spaceSpace and Physics

Protons Are Very Slightly Lighter Than We Thought


Jonathan O'Callaghan

Senior Staff Writer


How heavy is a proton? It might sound like a bit of a stupid question, but we can actually figure it out with some sciency hocus pocus. And some researchers have done just that, finding it to be lighter than we thought.

Researchers from the Max Planck Institute for Nuclear Physics in Germany found it was lighter by about one part in 10 billion. The result itself has a precision to about 32 parts per trillion. Yep, that’s pretty small.


“We performed the most precise measurement of the atomic mass of the proton,” the researchers write in their paper, which is pre-printed on arXiv. “Our measurement is a factor of three more precise compared to the current literature value.”

To make the measurement, the scientists held a proton in a Penning trap, which involves using magnetic and electric fields to keep it trapped. It follows a looping orbit, and by comparing its velocity to an atom of carbon-12, they were able to work out the mass of the proton in atomic units. That came in at 1.007276446583 atomic units.

“These are very, very precise experiments, and they use very sophisticated methods,” Peter Mohr from the Committee on Data for Science and Technology (CODATA), who was not involved in the research, told New Scientist.

The experiment was performed inside a 1.5-liter can, which had its air pumped out and had been cooled to nearly absolute zero. To free protons inside the can and ultimately trap one, an electron beam was fired at a plastic target.


Aside from just being kind of interesting, there are a number of applications for this research. Physics World notes it could help solve the “proton radius puzzle”, namely why the radius of a proton appears smaller than expected. It could also tell us why there’s more matter than antimatter in the universe.

Don’t go ripping up your old proton measurement textbooks just yet, though. It’s noted that this result could be simply an experimental issue, rather than some new physics. The researchers don’t yet have a good explanation for why this measurement differs from previous ones. However, at the moment they are standing by their measurement.

“In a set of carefully conducted cross-check measurements we have confirmed a series of other literature values and were not able to track any yet uncovered systematic effects imposed by our method,” they wrote.


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