A search for differences in the charge-to-mass ratio of protons and antiprotons, conducted at unprecedented levels of precision, results in stringent limits to the validity of fundamental physical symmetries. See Letter p.196
The standard model1 of particle physics is considered to be the best physical theory that we have. It is built on symmetries and can describe all the experiments and observations concerning the known subatomic particles. However, the model includes some 30 free parameters and is not fully explanatory. For example, it cannot explain a profound mystery of physics and cosmology2, the fact that there is no antimatter in the Universe. When matter and antimatter mutually annihilated each other following the Big Bang, any pre-existing symmetry between them was broken. Matter but no antimatter was left behind, and we lack a satisfactory explanation as to how this occurred3. Research on the fundamental differences between particles and antiparticles may provide an answer. In this vein, Ulmer et al.4 (page 196) perform a high-precision, comparative study of the properties of protons and antiprotons.
The authors used negatively charged hydrogen atoms (which represent protons for technical reasons) and individual antiprotons, the latter generated by the antiproton decelerator facility at CERN, Europe's particle-physics laboratory near Geneva, Switzerland. These species were stored in a sophisticated device known as a Penning trap, which consists of metal electrodes placed at defined electric potentials inside a strong and stable magnetic field (Fig. 1). In the trap, which has a diameter of just a few millimetres, the motion of electrically charged particles is similar to that in an accelerator such as the Large Hadron Collider at CERN, but the energies attained are 1015 times smaller.
A particle's cyclical motion in the Penning trap has a characteristic frequency (known as the cyclotron frequency), which is proportional to the magnetic field strength and the particle's charge-to-mass ratio. Ulmer et al. determined the cyclotron-frequency ratio for the antiproton and the negative hydrogen ion, alternately recycling the same individual particles at intervals of a few minutes from each other in the same experiment. The authors repeated this procedure 6,500 times within 35 days and scrutinized the results for systematic errors. Finally, they found that the charge-to-mass ratios of protons and antiprotons are equal to within 69 parts per trillion.
This result is four times more accurate than previous measurements5 of these ratios, and has implications for the validity of fundamental physical symmetries and theories that have been proposed to address unexplained aspects of the standard model. Symmetries have a central role in physics. A symmetry that holds across the Universe is an indication that a conservation law is at work. For example, adjusting a clock by an arbitrary time interval leaves all physical processes completely unaffected. A consequence of this is that energy can neither be created nor destroyed. But, when a symmetry is violated or a quantity is not conserved, a symmetry-breaking process must be at work.
In the process known as nuclear β-decay, for instance, a neutron is transformed into a proton, an electron and an antineutrino, but only antineutrinos of 'right-handed' nature appear. As a consequence, the electron is emitted into a preferred direction with respect to the neutron spin. This asymmetry is an example of parity (P) violation6, which means that β-decay would not proceed in exactly the same way in a mirrored version of the world. Similar symmetry violations are observed only in some processes that involve the weak force. They can appear if the signs of electric charges are reversed (charge conjugation, C), or if the arrow of time changes direction (time reversal, T). Symmetry violations also occur when the combination of C and P symmetries (CP symmetry) breaks down; these become evident for physical processes that occur differently when the signs of charges and handedness are changed simultaneously.
The physicist Andrei Sakharov offered7 an explanation for the observed dominance of matter, based on such a CP-symmetry violation. However, all the known CP-violating processes cannot sufficiently explain the preponderance of matter over antimatter. Furthermore, at current levels of precision, no physical process has been found to violate the combination of C, P and T symmetries (CPT symmetry), which relates to fundamental physical principles. In quantum mechanics, for example, this combined symmetry ensures that particle spins take only integer and half-integer values. Moreover, the invariance of physical laws in different moving frames of reference (known as the Lorentz invariance) implies CPT symmetry8,9.
Physicist Alan Kostelecký and colleagues have suggested that a violation of this symmetry might provide an alternative explanation for the missing antimatter10. Unlike Sakharov's model, which requires the disappearance of antimatter in the early, thermally unstable Universe, the latter model does not have this additional stringent condition. Under CPT symmetry, particles and antiparticles are strictly identical except for the sign of their charge. Ulmer and colleagues' measurements of the proton and antiproton charge-to-mass-ratios place limits on the differences between the properties of particles and antiparticles and establish a tighter boundary on a possible CPT-symmetry violation.
The charge-to-mass ratios measured by the authors do not vary by more than 720 parts per trillion during a sidereal day, which is the duration of a day with respect to the fixed positions of stars rather than to the Sun. Therefore, this level of accuracy excludes a violation of the CPT symmetry or of the related Lorentz invariance that could be attributed to a preferred frame of reference, such as the one provided by the cosmological microwave background (the Big Bang's relic radiation). It should also be noted that because the cyclotron frequency measurements took place in Earth's gravitational field, any difference in the way that protons and antiprotons interact with gravity would modify their respective frequencies11. However, the authors found no such difference larger than 870 parts per billion. This means that the weak equivalence principle — which states that all bodies in a given gravitational field undergo the same acceleration independently of their properties — holds at this level of accuracy.
Ulmer and colleagues' experiment has improved our understanding of fundamental physical principles by placing important limits on several processes. This experiment is a highlight of research on the central question of the prevailing matter–antimatter asymmetry, which the researchers approach by a promising route. Apart from the authors' tests of the CPT-symmetry invariance, there are other experiments12 that have searched for violations of the CP and T symmetries. The search for the former typically involved precise measurements of particle properties, including antiprotonic systems. The hunt for the latter included searches for the elusive permanent electric dipole moments of particles, and research on the correlations in the parameters of β-decaying nuclei and their decay products, such as neutrinos, electrons and daughter nuclei.
Highly precise experiments at low energies, such as this, are complementary to searches for evidence of fundamental symmetry violations in high-energy particle colliders. There is still no indication whether CPT- or CP-symmetry violations may be responsible for the matter–antimatter asymmetry and for any possible, but as yet unknown, differences between particles and antiparticles. Scientists therefore look forward to improved results from ongoing, well-motivated precision experiments3, involving antiprotons in particular13, which sustain the attack on one of the most intriguing questions in physics.