Supported by the National Natural Science Foundation of China (Grant No. 11935018，12075107, 12005113), the Beijing Spectrometer (BESIII) Collaboration has reported a new method to probe differences between matter and antimatter with extreme sensitivity. The results were published in Nature on June 2, 2022, entitled “Probing CP symmetry and weak phases with entangled double-strange baryons”. Here’s the link to this article: https://www.nature.com/articles/s41586-022-04624-1.

In particle physics, every kind of particle has a corresponding antiparticle. The standard Big Bang tells us that the Universe should have had the same amounts of matter and antimatter at the beginning. However, all the available data point to the fact that the observable Universe is predominantly composed of baryons rather than antibaryons, which has puzzled the scientific community for more than half a century. Do matter and antimatter follow different laws of physics?

Nowadays, physicists believe that to explain the dynamic origin of the baryon-antibaryon asymmetry the laws of physics must accommodate processes that violate charge conjugation and parity (CP) symmetry. In short, CP symmetry means that particles and antiparticles follow the same laws. For example, the decay patterns of particles and antiparticles should be the same. To explain the baryon-antibaryon asymmetry, CP symmetry has to be violated to a larger amount than predicted by the hitherto immensely successful Standard Model of particle physics.

The BESIII collaboration has exploited strange baryons to shed light on CP violation. The strange baryons consist of three quarks, just like protons, but contain one or more heavier and unstable strange quarks. By observing the decay of the strange quark, the spin orientation of the baryon can be determined. At BESIII (Figure 1), systems of double-strange baryon-antibaryon are created in electrons annihilations with positrons. The new BESIII results show that the produced baryons and antibaryons have a preferred direction. Moreover, the spin direction of the baryon and antibaryon are correlated, due to quantum entanglement (Figure 2). Studying angular distributions of the decays products of such systems allows for a separation of the contribution from CP violating processes, which are described by the nonzero value of the so-called weak phase. This phase had never been directly measured until this result was released by BESIII as described in the Nature article.  

Although no sign of CP violation was observed in the analyzed data sample, this experimental method can be applied to larger data sets collected at BESIII and other facilities in the future. There is hope to observe a CP violation signal with a size that either confirms or rules out the Standard Model predictions.

Figure 1. The BESIII detector at Institute of High Energy Physics in Beijing

Figure 2: Artistic interpretation of the decay cascade of a baryon-antibaryon pair. If matter

and antimatter follow the same laws, the decay pattern of a baryon should be the same as

that of an antibaryon, but with inverted spatial coordinates.