In high-energy physics, a pseudoscalar meson is a meson with total spin 0 and odd parity (usually notated as J = 0 ). Pseudoscalar mesons are commonly seen in proton -proton scattering and proton-antiproton annihilation, and include the pion (π), kaon (K), eta (η), and eta prime (η ′) particles, whose masses are known with great precision.