Observation of a Quadratic Term in the hfs Pressure Shift for Muonium and a New Precise Value for MuoniumΔν

Abstract
New measurements of Δν for muonium in argon at low pressures and at very weak magnetic field provide the first observation for muonium of a quadratic term in the hfs pressure shift and lead to a new precise value for the hfs interval in muonium, Δν=4463.311(12) MHz.