A phenomenon known as CP asymmetry, which explains our very existence, has been observed in the decays of B s 0 mesonic particles. The finding represents yet another triumph of the standard model of particle physics.

“The LHCb measurement reinforces the standard model's explanation of how the weak force distinguishes matter from antimatter.”

The first observation of matter–antimatter asymmetry in the decays of particles known as B s 0 mesons has been reported in Physical Review Letters by the LHCb collaboration1 (Aaij et al.) at the Large Hadron Collider at CERN, near Geneva, Switzerland. This measurement reinforces the standard model of particle physics and, in particular, its explanation of how the weak force, which governs radioactive decays, distinguishes matter from antimatter.

Matter particles and their antiparticles have opposite charge (C) and parity (P), the latter meaning that, under spatial-inversion transformation, the direction of a particle's spin relative to its direction of motion is reversed. The difference in the laws of physics obeyed by matter and antimatter is experimentally explored by comparing the rate of a process — such as the rate at which a particle will decay into other, lighter particles — with the rate of its CP-related process, in which all the initial and final particles are replaced with the corresponding antiparticles. If these rates are measured to be different, then a 'CP asymmetry' has been observed. Measurements of CP asymmetries are interesting for two reasons. First, they provide stringent tests of the standard model of particle physics. Second, they may provide hints that could point scientists in the direction of a solution to the mystery of the Universe's matter–antimatter imbalance.

Among the four known forces of nature — the standard model's weak, strong and electromagnetic forces, and the gravitational force — the weak force is the only one that distinguishes between matter and antimatter. But even the weak force would not do so if it were not that three quark particles exist for each of two charge types: the down, strange and bottom quarks, which have a charge that is minus one-third of the proton's charge; and the up, charm and top quarks, which have a charge that is two-thirds of the proton's charge. With three quarks of either charge, the weak force has a single coupling constant (a parameter that quantifies the force's strength), which is different for particles and antiparticles2,3. If, as the standard model predicts, all CP asymmetries are proportional to this single coupling constant, then the sizes of these asymmetries are correlated. By contrast, most theories that go beyond the standard model have many independent coupling constants that distinguish particles from antiparticles, and thus predict deviations from the standard-model correlations.

Mesons are bound states of one quark and one antiquark. Until the LHCb measurement was made, CP asymmetries had been observed in the decays of three types of meson: K0 mesons, B0 mesons, and B± mesons. The LHCb experiment is the first to measure CP asymmetry in the decay of a fourth type of meson, the B s 0 meson. The fact that, within the standard model, the difference in the laws of physics followed by matter and antimatter is encoded in a single parameter provides a particularly strong correlation between this asymmetry and an asymmetry in B0 decay that has been measured by several experiments with high accuracy. Although neither of these asymmetries, nor the corresponding decay rates, can be theoretically predicted with any accuracy, the product of the CP asymmetry with the decay rate should, according to the standard model, be equal (to a good approximation) between the B s 0 decay and the B0 decay4,5. The LHCb measurement is consistent with the predicted equality at the level of a few per cent, implying yet another triumph of the standard model.

In many extensions of the standard model, new particles are predicted to interact more strongly with the heavier quarks (strange, bottom, charm and top) than with the lighter ones (up and down). The K0 mesons, the B0 mesons and the B± mesons all have either a down (anti)quark or an up (anti)quark. But the B s 0 mesons are different: they are made up of a strange quark and a bottom antiquark. Their antiparticles are composed of a bottom quark and a strange antiquark. Thus, scientists had hoped that although the effects of new physics were negligibly small in all previously measured CP asymmetries, they would be large enough to be observed in B s 0 decays. Frustratingly, this seems not to be the case.

The standard-model prediction2 of how the weak force distinguishes between matter and antimatter therefore continues to successfully describe all measurements of CP asymmetries in meson decays. However, the standard model fails to explain the Universe's matter–antimatter imbalance. All structures in the Universe, from clusters of galaxies to human cells, are made of matter: protons, neutrons and electrons. Their antiparticles — the antiprotons, antineutrons and positrons — are not found in the Universe at large. If the laws of nature were identical for matter and antimatter, then particles and antiparticles would have been created in equal amounts and would then have annihilated each other, leaving only pure radiation and no matter structures in the Universe. Our very existence is possible only because of CP asymmetries. The standard model allows all antimatter to disappear from the Universe, but it predicts that the amount of surviving matter is many orders of magnitude smaller than observed.

Therefore, there must exist a force, as yet unknown to us, that distinguishes matter from antimatter in a way that is much stronger than that of the weak force. Theorists have made various suggestions as to what this new force might be. To disclose the nature of this force, further hints from experiments are needed. Searching for CP asymmetries in neutrino 'oscillations' of one type into another and for the electric-dipole moments of the neutron and the electron, in addition to measuring CP asymmetries in B s 0-meson decays, seem the most promising avenues through which to obtain such hints. The consistency of the LHCb measurement with the standard model provides further motivation to pursue the other searches even more vigorously.

References 1 Aaij, R. et al. Phys. Rev. Lett. 110, 221601 (2013). 2 Kobayashi, M. & Maskawa, T. Prog. Theor. Phys. 49, 652–657 (1973). 3 Jarlskog, C. Phys. Rev. Lett. 55, 1039–1042 (1985). 4 Lipkin, H. J. Phys. Lett. B 621, 126–132 (2005). 5 Gronau, M. & Rosner, J. L. Phys. Lett. B 482, 71–76 (2000). Download references

Author information Affiliations Yosef Nir is in the Department of Particle Physics, Weizmann Institute of Science, Rehovot 76100, Israel. Yosef Nir Authors Yosef Nir View author publications You can also search for this author in PubMed Google Scholar Corresponding author Correspondence to Yosef Nir.

Rights and permissions Reprints and Permissions