Form factors for this particle are parametrized by the Gounaris-Sakurai formula. Greisin, Kuzmin, and Zatsepin predicted that cosmic rays above a certain energy will interact 
Form factors for this particle are parametrized by the Gounaris-Sakurai formula. Greisin, Kuzmin, and Zatsepin predicted that cosmic rays above a certain energy will interact with CMB photons to make an unstable delta resonance, which would then decay to produce these particles. Kaons can decay into three of these particles and negative ones usually decay into a muon and muon antineutrino. They're composite particles, but proton-antiproton annihilation would shoot out a bunch of them. Before gluons, these particles were said to mediate the strong force by an interaction named for Yukawa, and this particle consists of only first generation quarks and their antiparticles. For 10 points, name this lightest meson.
Form factors for this particle are parametrized by the Gounaris-Sakurai formula.
pi meson [or pion; prompt on mesons]