New Measurement of the Positronium Hyperfine Interval

The hyperfine interval Δν of the positronium ground state has been measured with significantly improved accuracy. Using a SF₆ buffer gas we obtain Δν=203.3875±0.0017 GHz and ${({Δ}{ν})}^{{-}1}{{∂}{Δ}{ν}}{{∂}D}=({-}8.0±{}0.4)×{}{10}^{{-}5}$ ${amagat}^{{-}1}$. Using a ${N}₂$ buffer gas we obtain ${Δ}{ν}=203.3854±{}0.0029$ GHz and ${({Δ}{ν})}^{{-}1}{{∂}{Δ}{ν}}{{∂}D}=({-}2.1±{}0.6)×{}{10}^{{-}5}$ ${amagat}^{{-}1}$. No evidence is found for a quadratic dependence of ${Δ}{ν}$ on density in either gas.