Alpha and helion particle charge radius difference determined from quantum-degenerate helium

Accurate spectroscopic measurements of calculable systems provide a powerful method for testing the Standard Model and extracting fundamental constants. Recently, spectroscopic measurements of finite nuclear size effects in normal and muonic hydrogen resulted in unexpectedly large adjustments of the proton charge radius and the Rydberg constant. We measured the 23S→21S transition frequency in a Fermi gas of 3He with an order of magnitude higher accuracy than determined previously. Together with a previous measurement in a 4He Bose-Einstein condensate, a squared charge radius difference rh2 – rα2 = 1.0757(12)exp(9)theo fm2 was determined between the helion and alpha particle. This measurement provides a stringent benchmark for nuclear structure calculations.