antiK0

antiK0, or K0-bar, is the antiparticle of the neutral kaon K0. In quark terms, K0 has the composition down quark (d) and anti-strange quark (s-bar), so antiK0 consists of an anti-down quark (anti-d) and strange quark (s). It is a neutral meson with spin zero and participates in strong and electromagnetic interactions like other hadrons, but its weak decay makes it unstable.

In strong interactions, K0 and antiK0 are flavor eigenstates with opposite strangeness: K0 has S = +1 and antiK0 has S = -1. The weak interaction, however, allows transitions between these states, causing a mixing that leads to physical neutral-kaon states known as K_S (short-lived) and K_L (long-lived). These states are quantum superpositions of K0 and antiK0, with lifetimes of about 9×10^-11 seconds for K_S and 5×10^-8 seconds for K_L.

Decays of the neutral kaon system proceed via weak interactions. K_S predominantly decays into two pions, while K_L decays most commonly into three pions or into semileptonic channels, with CP violation allowing rare two-pion decays of K_L as well. The study of antiK0 and the K0–antiK0 system was pivotal in uncovering CP violation, a fundamental asymmetry between matter and antimatter.

Significance: antiK0 is central to investigations of CP violation, strangeness oscillations, and quark mixing described by the CKM matrix. It is used in experiments exploring matter–antimatter asymmetries, regeneration phenomena in matter, and tests of fundamental symmetries in particle physics.