Observation Of A Quadratic Term In Hfs Pressure Shift For Muonium And A New Precise Value For Muonium Deltav

New measurements of $\\ensuremath{\\Delta}\\ensuremath{\\nu}$ 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, $\\ensuremath{\\Delta}\\ensuremath{\\nu}=4463.311(12)$ MHz.