Higher-Precision Determination of the Fine-Structure Interval in the Ground State of Positronium, and the Fine-Structure Density Shift in Nitrogen

The fine-structure interval Δν in the ground state of positronium has been remeasured with higher precision to obtain Δνₑₓₚₜ=203.396±0.005 GHz (24 ppm). The current theoretical value is Δνₜₕₑₒᵣ=203.4155 GHz. The difference Δνₑₓₚₜ-Δνₜₕₑₒᵣ=-19±5 MHz is about 4 standard deviations of the experimental error, indicating the need for calculations of higher-order radiative contributions. Values are given for both the linear and quadratic coefficients of the fine-structure density shift in N₂.