Quadratic hfs Pressure Shifts of Muonium in Argon and Krypton

The very precise, low-pressure measurements of the hfs interval $\Delta \nu \mathrm{}\left(p\right)\mathrm{}$ of muonium in Ar and Kr described in the preceding Letter are combined with the high-pressure measurements from Yale to determine the magnitude of quadratic pressure shifts. Fitting by the form $\Delta \nu =\Delta \nu \mathrm{}\left(0\right)[1+ap+b{p}^2]$, we find $b\left(\mathrm{Ar}\right)=7.1\mathrm{}\left(2.3\right)\mathrm{}$ and $b\left(\mathrm{Kr}\right)=8.7\mathrm{}\left(3.1\right)\mathrm{}$ (in units of ${10}^{-15\mathrm{}}$ ${\mathrm{Torr}}^{-2\mathrm{}}$). Older evidence for these quadratic shifts from similar fits involving earlier Chicago data is reviewed.