Alpha and helion particle charge radius difference determined from quantum-degenerate helium
Yuri van der Werf, Kees Steinebach, Raphaël Jannin, Hendrick L. Bethlem, K. S. E. Eikema
Abstract
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 2 3 S →2 1 S transition frequency in a Fermi gas of 3 He with an order of magnitude higher accuracy than determined previously. Together with a previous measurement in a 4 He Bose-Einstein condensate, a squared charge radius difference r h 2 – r α 2 = 1.0757(12) exp (9) theo fm 2 was determined between the helion and alpha particle. This measurement provides a stringent benchmark for nuclear structure calculations.