Protons aren’t as heavy as we thought.

New Scientist looks closely at a tiny change with some big ramifications:

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.

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.

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.

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