Thousandfold improvement in the measured antiproton mass

Comparisons of antiproton and proton cyclotron frequencies yield the ratio of inertial masses M(p¯)/M(p)=0.999 999 977 ±0.000 000 042. The fractional uncertainty of 4×${10}^{\mathrm{-}8}$ is 1000 times more accurate than previous measurements of this ratio using exotic atoms and is the most precise test of CPT invariance with baryons. Independent comparisons to electrons yield the mass ratios M(p¯)/M(${\mathit{e}}^{\mathrm{-}}$)=1836.152 660±0.000 083 and M(p)/M(${\mathit{e}}^{\mathrm{-}}$) =1836.152 680±0.000 088. Cryogenic antiprotons (near 4 K) stored in a Penning trap for 2 months establish directly a lifetime greater than 3.4 months.