with the values obtained by applying the same technique to atoms with electrons rather than muons,” the report said, suggesting that the discrepancies are found in both deuterons and protons.
As a result of the findings, the team of scientists are left scratching their heads, and have concluded there’s no way of solving the problem with existing theories.
“This independent discrepancy points to experimental or theoretical error or even to physics beyond the standard model,” says the report.
Rather than simply proving Codata’s findings are mis-measured, the research increases the chance that something is amiss in the heart of atoms, and only makes the proton radius puzzle even more...puzzling.
This article was originally published by WIRED UK
