Quadratic hfs Pressure Shifts of Muonium in Argon and Krypton

The very precise, low-pressure measurements of the hfs interval Δν(p) 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 Δν=Δν(0)[1+ap+bp²], we find b(Ar)=7.1(2.3) and b(Kr)=8.7(3.1) (in units of 10^-15 Torr^-2). Older evidence for these quadratic shifts from similar fits involving earlier Chicago data is reviewed.