Earlier techniques cooled antiprotons with cold electrons, but the coldest antiproton temperature recorded with this method was about 100 kelvin. To turn down the heat, Hangst and colleagues used a technique called evaporative cooling, which had previously been used only for neutral atoms. “It’s exactly how your coffee cools itself,” Hangst says. “The steam above your coffee, those molecules are the hottest ones. They can escape from the coffee and carry away energy, so the coffee is absolutely colder.”
Meanwhile, protons are suddenly smaller: The proton shrinks in size
Pohl and his team have a come up with a smaller number by using a cousin of the electron, known as the muon. Muons are about 200 times heavier than electrons, making them more sensitive to the proton’s size. To measure the proton radius using the muon, Pohl and his colleagues fired muons from a particle accelerator at a cloud of hydrogen. Hydrogen nuclei each consist of a single proton, orbited by an electron. Sometimes a muon replaces an electron and orbits around a proton. Using lasers, the team measured relevant muonic energy levels with extremely high accuracy and found that the proton was around 4% smaller than previously thought.
Update (7/9) Chad has posted an excellent summary of the paper