In 1947, the first of what have come to be known as "strange particles" were detected. As the number and variety of these particles proliferated, physicists began to try to make sense of them. Some seemed to have masses about 900 times that of the electron, and existed in both charged and neutral varieties. These particles are now called kaons (or K mesons), and they have become the subject of some of the most exciting research in particle physics. Kaon Physics at the Turn of the Millennium presents cutting-edge papers by leading theorists and experimentalists that synthesize the current state of the field and suggest promising new directions for the future study of kaons. Topics covered include the history of kaon physics, direct CP violation in kaon decays, time reversal violation, CPT studies, theoretical aspects of kaon physics, rare kaon decays, hyperon physics, charm: CP violation and mixing, the physics of B mesons, and future opportunities for kaon physics in the twenty-first century.

This book reports on a new result from the KL→π0νν search at the J-PARC KOTO experiment, which sets an upper limit of 3×10-9 for the branching fraction of the decay at the 90% confidence level, improving the previous best limit by an order of magnitude. To explain the matter–antimatter asymmetry in the universe, still unknown new physics beyond the standard model (SM) that breaks CP symmetry is necessary. The rare decay of a long-lived neutral K meson, KL→π0νν, is a CP-violating decay. It is an excellent probe to search for new physics because new physics can contribute to the decay and change its branching fraction, while the SM is as small as 3×10-11. However, it is extremely difficult to search for because all of the decay products are neutral and two neutrinos are undetectable. The KL→π0νν signal is identified by measuring two photons from a π0 with a calorimeter and confirming the absence of any other detectable particles with hermetic veto counters. The book contributes to the analysis of neutron-induced backgrounds which were the dominant background sources in the search. For the background caused by two consecutive hadronic showers in the calorimeter due to a neutron, the author evaluated the background yield using a data-driven approach. For another background caused by an η meson production—η decays two photons—by a neutron that hits a veto counter near the calorimeter, the author developed an original analysis technique to reduce it. The book also contributes to the analysis of the normalization modes (KL→3π0, KL→2π0, KL→2γ) to measure KL yield, the estimation of the signal acceptance based on a simulation, and the evaluation of the trigger efficiency. As a result, significant improvements in the measurement were achieved, and this is an important step in the continuing higher sensitivity search, which can reach new physics with the energy scales up to O(100-1000 TeV).

This book is not just a conference proceedings covering the full spectrum of physics disciplines. It is also a historic retrospective on the past generation of giants in Chinese physics. It covers the historical tributes by Nobel Laureates Lee and Yang and others to the life and works of Professors Ta-You Wu, Chien-Shiung Wu and Xie Xi-de. In the words of the title in Chinese, as we drink the water let us ponder the source.