The $2{}^{3}S–2{}^{3}P$ transition of ${}^4\mathrm{He}$ was measured by comb-linked laser spectroscopy using a transverse-cooled atomic beam. The centroid frequency was determined to be 276 736 495 600.0(1.4) kHz, with a fractional uncertainty of $5.1\times {10}^{-12}$. This value is not only more accurate but also differs by as much as $-49.5\mathrm{kHz}$ ($20\sigma$) from the previous result given by [Cancio Pastor et al., Phys. Rev. Lett. 92, 023001 (2004); Cancio Pastor et al.Phys. Rev. Lett.97, 139903(E) (2006); Cancio Pastor et al.Phys. Rev. Lett.108, 143001 (2012)]. In combination with ongoing theoretical calculations, this work may allow the most accurate determination of the nuclear charge radius of helium.

• Received 18 September 2017

DOI:https://doi.org/10.1103/PhysRevLett.119.263002