Major questions in physics hang on the proton’s mass William Andrew/Getty

The proton has lost a little of its bulk. A fresh attempt to pin down its mass, with three times the precision of the previous best try, finds that the subatomic particle is 30 billionths of a per cent lighter than we thought.

All atoms contain at least one proton, which means measurements of its simplest characteristics – its size, charge and mass – can help answer some of the big questions in physics, including why the universe contains more matter than antimatter.

The international team behind the new result used instruments sensitive to parts per trillion. That’s comparable to a scale designed to weigh a grand piano being able to detect an eyelash falling on it.


The measurement took place in a 1.5-litre can with the air pumped out and cooled to nearly absolute zero. “The can is hermetically sealed, so there is no connection to outside world at all,” says Sven Sturm of the Max Planck Institute for Nuclear Physics in Germany, who led the effort.

An electron beam bombarded a plastic target inside the can, freeing protons. The team was able to trap a single proton in a combination of electric and magnetic fields, using a set-up known as a Penning trap.

The proton moved in circles in the magnetic field, and by measuring its velocity, the team could calculate its mass.

“These are very, very precise experiments, and they use very sophisticated methods,” says Peter Mohr, who was not involved in the work. Mohr is a member of the Committee on Data for Science and Technology (CODATA), the group that collects fundamental physics measurements and regularly publishes standard values for the scientific community to use.

Fine-tuning

The slimming down of the proton could help us fine-tune experiments that aim to understand why the amount of matter in the universe dwarfs the amount of antimatter, says Makoto Fujiwara, who works on CERN’s ALPHA experiment, seeking differences between hydrogen and its antimatter counterpart.

More precise measurements on the proton will allow researchers to look for smaller discrepancies between it and the antiproton, although Fujiwara points out similar precision in antiproton measurements will be needed for that.

What is 95% of the universe made of? Learn more at New Scientist Live in London

As for the tiny discrepancy between the new proton mass and the previously reported value, “in precision measurement, this is not so unusual”, Sturm says.

But no one is yet sure why the results disagree. It could be an indication of new physics – or simply an experimental error that the researchers overlooked, Mohr says. “Of course, 99 percent of the time, it’s an experimental issue,” he says. “We don’t break through new principles that often.”

Sturm’s group produced its measurement in time for CODATA’s latest physics standards, which will be published in a few months. Since we don’t know why this measurement differs from the last, CODATA has to carefully consider how to make use of the new value, Mohr says.

Sturm’s group plans to repeat and refine the measurement. “We will try to implement some new techniques which should improve the precision by a factor of six,” he says.

Reference: arxiv.org/abs/1706.06780