Special Relativity and the Single Antiproton: Fortyfold Improved Comparison ofp¯andpCharge-to-Mass Ratios

The measured ratio of charge-to-mass ratios for the antiproton and proton is $1.0000000015\pm 0.0000000011$. This $1$ part in ${10}^9$ comparison ( $1$ ppb) is possible because a single $\overline{p}$ or $p$ is now directly observed while trapped in an open access Penning trap. The comparison is the most accurate mass spectrometry of particles with opposite charge, and is the most sensitive test of $\mathrm{CPT}$ invariance for a baryon system. It is 40 times more accurate than our earlier comparison with many trapped antiprotons and protons, and is more than 45 000 times more accurate than earlier comparisons made with other techniques.