Antiprotons in a Penning trap: A new measurement of the inertial mass

Antiprotons from the Low Energy Antiproton Ring (LEAR) at CERN with a kinetic energy of 5.9 MeV are passed through matter and captured in a cylindrical Penning trap. Using electron cooling, they are brought into thermal equilibrium with the 4 K trapping environment and have been stored for up to 2 months directly establishing the antiproton lifetime to be greater than 103 days. The inertial masses of the antiproton are compared by direct nondestructive measurements of their oscillatory motions in a Penning trap. An open endcap cylindrical trap produces a high purity quadrupole potential and allows antiprotons to be trapped and cooled. A self-shielding superconducting solenoid compensates the fluctuating magnetic field in the accelerator hall. The measured mass ratio is mbarp/mp = 0.999 999 977(42). The achieved accuracy of 4.2 x 10-8 is one thousand times more precise than obtained with previous techniques, and is the most stringent test of CPT invariance with baryons. As part of a thorough systematic study, independent comparisons to electrons yield the mass ratios mbarp/me- = 1836.152 648(89) mp/me- = 1836.152693(88)