The universe's survival mystery
The Big Bang should have created equal amounts of matter and antimatter. When matter and antimatter meet, they annihilate each other in a flash of energy. A perfectly symmetrical beginning would have resulted in an empty universe containing nothing but leftover radiation. Yet, here we are. The answer to this paradox lies in a subtle difference in their behavior, a phenomenon confirmed at Japan's High Energy Accelerator Research Organization (KEK).
From 1999 to 2010, KEK operated an experiment called Belle to investigate this asymmetry. It used the KEKB accelerator, a "B-factory" with a circumference of 3.016 kilometers, designed to produce enormous numbers of specific particles. The machine collided high-energy electrons (at 8 GeV) with lower-energy positrons (at 3.5 GeV). These collisions generated pairs of B mesons and their antimatter counterparts, anti-B mesons. The Belle detector, a massive instrument at the collision point, recorded the subsequent decays of over 771 million of these particle pairs. The results were clear: B mesons and anti-B mesons decay at slightly different rates. This violation of charge-parity (CP) symmetry proved that matter and antimatter are not perfect mirror images. The discovery provided the experimental proof for the theory developed by physicists Makoto Kobayashi and Toshihide Maskawa in 1973, who were awarded the Nobel Prize in Physics in 2008 for their work.
Pushing the luminosity frontier
The work at KEK did not stop with Belle. The facility has been upgraded to push the boundaries of physics even further. The accelerator complex is now known as SuperKEKB, and it began its first collisions in 2018. This upgraded machine collides 7 GeV electrons with 4 GeV positrons. Its primary goal is to achieve an instantaneous luminosity—the rate of particle collisions—approximately 40 times higher than its predecessor. On June 15, 2020, SuperKEKB achieved the world's highest instantaneous luminosity for a colliding-beam accelerator, a record it has since surpassed.
Inside SuperKEKB sits the new Belle II detector, a 1,400-ton instrument designed to record the flood of new data with unprecedented precision. By generating and analyzing billions more B mesons, physicists are searching for deviations from the Standard Model of particle physics. Such a discovery could show new particles or forces, further refining our understanding of the universe's fundamental laws. KEK also hosts other major research facilities. The Photon Factory generates intense X-rays using synchrotron radiation to study the structure of materials. The Japan Proton Accelerator Research Complex (J-PARC), a joint project with the Japan Atomic Energy Agency (JAEA), uses high-intensity proton beams for research in materials science, life science, and nuclear and particle physics, including creating neutrino beams for other experiments.