High-precision comparison of the antiproton-to-proton charge-to-mass ratio

The CPT theorem (the assumption that physical laws are invariant under simultaneous charge conjugation, parity transformation and time reversal) is central to the standard model of particle physics; here the charge-to-mass ratio of the antiproton is compared to that of the proton, with a precision of 69 parts per trillion, and the result supports the CPT theorem at the atto-electronvolt scale. The CPT theorem, the assumption that physical laws are invariant under simultaneous charge conjugation, parity transformation and time reversal, is central to the standard model of particle physics. Consequently, precision tests of the CPT theorem are a window onto the physics beyond the standard model. Here, Stefan Ulmer et al. test the CPT theorem by measuring whether particles and antiparticles, apart from a sign change, are identical. In a Penning-trap measurement, they compare the antiproton charge-to-mass ratio to its proton counterpart, showing that the CPT theorem holds at the atto-electronvolt scale. Their experiment improves the precision of previous proton–antiproton mass comparisons by a factor of four. Invariance under the charge, parity, time-reversal (CPT) transformation1 is one of the fundamental symmetries of the standard model of particle physics. This CPT invariance implies that the fundamental properties of antiparticles and their matter-conjugates are identical, apart from signs. There is a deep link between CPT invariance and Lorentz symmetry—that is, the laws of nature seem to be invariant under the symmetry transformation of spacetime—although it is model dependent2. A number of high-precision CPT and Lorentz invariance tests—using a co-magnetometer, a torsion pendulum and a maser, among others—have been performed3, but only a few direct high-precision CPT tests that compare the fundamental properties of matter and antimatter are available4,5,6,7,8. Here we report high-precision cyclotron frequency comparisons of a single antiproton and a negatively charged hydrogen ion (H−) carried out in a Penning trap system. From 13,000 frequency measurements we compare the charge-to-mass ratio for the antiproton to that for the proton and obtain . The measurements were performed at cyclotron frequencies of 29.6 megahertz, so our result shows that the CPT theorem holds at the atto-electronvolt scale. Our precision of 69 parts per trillion exceeds the energy resolution of previous antiproton-to-proton mass comparisons7,9 as well as the respective figure of merit of the standard model extension10 by a factor of four. In addition, we give a limit on sidereal variations in the measured ratio of <720 parts per trillion. By following the arguments of ref. 11, our result can be interpreted as a stringent test of the weak equivalence principle of general relativity using baryonic antimatter, and it sets a new limit on the gravitational anomaly parameter of < 8.7 × 10−7.