The size of the alpha particle, the nucleus of the helium atom, has been measured more accurately than ever before. Results now indicate a size 1.67824(83) femtometers, which is 4.8 times more precise than previous measurements. The researchers used laser spectroscopy of exotic helium ions, where the electron is replaced by a 200 times heavier muon.
How big is the alpha particle? Laser spectroscopy provides new results with record precision
