BESIII Collaboration reports a new method to probe differences between matter and antimatter with an extreme sensitivity. The results are published in the journal Nature on June 2^nd 2022.

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, 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. 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 has never been directly measured until this result by BESIII as described in the Nature article. “The weak phase measurement is the first real limit ever of any kind on CP phase of a P-wave amplitude and, even though it is the first one of all times, it is really stringent one, something like less than 3 degrees. Incredible, BESIII is now seriously in the CP Business”says Prof. Steve Olsen.

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